[aarch64,middle-end] v1: Move pair_fusion pass from aarch64 to middle-end

Message ID a50a7569-a8d2-46a2-b5b7-0832b782f994@linux.ibm.com
State New
Headers
Series [aarch64,middle-end] v1: Move pair_fusion pass from aarch64 to middle-end |

Checks

Context Check Description
linaro-tcwg-bot/tcwg_gcc_build--master-aarch64 success Testing passed
linaro-tcwg-bot/tcwg_gcc_check--master-aarch64 success Testing passed
linaro-tcwg-bot/tcwg_gcc_build--master-arm success Testing passed
linaro-tcwg-bot/tcwg_gcc_check--master-arm success Testing passed

Commit Message

Ajit Agarwal May 21, 2024, 10:24 a.m. UTC
  Hello Alex/Richard:

All comments are addressed.

Move pair fusion pass from aarch64-ldp-fusion.cc to middle-end
to support multiple targets.

Common infrastructure of load store pair fusion is divided into
target independent and target dependent code.

Target independent code is structured in the following files.
gcc/pair-fusion.h
gcc/pair-fusion.cc

Target independent code is the Generic code with pure virtual
function to interface betwwen target independent and dependent
code.

Bootstrapped and regtested on aarch64-linux-gnu.

Thanks & Regards
Ajit


aarch64, middle-end: Move pair_fusion pass from aarch64 to middle-end

Move pair fusion pass from aarch64-ldp-fusion.cc to middle-end
to support multiple targets.

Common infrastructure of load store pair fusion is divided into
target independent and target dependent code.

Target independent code is structured in the following files.
gcc/pair-fusion.h
gcc/pair-fusion.cc

Target independent code is the Generic code with pure virtual
function to interface betwwen target independent and dependent
code.

2024-05-21  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>

gcc/ChangeLog:

	* pair-fusion.h: Generic header code for load store pair fusion
	that can be shared across different architectures.
	* pair-fusion.cc: Generic source code implementation for
	load store pair fusion that can be shared across different architectures.
	* Makefile.in: Add new object file pair-fusion.o.
	* config/aarch64/aarch64-ldp-fusion.cc: Delete generic code and move it
	to pair-fusion.cc in the middle-end.
---
 gcc/Makefile.in                          |    1 +
 gcc/config/aarch64/aarch64-ldp-fusion.cc | 3295 +---------------------
 gcc/pair-fusion.cc                       | 2992 ++++++++++++++++++++
 gcc/pair-fusion.h                        |  201 ++
 4 files changed, 3269 insertions(+), 3220 deletions(-)
 create mode 100644 gcc/pair-fusion.cc
 create mode 100644 gcc/pair-fusion.h
  

Patch

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index a7f15694c34..643342f623d 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1563,6 +1563,7 @@  OBJS = \
 	ipa-strub.o \
 	ipa.o \
 	ira.o \
+	pair-fusion.o \
 	ira-build.o \
 	ira-costs.o \
 	ira-conflicts.o \
diff --git a/gcc/config/aarch64/aarch64-ldp-fusion.cc b/gcc/config/aarch64/aarch64-ldp-fusion.cc
index 085366cdf68..9fdb2a5f323 100644
--- a/gcc/config/aarch64/aarch64-ldp-fusion.cc
+++ b/gcc/config/aarch64/aarch64-ldp-fusion.cc
@@ -17,18 +17,7 @@ 
 // along with GCC; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-#define INCLUDE_ALGORITHM
-#define INCLUDE_FUNCTIONAL
-#define INCLUDE_LIST
-#define INCLUDE_TYPE_TRAITS
-#include "config.h"
-#include "system.h"
-#include "coretypes.h"
-#include "backend.h"
-#include "rtl.h"
-#include "df.h"
-#include "rtl-iter.h"
-#include "rtl-ssa.h"
+#include "pair-fusion.h"
 #include "cfgcleanup.h"
 #include "tree-pass.h"
 #include "ordered-hash-map.h"
@@ -38,264 +27,11 @@ 
 #include "print-tree.h"
 #include "insn-attr.h"
 
-using namespace rtl_ssa;
-
 static constexpr HOST_WIDE_INT LDP_IMM_BITS = 7;
 static constexpr HOST_WIDE_INT LDP_IMM_SIGN_BIT = (1 << (LDP_IMM_BITS - 1));
 static constexpr HOST_WIDE_INT LDP_MAX_IMM = LDP_IMM_SIGN_BIT - 1;
 static constexpr HOST_WIDE_INT LDP_MIN_IMM = -LDP_MAX_IMM - 1;
 
-// We pack these fields (load_p, fpsimd_p, and size) into an integer
-// (LFS) which we use as part of the key into the main hash tables.
-//
-// The idea is that we group candidates together only if they agree on
-// the fields below.  Candidates that disagree on any of these
-// properties shouldn't be merged together.
-struct lfs_fields
-{
-  bool load_p;
-  bool fpsimd_p;
-  unsigned size;
-};
-
-using insn_list_t = std::list<insn_info *>;
-using insn_iter_t = insn_list_t::iterator;
-
-// Information about the accesses at a given offset from a particular
-// base.  Stored in an access_group, see below.
-struct access_record
-{
-  poly_int64 offset;
-  std::list<insn_info *> cand_insns;
-  std::list<access_record>::iterator place;
-
-  access_record (poly_int64 off) : offset (off) {}
-};
-
-// A group of accesses where adjacent accesses could be ldp/stp
-// candidates.  The splay tree supports efficient insertion,
-// while the list supports efficient iteration.
-struct access_group
-{
-  splay_tree<access_record *> tree;
-  std::list<access_record> list;
-
-  template<typename Alloc>
-  inline void track (Alloc node_alloc, poly_int64 offset, insn_info *insn);
-};
-
-// Information about a potential base candidate, used in try_fuse_pair.
-// There may be zero, one, or two viable RTL bases for a given pair.
-struct base_cand
-{
-  // DEF is the def of the base register to be used by the pair.
-  def_info *def;
-
-  // FROM_INSN is -1 if the base candidate is already shared by both
-  // candidate insns.  Otherwise it holds the index of the insn from
-  // which the base originated.
-  //
-  // In the case that the base is shared, either DEF is already used
-  // by both candidate accesses, or both accesses see different versions
-  // of the same regno, in which case DEF is the def consumed by the
-  // first candidate access.
-  int from_insn;
-
-  // To form a pair, we do so by moving the first access down and the second
-  // access up.  To determine where to form the pair, and whether or not
-  // it is safe to form the pair, we track instructions which cannot be
-  // re-ordered past due to either dataflow or alias hazards.
-  //
-  // Since we allow changing the base used by an access, the choice of
-  // base can change which instructions act as re-ordering hazards for
-  // this pair (due to different dataflow).  We store the initial
-  // dataflow hazards for this choice of base candidate in HAZARDS.
-  //
-  // These hazards act as re-ordering barriers to each candidate insn
-  // respectively, in program order.
-  //
-  // Later on, when we take alias analysis into account, we narrow
-  // HAZARDS accordingly.
-  insn_info *hazards[2];
-
-  base_cand (def_info *def, int insn)
-    : def (def), from_insn (insn), hazards {nullptr, nullptr} {}
-
-  base_cand (def_info *def) : base_cand (def, -1) {}
-
-  // Test if this base candidate is viable according to HAZARDS.
-  bool viable () const
-  {
-    return !hazards[0] || !hazards[1] || (*hazards[0] > *hazards[1]);
-  }
-};
-
-// Information about an alternate base.  For a def_info D, it may
-// instead be expressed as D = BASE + OFFSET.
-struct alt_base
-{
-  def_info *base;
-  poly_int64 offset;
-};
-
-// Virtual base class for load/store walkers used in alias analysis.
-struct alias_walker
-{
-  virtual bool conflict_p (int &budget) const = 0;
-  virtual insn_info *insn () const = 0;
-  virtual bool valid () const = 0;
-  virtual void advance () = 0;
-};
-
-// When querying should_handle_writeback, this enum is used to
-// qualify which opportunities we are asking about.
-enum class writeback {
-  // Only those writeback opportunities that arise from existing
-  // auto-increment accesses.
-  EXISTING,
-
-  // All writeback opportunities, including those that involve folding
-  // base register updates into a non-writeback pair.
-  ALL
-};
-
-// This class can be overriden by targets to give a pass that fuses
-// adjacent loads and stores into load/store pair instructions.
-//
-// The target can override the various virtual functions to customize
-// the behaviour of the pass as appropriate for the target.
-struct pair_fusion {
-  pair_fusion ();
-
-  // Given:
-  // - an rtx REG_OP, the non-memory operand in a load/store insn,
-  // - a machine_mode MEM_MODE, the mode of the MEM in that insn, and
-  // - a boolean LOAD_P (true iff the insn is a load), then:
-  // return true if the access should be considered an FP/SIMD access.
-  // Such accesses are segregated from GPR accesses, since we only want
-  // to form pairs for accesses that use the same register file.
-  virtual bool fpsimd_op_p (rtx, machine_mode, bool)
-  {
-    return false;
-  }
-
-  // Return true if we should consider forming pairs from memory
-  // accesses with operand mode MODE at this stage in compilation.
-  virtual bool pair_operand_mode_ok_p (machine_mode mode) = 0;
-
-  // Return true iff REG_OP is a suitable register operand for a paired
-  // memory access, where LOAD_P is true if we're asking about loads and
-  // false for stores.  MODE gives the mode of the operand.
-  virtual bool pair_reg_operand_ok_p (bool load_p, rtx reg_op,
-				      machine_mode mode) = 0;
-
-  // Return alias check limit.
-  // This is needed to avoid unbounded quadratic behaviour when
-  // performing alias analysis.
-  virtual int pair_mem_alias_check_limit () = 0;
-
-  // Return true if we should try to handle writeback opportunities.
-  // WHICH determines the kinds of writeback opportunities the caller
-  // is asking about.
-  virtual bool should_handle_writeback (enum writeback which) = 0;
-
-  // Given BASE_MEM, the mem from the lower candidate access for a pair,
-  // and LOAD_P (true if the access is a load), check if we should proceed
-  // to form the pair given the target's code generation policy on
-  // paired accesses.
-  virtual bool pair_mem_ok_with_policy (rtx base_mem, bool load_p) = 0;
-
-  // Generate the pattern for a paired access.  PATS gives the patterns
-  // for the individual memory accesses (which by this point must share a
-  // common base register).  If WRITEBACK is non-NULL, then this rtx
-  // describes the update to the base register that should be performed by
-  // the resulting insn.  LOAD_P is true iff the accesses are loads.
-  virtual rtx gen_pair (rtx *pats, rtx writeback, bool load_p) = 0;
-
-  // Return true if INSN is a paired memory access.  If so, set LOAD_P to
-  // true iff INSN is a load pair.
-  virtual bool pair_mem_insn_p (rtx_insn *insn, bool &load_p) = 0;
-
-  // Return true if we should track loads.
-  virtual bool track_loads_p ()
-  {
-    return true;
-  }
-
-  // Return true if we should track stores.
-  virtual bool track_stores_p ()
-  {
-    return true;
-  }
-
-  // Return true if OFFSET is in range for a paired memory access.
-  virtual bool pair_mem_in_range_p (HOST_WIDE_INT offset) = 0;
-
-  // Given a load/store pair insn in PATTERN, unpack the insn, storing
-  // the register operands in REGS, and returning the mem.  LOAD_P is
-  // true for loads and false for stores.
-  virtual rtx destructure_pair (rtx regs[2], rtx pattern, bool load_p) = 0;
-
-  // Given a pair mem in MEM, register operands in REGS, and an rtx
-  // representing the effect of writeback on the base register in WB_EFFECT,
-  // return an insn representing a writeback variant of this pair.
-  // LOAD_P is true iff the pair is a load.
-  // This is used when promoting existing non-writeback pairs to writeback
-  // variants.
-  virtual rtx gen_promote_writeback_pair (rtx wb_effect, rtx mem,
-					  rtx regs[2], bool load_p) = 0;
-
-  void process_block (bb_info *bb);
-  inline insn_info *find_trailing_add (insn_info *insns[2],
-				       const insn_range_info &pair_range,
-				       int initial_writeback,
-				       rtx *writeback_effect,
-				       def_info **add_def,
-				       def_info *base_def,
-				       poly_int64 initial_offset,
-				       unsigned access_size);
-  inline int get_viable_bases (insn_info *insns[2],
-			       vec<base_cand> &base_cands,
-			       rtx cand_mems[2],
-			       unsigned access_size,
-			       bool reversed);
-  inline void do_alias_analysis (insn_info *alias_hazards[4],
-				 alias_walker *walkers[4],
-				 bool load_p);
-  inline void try_promote_writeback (insn_info *insn, bool load_p);
-  inline void run ();
-  ~pair_fusion ();
-};
-
-pair_fusion::pair_fusion ()
-{
-  calculate_dominance_info (CDI_DOMINATORS);
-  df_analyze ();
-  crtl->ssa = new rtl_ssa::function_info (cfun);
-}
-
-pair_fusion::~pair_fusion ()
-{
-  if (crtl->ssa->perform_pending_updates ())
-    cleanup_cfg (0);
-
-  free_dominance_info (CDI_DOMINATORS);
-
-  delete crtl->ssa;
-  crtl->ssa = nullptr;
-}
-
-// This is the main function to start the pass.
-void
-pair_fusion::run ()
-{
-  if (!track_loads_p () && !track_stores_p ())
-    return;
-
-  for (auto bb : crtl->ssa->bbs ())
-    process_block (bb);
-}
-
 struct aarch64_pair_fusion : public pair_fusion
 {
   bool fpsimd_op_p (rtx reg_op, machine_mode mem_mode,
@@ -367,83 +103,6 @@  struct aarch64_pair_fusion : public pair_fusion
   rtx destructure_pair (rtx regs[2], rtx pattern, bool load_p) override final;
 };
 
-// State used by the pass for a given basic block.
-struct pair_fusion_bb_info
-{
-  using def_hash = nofree_ptr_hash<def_info>;
-  using expr_key_t = pair_hash<tree_operand_hash, int_hash<int, -1, -2>>;
-  using def_key_t = pair_hash<def_hash, int_hash<int, -1, -2>>;
-
-  // Map of <tree base, LFS> -> access_group.
-  ordered_hash_map<expr_key_t, access_group> expr_map;
-
-  // Map of <RTL-SSA def_info *, LFS> -> access_group.
-  ordered_hash_map<def_key_t, access_group> def_map;
-
-  // Given the def_info for an RTL base register, express it as an offset from
-  // some canonical base instead.
-  //
-  // Canonicalizing bases in this way allows us to identify adjacent accesses
-  // even if they see different base register defs.
-  hash_map<def_hash, alt_base> canon_base_map;
-
-  static const size_t obstack_alignment = sizeof (void *);
-
-  pair_fusion_bb_info (bb_info *bb, pair_fusion *d)
-    : m_bb (bb), m_pass (d), m_emitted_tombstone (false)
-  {
-    obstack_specify_allocation (&m_obstack, OBSTACK_CHUNK_SIZE,
-				obstack_alignment, obstack_chunk_alloc,
-				obstack_chunk_free);
-  }
-  ~pair_fusion_bb_info ()
-  {
-    obstack_free (&m_obstack, nullptr);
-
-    if (m_emitted_tombstone)
-      {
-	bitmap_release (&m_tombstone_bitmap);
-	bitmap_obstack_release (&m_bitmap_obstack);
-      }
-  }
-
-  inline void track_access (insn_info *, bool load, rtx mem);
-  inline void transform ();
-  inline void cleanup_tombstones ();
-
-private:
-  obstack m_obstack;
-  bb_info *m_bb;
-  pair_fusion *m_pass;
-
-  // State for keeping track of tombstone insns emitted for this BB.
-  bitmap_obstack m_bitmap_obstack;
-  bitmap_head m_tombstone_bitmap;
-  bool m_emitted_tombstone;
-
-  inline splay_tree_node<access_record *> *node_alloc (access_record *);
-
-  template<typename Map>
-  inline void traverse_base_map (Map &map);
-  inline void transform_for_base (int load_size, access_group &group);
-
-  inline void merge_pairs (insn_list_t &, insn_list_t &,
-			   bool load_p, unsigned access_size);
-
-  inline bool try_fuse_pair (bool load_p, unsigned access_size,
-			     insn_info *i1, insn_info *i2);
-
-  inline bool fuse_pair (bool load_p, unsigned access_size,
-			 int writeback,
-			 insn_info *i1, insn_info *i2,
-			 base_cand &base,
-			 const insn_range_info &move_range);
-
-  inline void track_tombstone (int uid);
-
-  inline bool track_via_mem_expr (insn_info *, rtx mem, lfs_fields lfs);
-};
-
 bool
 aarch64_pair_fusion::pair_mem_insn_p (rtx_insn *rti, bool &load_p)
 {
@@ -483,103 +142,6 @@  aarch64_pair_fusion::gen_pair (rtx *pats, rtx writeback, bool load_p)
   return pair_pat;
 }
 
-splay_tree_node<access_record *> *
-pair_fusion_bb_info::node_alloc (access_record *access)
-{
-  using T = splay_tree_node<access_record *>;
-  void *addr = obstack_alloc (&m_obstack, sizeof (T));
-  return new (addr) T (access);
-}
-
-// Given a mem MEM, if the address has side effects, return a MEM that accesses
-// the same address but without the side effects.  Otherwise, return
-// MEM unchanged.
-static rtx
-drop_writeback (rtx mem)
-{
-  rtx addr = XEXP (mem, 0);
-
-  if (!side_effects_p (addr))
-    return mem;
-
-  switch (GET_CODE (addr))
-    {
-    case PRE_MODIFY:
-      addr = XEXP (addr, 1);
-      break;
-    case POST_MODIFY:
-    case POST_INC:
-    case POST_DEC:
-      addr = XEXP (addr, 0);
-      break;
-    case PRE_INC:
-    case PRE_DEC:
-    {
-      poly_int64 adjustment = GET_MODE_SIZE (GET_MODE (mem));
-      if (GET_CODE (addr) == PRE_DEC)
-	adjustment *= -1;
-      addr = plus_constant (GET_MODE (addr), XEXP (addr, 0), adjustment);
-      break;
-    }
-    default:
-      gcc_unreachable ();
-    }
-
-  return change_address (mem, GET_MODE (mem), addr);
-}
-
-// Convenience wrapper around strip_offset that can also look through
-// RTX_AUTOINC addresses.  The interface is like strip_offset except we take a
-// MEM so that we know the mode of the access.
-static rtx
-pair_mem_strip_offset (rtx mem, poly_int64 *offset)
-{
-  rtx addr = XEXP (mem, 0);
-
-  switch (GET_CODE (addr))
-    {
-    case PRE_MODIFY:
-    case POST_MODIFY:
-      addr = strip_offset (XEXP (addr, 1), offset);
-      gcc_checking_assert (REG_P (addr));
-      gcc_checking_assert (rtx_equal_p (XEXP (XEXP (mem, 0), 0), addr));
-      break;
-    case PRE_INC:
-    case POST_INC:
-      addr = XEXP (addr, 0);
-      *offset = GET_MODE_SIZE (GET_MODE (mem));
-      gcc_checking_assert (REG_P (addr));
-      break;
-    case PRE_DEC:
-    case POST_DEC:
-      addr = XEXP (addr, 0);
-      *offset = -GET_MODE_SIZE (GET_MODE (mem));
-      gcc_checking_assert (REG_P (addr));
-      break;
-
-    default:
-      addr = strip_offset (addr, offset);
-    }
-
-  return addr;
-}
-
-// Return true if X is a PRE_{INC,DEC,MODIFY} rtx.
-static bool
-any_pre_modify_p (rtx x)
-{
-  const auto code = GET_CODE (x);
-  return code == PRE_INC || code == PRE_DEC || code == PRE_MODIFY;
-}
-
-// Return true if X is a POST_{INC,DEC,MODIFY} rtx.
-static bool
-any_post_modify_p (rtx x)
-{
-  const auto code = GET_CODE (x);
-  return code == POST_INC || code == POST_DEC || code == POST_MODIFY;
-}
-
 // Return true if we should consider forming ldp/stp insns from memory
 // accesses with operand mode MODE at this stage in compilation.
 bool
@@ -594,2816 +156,109 @@  aarch64_pair_fusion::pair_operand_mode_ok_p (machine_mode mode)
   return reload_completed || mode != TImode;
 }
 
-// Given LFS (load_p, fpsimd_p, size) fields in FIELDS, encode these
-// into an integer for use as a hash table key.
-static int
-encode_lfs (lfs_fields fields)
-{
-  int size_log2 = exact_log2 (fields.size);
-  gcc_checking_assert (size_log2 >= 2 && size_log2 <= 4);
-  return ((int)fields.load_p << 3)
-    | ((int)fields.fpsimd_p << 2)
-    | (size_log2 - 2);
-}
-
-// Inverse of encode_lfs.
-static lfs_fields
-decode_lfs (int lfs)
-{
-  bool load_p = (lfs & (1 << 3));
-  bool fpsimd_p = (lfs & (1 << 2));
-  unsigned size = 1U << ((lfs & 3) + 2);
-  return { load_p, fpsimd_p, size };
-}
-
-// Track the access INSN at offset OFFSET in this access group.
-// ALLOC_NODE is used to allocate splay tree nodes.
-template<typename Alloc>
-void
-access_group::track (Alloc alloc_node, poly_int64 offset, insn_info *insn)
+// Given a pair mode MODE, return a canonical mode to be used for a single
+// operand of such a pair.  Currently we only use this when promoting a
+// non-writeback pair into a writeback pair, as it isn't otherwise clear
+// which mode to use when storing a modeless CONST_INT.
+static machine_mode
+aarch64_operand_mode_for_pair_mode (machine_mode mode)
 {
-  auto insert_before = [&](std::list<access_record>::iterator after)
-    {
-      auto it = list.emplace (after, offset);
-      it->cand_insns.push_back (insn);
-      it->place = it;
-      return &*it;
-    };
-
-  if (!list.size ())
-    {
-      auto access = insert_before (list.end ());
-      tree.insert_max_node (alloc_node (access));
-      return;
-    }
-
-  auto compare = [&](splay_tree_node<access_record *> *node)
-    {
-      return compare_sizes_for_sort (offset, node->value ()->offset);
-    };
-  auto result = tree.lookup (compare);
-  splay_tree_node<access_record *> *node = tree.root ();
-  if (result == 0)
-    node->value ()->cand_insns.push_back (insn);
-  else
+  switch (mode)
     {
-      auto it = node->value ()->place;
-      auto after = (result > 0) ? std::next (it) : it;
-      auto access = insert_before (after);
-      tree.insert_child (node, result > 0, alloc_node (access));
+    case E_V2x4QImode:
+      return SImode;
+    case E_V2x8QImode:
+      return DImode;
+    case E_V2x16QImode:
+      return V16QImode;
+    default:
+      gcc_unreachable ();
     }
 }
 
-// Given a candidate access INSN (with mem MEM), see if it has a suitable
-// MEM_EXPR base (i.e. a tree decl) relative to which we can track the access.
-// LFS is used as part of the key to the hash table, see track_access.
-bool
-pair_fusion_bb_info::track_via_mem_expr (insn_info *insn, rtx mem,
-					 lfs_fields lfs)
+// Given a load pair insn in PATTERN, unpack the insn, storing
+// the registers in REGS and returning the mem.
+static rtx
+aarch64_destructure_load_pair (rtx regs[2], rtx pattern)
 {
-  if (!MEM_EXPR (mem) || !MEM_OFFSET_KNOWN_P (mem))
-    return false;
-
-  poly_int64 offset;
-  tree base_expr = get_addr_base_and_unit_offset (MEM_EXPR (mem),
-						  &offset);
-  if (!base_expr || !DECL_P (base_expr))
-    return false;
-
-  offset += MEM_OFFSET (mem);
-
-  const machine_mode mem_mode = GET_MODE (mem);
-  const HOST_WIDE_INT mem_size = GET_MODE_SIZE (mem_mode).to_constant ();
-
-  // Punt on misaligned offsets.  Paired memory access instructions require
-  // offsets to be a multiple of the access size, and we believe that
-  // misaligned offsets on MEM_EXPR bases are likely to lead to misaligned
-  // offsets w.r.t. RTL bases.
-  if (!multiple_p (offset, mem_size))
-    return false;
-
-  const auto key = std::make_pair (base_expr, encode_lfs (lfs));
-  access_group &group = expr_map.get_or_insert (key, NULL);
-  auto alloc = [&](access_record *access) { return node_alloc (access); };
-  group.track (alloc, offset, insn);
+  rtx mem = NULL_RTX;
 
-  if (dump_file)
+  for (int i = 0; i < 2; i++)
     {
-      fprintf (dump_file, "[bb %u] tracking insn %d via ",
-	       m_bb->index (), insn->uid ());
-      print_node_brief (dump_file, "mem expr", base_expr, 0);
-      fprintf (dump_file, " [L=%d FP=%d, %smode, off=",
-	       lfs.load_p, lfs.fpsimd_p, mode_name[mem_mode]);
-      print_dec (offset, dump_file);
-      fprintf (dump_file, "]\n");
+      rtx pat = XVECEXP (pattern, 0, i);
+      regs[i] = XEXP (pat, 0);
+      rtx unspec = XEXP (pat, 1);
+      gcc_checking_assert (GET_CODE (unspec) == UNSPEC);
+      rtx this_mem = XVECEXP (unspec, 0, 0);
+      if (mem)
+	gcc_checking_assert (rtx_equal_p (mem, this_mem));
+      else
+	{
+	  gcc_checking_assert (MEM_P (this_mem));
+	  mem = this_mem;
+	}
     }
 
-  return true;
+  return mem;
 }
 
-// Main function to begin pair discovery.  Given a memory access INSN,
-// determine whether it could be a candidate for fusing into a paired
-// access, and if so, track it in the appropriate data structure for
-// this basic block.  LOAD_P is true if the access is a load, and MEM
-// is the mem rtx that occurs in INSN.
-void
-pair_fusion_bb_info::track_access (insn_info *insn, bool load_p, rtx mem)
+// Given a store pair insn in PATTERN, unpack the insn, storing
+// the register operands in REGS, and returning the mem.
+static rtx
+aarch64_destructure_store_pair (rtx regs[2], rtx pattern)
 {
-  // We can't combine volatile MEMs, so punt on these.
-  if (MEM_VOLATILE_P (mem))
-    return;
-
-  // Ignore writeback accesses if the hook says to do so.
-  if (!m_pass->should_handle_writeback (writeback::EXISTING)
-      && GET_RTX_CLASS (GET_CODE (XEXP (mem, 0))) == RTX_AUTOINC)
-    return;
-
-  const machine_mode mem_mode = GET_MODE (mem);
-  if (!m_pass->pair_operand_mode_ok_p (mem_mode))
-    return;
-
-  rtx reg_op = XEXP (PATTERN (insn->rtl ()), !load_p);
-
-  if (!m_pass->pair_reg_operand_ok_p (load_p, reg_op, mem_mode))
-    return;
-
-  // We want to segregate FP/SIMD accesses from GPR accesses.
-  const bool fpsimd_op_p = m_pass->fpsimd_op_p (reg_op, mem_mode, load_p);
-
-  // Note pair_operand_mode_ok_p already rejected VL modes.
-  const HOST_WIDE_INT mem_size = GET_MODE_SIZE (mem_mode).to_constant ();
-  const lfs_fields lfs = { load_p, fpsimd_op_p, mem_size };
-
-  if (track_via_mem_expr (insn, mem, lfs))
-    return;
-
-  poly_int64 mem_off;
-  rtx addr = XEXP (mem, 0);
-  const bool autoinc_p = GET_RTX_CLASS (GET_CODE (addr)) == RTX_AUTOINC;
-  rtx base = pair_mem_strip_offset (mem, &mem_off);
-  if (!REG_P (base))
-    return;
+  rtx mem = XEXP (pattern, 0);
+  rtx unspec = XEXP (pattern, 1);
+  gcc_checking_assert (GET_CODE (unspec) == UNSPEC);
+  for (int i = 0; i < 2; i++)
+    regs[i] = XVECEXP (unspec, 0, i);
+  return mem;
+}
 
-  // Need to calculate two (possibly different) offsets:
-  //  - Offset at which the access occurs.
-  //  - Offset of the new base def.
-  poly_int64 access_off;
-  if (autoinc_p && any_post_modify_p (addr))
-    access_off = 0;
+rtx
+aarch64_pair_fusion::destructure_pair (rtx regs[2], rtx pattern, bool load_p)
+{
+  if (load_p)
+    return aarch64_destructure_load_pair (regs, pattern);
   else
-    access_off = mem_off;
-
-  poly_int64 new_def_off = mem_off;
-
-  // Punt on accesses relative to eliminable regs.  Since we don't know the
-  // elimination offset pre-RA, we should postpone forming pairs on such
-  // accesses until after RA.
-  //
-  // As it stands, addresses in range for an individual load/store but not
-  // for a paired access are currently reloaded inefficiently,
-  // ending up with a separate base register for each pair.
-  //
-  // In theory LRA should make use of
-  // targetm.legitimize_address_displacement to promote sharing of
-  // bases among multiple (nearby) address reloads, but the current
-  // LRA code returns early from process_address_1 for operands that
-  // satisfy "m", even if they don't satisfy the real (relaxed) address
-  // constraint; this early return means we never get to the code
-  // that calls targetm.legitimize_address_displacement.
-  //
-  // So for now, it's better to punt when we can't be sure that the
-  // offset is in range for paired access.  On aarch64, out-of-range cases
-  // can then be handled after RA by the out-of-range LDP/STP peepholes.
-  // Eventually, it would be nice to handle known out-of-range opportunities
-  // in the pass itself (for stack accesses, this would be in the post-RA pass).
-  if (!reload_completed
-      && (REGNO (base) == FRAME_POINTER_REGNUM
-	  || REGNO (base) == ARG_POINTER_REGNUM))
-    return;
-
-  // Now need to find def of base register.
-  use_info *base_use = find_access (insn->uses (), REGNO (base));
-  gcc_assert (base_use);
-  def_info *base_def = base_use->def ();
-  if (!base_def)
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "base register (regno %d) of insn %d is undefined",
-		 REGNO (base), insn->uid ());
-      return;
-    }
-
-  alt_base *canon_base = canon_base_map.get (base_def);
-  if (canon_base)
-    {
-      // Express this as the combined offset from the canonical base.
-      base_def = canon_base->base;
-      new_def_off += canon_base->offset;
-      access_off += canon_base->offset;
-    }
-
-  if (autoinc_p)
-    {
-      auto def = find_access (insn->defs (), REGNO (base));
-      gcc_assert (def);
-
-      // Record that DEF = BASE_DEF + MEM_OFF.
-      if (dump_file)
-	{
-	  pretty_printer pp;
-	  pp_access (&pp, def, 0);
-	  pp_string (&pp, " = ");
-	  pp_access (&pp, base_def, 0);
-	  fprintf (dump_file, "[bb %u] recording %s + ",
-		   m_bb->index (), pp_formatted_text (&pp));
-	  print_dec (new_def_off, dump_file);
-	  fprintf (dump_file, "\n");
-	}
-
-      alt_base base_rec { base_def, new_def_off };
-      if (canon_base_map.put (def, base_rec))
-	gcc_unreachable (); // Base defs should be unique.
-    }
-
-  // Punt on misaligned offsets.  Paired memory accesses require offsets
-  // to be a multiple of the access size.
-  if (!multiple_p (mem_off, mem_size))
-    return;
-
-  const auto key = std::make_pair (base_def, encode_lfs (lfs));
-  access_group &group = def_map.get_or_insert (key, NULL);
-  auto alloc = [&](access_record *access) { return node_alloc (access); };
-  group.track (alloc, access_off, insn);
-
-  if (dump_file)
-    {
-      pretty_printer pp;
-      pp_access (&pp, base_def, 0);
-
-      fprintf (dump_file, "[bb %u] tracking insn %d via %s",
-	       m_bb->index (), insn->uid (), pp_formatted_text (&pp));
-      fprintf (dump_file,
-	       " [L=%d, WB=%d, FP=%d, %smode, off=",
-	       lfs.load_p, autoinc_p, lfs.fpsimd_p, mode_name[mem_mode]);
-      print_dec (access_off, dump_file);
-      fprintf (dump_file, "]\n");
-    }
+    return aarch64_destructure_store_pair (regs, pattern);
 }
 
-// Dummy predicate that never ignores any insns.
-static bool no_ignore (insn_info *) { return false; }
-
-// Return the latest dataflow hazard before INSN.
-//
-// If IGNORE is non-NULL, this points to a sub-rtx which we should ignore for
-// dataflow purposes.  This is needed when considering changing the RTL base of
-// an access discovered through a MEM_EXPR base.
-//
-// If IGNORE_INSN is non-NULL, we should further ignore any hazards arising
-// from that insn.
-//
-// N.B. we ignore any defs/uses of memory here as we deal with that separately,
-// making use of alias disambiguation.
-static insn_info *
-latest_hazard_before (insn_info *insn, rtx *ignore,
-		      insn_info *ignore_insn = nullptr)
+rtx
+aarch64_pair_fusion::gen_promote_writeback_pair (rtx wb_effect, rtx pair_mem,
+						 rtx regs[2],
+						 bool load_p)
 {
-  insn_info *result = nullptr;
-
-  // If the insn can throw then it is at the end of a BB and we can't
-  // move it, model this by recording a hazard in the previous insn
-  // which will prevent moving the insn up.
-  if (cfun->can_throw_non_call_exceptions
-      && find_reg_note (insn->rtl (), REG_EH_REGION, NULL_RTX))
-    return insn->prev_nondebug_insn ();
+  auto op_mode = aarch64_operand_mode_for_pair_mode (GET_MODE (pair_mem));
 
-  // Return true if we registered the hazard.
-  auto hazard = [&](insn_info *h) -> bool
+  machine_mode modes[2];
+  for (int i = 0; i < 2; i++)
     {
-      gcc_checking_assert (*h < *insn);
-      if (h == ignore_insn)
-	return false;
+      machine_mode mode = GET_MODE (regs[i]);
+      if (load_p)
+	gcc_checking_assert (mode != VOIDmode);
+      else if (mode == VOIDmode)
+	mode = op_mode;
 
-      if (!result || *h > *result)
-	result = h;
+      modes[i] = mode;
+    }
 
-      return true;
-    };
+  const auto op_size = GET_MODE_SIZE (modes[0]);
+  gcc_checking_assert (known_eq (op_size, GET_MODE_SIZE (modes[1])));
 
-  rtx pat = PATTERN (insn->rtl ());
-  auto ignore_use = [&](use_info *u)
+  rtx pats[2];
+  for (int i = 0; i < 2; i++)
     {
-      if (u->is_mem ())
-	return true;
-
-      return !refers_to_regno_p (u->regno (), u->regno () + 1, pat, ignore);
-    };
-
-  // Find defs of uses in INSN (RaW).
-  for (auto use : insn->uses ())
-    if (!ignore_use (use) && use->def ())
-      hazard (use->def ()->insn ());
-
-  // Find previous defs (WaW) or previous uses (WaR) of defs in INSN.
-  for (auto def : insn->defs ())
-    {
-      if (def->is_mem ())
-	continue;
-
-      if (def->prev_def ())
-	{
-	  hazard (def->prev_def ()->insn ()); // WaW
-
-	  auto set = dyn_cast<set_info *> (def->prev_def ());
-	  if (set && set->has_nondebug_insn_uses ())
-	    for (auto use : set->reverse_nondebug_insn_uses ())
-	      if (use->insn () != insn && hazard (use->insn ())) // WaR
-		break;
-	}
-
-      if (!HARD_REGISTER_NUM_P (def->regno ()))
-	continue;
-
-      // Also need to check backwards for call clobbers (WaW).
-      for (auto call_group : def->ebb ()->call_clobbers ())
-	{
-	  if (!call_group->clobbers (def->resource ()))
-	    continue;
-
-	  auto clobber_insn = prev_call_clobbers_ignoring (*call_group,
-							   def->insn (),
-							   no_ignore);
-	  if (clobber_insn)
-	    hazard (clobber_insn);
-	}
-
-    }
-
-  return result;
-}
-
-// Return the first dataflow hazard after INSN.
-//
-// If IGNORE is non-NULL, this points to a sub-rtx which we should ignore for
-// dataflow purposes.  This is needed when considering changing the RTL base of
-// an access discovered through a MEM_EXPR base.
-//
-// N.B. we ignore any defs/uses of memory here as we deal with that separately,
-// making use of alias disambiguation.
-static insn_info *
-first_hazard_after (insn_info *insn, rtx *ignore)
-{
-  insn_info *result = nullptr;
-  auto hazard = [insn, &result](insn_info *h)
-    {
-      gcc_checking_assert (*h > *insn);
-      if (!result || *h < *result)
-	result = h;
-    };
-
-  rtx pat = PATTERN (insn->rtl ());
-  auto ignore_use = [&](use_info *u)
-    {
-      if (u->is_mem ())
-	return true;
-
-      return !refers_to_regno_p (u->regno (), u->regno () + 1, pat, ignore);
-    };
-
-  for (auto def : insn->defs ())
-    {
-      if (def->is_mem ())
-	continue;
-
-      if (def->next_def ())
-	hazard (def->next_def ()->insn ()); // WaW
-
-      auto set = dyn_cast<set_info *> (def);
-      if (set && set->has_nondebug_insn_uses ())
-	hazard (set->first_nondebug_insn_use ()->insn ()); // RaW
-
-      if (!HARD_REGISTER_NUM_P (def->regno ()))
-	continue;
-
-      // Also check for call clobbers of this def (WaW).
-      for (auto call_group : def->ebb ()->call_clobbers ())
-	{
-	  if (!call_group->clobbers (def->resource ()))
-	    continue;
-
-	  auto clobber_insn = next_call_clobbers_ignoring (*call_group,
-							   def->insn (),
-							   no_ignore);
-	  if (clobber_insn)
-	    hazard (clobber_insn);
-	}
-    }
-
-  // Find any subsequent defs of uses in INSN (WaR).
-  for (auto use : insn->uses ())
-    {
-      if (ignore_use (use))
-	continue;
-
-      if (use->def ())
-	{
-	  auto def = use->def ()->next_def ();
-	  if (def && def->insn () == insn)
-	    def = def->next_def ();
-
-	  if (def)
-	    hazard (def->insn ());
-	}
-
-      if (!HARD_REGISTER_NUM_P (use->regno ()))
-	continue;
-
-      // Also need to handle call clobbers of our uses (again WaR).
-      //
-      // See restrict_movement_for_uses_ignoring for why we don't
-      // need to check backwards for call clobbers.
-      for (auto call_group : use->ebb ()->call_clobbers ())
-	{
-	  if (!call_group->clobbers (use->resource ()))
-	    continue;
-
-	  auto clobber_insn = next_call_clobbers_ignoring (*call_group,
-							   use->insn (),
-							   no_ignore);
-	  if (clobber_insn)
-	    hazard (clobber_insn);
-	}
-    }
-
-  return result;
-}
-
-// Return true iff R1 and R2 overlap.
-static bool
-ranges_overlap_p (const insn_range_info &r1, const insn_range_info &r2)
-{
-  // If either range is empty, then their intersection is empty.
-  if (!r1 || !r2)
-    return false;
-
-  // When do they not overlap? When one range finishes before the other
-  // starts, i.e. (*r1.last < *r2.first || *r2.last < *r1.first).
-  // Inverting this, we get the below.
-  return *r1.last >= *r2.first && *r2.last >= *r1.first;
-}
-
-// Get the range of insns that def feeds.
-static insn_range_info get_def_range (def_info *def)
-{
-  insn_info *last = def->next_def ()->insn ()->prev_nondebug_insn ();
-  return { def->insn (), last };
-}
-
-// Given a def (of memory), return the downwards range within which we
-// can safely move this def.
-static insn_range_info
-def_downwards_move_range (def_info *def)
-{
-  auto range = get_def_range (def);
-
-  auto set = dyn_cast<set_info *> (def);
-  if (!set || !set->has_any_uses ())
-    return range;
-
-  auto use = set->first_nondebug_insn_use ();
-  if (use)
-    range = move_earlier_than (range, use->insn ());
-
-  return range;
-}
-
-// Given a def (of memory), return the upwards range within which we can
-// safely move this def.
-static insn_range_info
-def_upwards_move_range (def_info *def)
-{
-  def_info *prev = def->prev_def ();
-  insn_range_info range { prev->insn (), def->insn () };
-
-  auto set = dyn_cast<set_info *> (prev);
-  if (!set || !set->has_any_uses ())
-    return range;
-
-  auto use = set->last_nondebug_insn_use ();
-  if (use)
-    range = move_later_than (range, use->insn ());
-
-  return range;
-}
-
-// Class that implements a state machine for building the changes needed to form
-// a store pair instruction.  This allows us to easily build the changes in
-// program order, as required by rtl-ssa.
-struct store_change_builder
-{
-  enum class state
-  {
-    FIRST,
-    INSERT,
-    FIXUP_USE,
-    LAST,
-    DONE
-  };
-
-  enum class action
-  {
-    TOMBSTONE,
-    CHANGE,
-    INSERT,
-    FIXUP_USE
-  };
-
-  struct change
-  {
-    action type;
-    insn_info *insn;
-  };
-
-  bool done () const { return m_state == state::DONE; }
-
-  store_change_builder (insn_info *insns[2],
-			insn_info *repurpose,
-			insn_info *dest)
-    : m_state (state::FIRST), m_insns { insns[0], insns[1] },
-      m_repurpose (repurpose), m_dest (dest), m_use (nullptr) {}
-
-  change get_change () const
-  {
-    switch (m_state)
-      {
-      case state::FIRST:
-	return {
-	  m_insns[0] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
-	  m_insns[0]
-	};
-      case state::LAST:
-	return {
-	  m_insns[1] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
-	  m_insns[1]
-	};
-      case state::INSERT:
-	return { action::INSERT, m_dest };
-      case state::FIXUP_USE:
-	return { action::FIXUP_USE, m_use->insn () };
-      case state::DONE:
-	break;
-      }
-
-    gcc_unreachable ();
-  }
-
-  // Transition to the next state.
-  void advance ()
-  {
-    switch (m_state)
-      {
-      case state::FIRST:
-	if (m_repurpose)
-	  m_state = state::LAST;
-	else
-	  m_state = state::INSERT;
-	break;
-      case state::INSERT:
-      {
-	def_info *def = memory_access (m_insns[0]->defs ());
-	while (*def->next_def ()->insn () <= *m_dest)
-	  def = def->next_def ();
-
-	// Now we know DEF feeds the insertion point for the new stp.
-	// Look for any uses of DEF that will consume the new stp.
-	gcc_assert (*def->insn () <= *m_dest
-		    && *def->next_def ()->insn () > *m_dest);
-
-	auto set = as_a<set_info *> (def);
-	for (auto use : set->nondebug_insn_uses ())
-	  if (*use->insn () > *m_dest)
-	    {
-	      m_use = use;
-	      break;
-	    }
-
-	if (m_use)
-	  m_state = state::FIXUP_USE;
-	else
-	  m_state = state::LAST;
-	break;
-      }
-      case state::FIXUP_USE:
-	m_use = m_use->next_nondebug_insn_use ();
-	if (!m_use)
-	  m_state = state::LAST;
-	break;
-      case state::LAST:
-	m_state = state::DONE;
-	break;
-      case state::DONE:
-	gcc_unreachable ();
-      }
-  }
-
-private:
-  state m_state;
-
-  // Original candidate stores.
-  insn_info *m_insns[2];
-
-  // If non-null, this is a candidate insn to change into an stp.  Otherwise we
-  // are deleting both original insns and inserting a new insn for the stp.
-  insn_info *m_repurpose;
-
-  // Destionation of the stp, it will be placed immediately after m_dest.
-  insn_info *m_dest;
-
-  // Current nondebug use that needs updating due to stp insertion.
-  use_info *m_use;
-};
-
-// Given candidate store insns FIRST and SECOND, see if we can re-purpose one
-// of them (together with its def of memory) for the stp insn.  If so, return
-// that insn.  Otherwise, return null.
-static insn_info *
-try_repurpose_store (insn_info *first,
-		     insn_info *second,
-		     const insn_range_info &move_range)
-{
-  def_info * const defs[2] = {
-    memory_access (first->defs ()),
-    memory_access (second->defs ())
-  };
-
-  if (move_range.includes (first)
-      || ranges_overlap_p (move_range, def_downwards_move_range (defs[0])))
-    return first;
-
-  if (move_range.includes (second)
-      || ranges_overlap_p (move_range, def_upwards_move_range (defs[1])))
-    return second;
-
-  return nullptr;
-}
-
-// Generate the RTL pattern for a "tombstone"; used temporarily during this pass
-// to replace stores that are marked for deletion where we can't immediately
-// delete the store (since there are uses of mem hanging off the store).
-//
-// These are deleted at the end of the pass and uses re-parented appropriately
-// at this point.
-static rtx
-gen_tombstone (void)
-{
-  return gen_rtx_CLOBBER (VOIDmode,
-			  gen_rtx_MEM (BLKmode, gen_rtx_SCRATCH (Pmode)));
-}
-
-// Given a pair mode MODE, return a canonical mode to be used for a single
-// operand of such a pair.  Currently we only use this when promoting a
-// non-writeback pair into a writeback pair, as it isn't otherwise clear
-// which mode to use when storing a modeless CONST_INT.
-static machine_mode
-aarch64_operand_mode_for_pair_mode (machine_mode mode)
-{
-  switch (mode)
-    {
-    case E_V2x4QImode:
-      return SImode;
-    case E_V2x8QImode:
-      return DImode;
-    case E_V2x16QImode:
-      return V16QImode;
-    default:
-      gcc_unreachable ();
-    }
-}
-
-// Go through the reg notes rooted at NOTE, dropping those that we should drop,
-// and preserving those that we want to keep by prepending them to (and
-// returning) RESULT.  EH_REGION is used to make sure we have at most one
-// REG_EH_REGION note in the resulting list.  FR_EXPR is used to return any
-// REG_FRAME_RELATED_EXPR note we find, as these can need special handling in
-// combine_reg_notes.
-static rtx
-filter_notes (rtx note, rtx result, bool *eh_region, rtx *fr_expr)
-{
-  for (; note; note = XEXP (note, 1))
-    {
-      switch (REG_NOTE_KIND (note))
-	{
-	case REG_DEAD:
-	  // REG_DEAD notes aren't required to be maintained.
-	case REG_EQUAL:
-	case REG_EQUIV:
-	case REG_UNUSED:
-	case REG_NOALIAS:
-	  // These can all be dropped.  For REG_EQU{AL,IV} they cannot apply to
-	  // non-single_set insns, and REG_UNUSED is re-computed by RTl-SSA, see
-	  // rtl-ssa/changes.cc:update_notes.
-	  //
-	  // Similarly, REG_NOALIAS cannot apply to a parallel.
-	case REG_INC:
-	  // When we form the pair insn, the reg update is implemented
-	  // as just another SET in the parallel, so isn't really an
-	  // auto-increment in the RTL sense, hence we drop the note.
-	  break;
-	case REG_EH_REGION:
-	  gcc_assert (!*eh_region);
-	  *eh_region = true;
-	  result = alloc_reg_note (REG_EH_REGION, XEXP (note, 0), result);
-	  break;
-	case REG_CFA_DEF_CFA:
-	case REG_CFA_OFFSET:
-	case REG_CFA_RESTORE:
-	  result = alloc_reg_note (REG_NOTE_KIND (note),
-				   copy_rtx (XEXP (note, 0)),
-				   result);
-	  break;
-	case REG_FRAME_RELATED_EXPR:
-	  gcc_assert (!*fr_expr);
-	  *fr_expr = copy_rtx (XEXP (note, 0));
-	  break;
-	default:
-	  // Unexpected REG_NOTE kind.
-	  gcc_unreachable ();
-	}
-    }
-
-  return result;
-}
-
-// Return the notes that should be attached to a combination of I1 and I2, where
-// *I1 < *I2.  LOAD_P is true for loads.
-static rtx
-combine_reg_notes (insn_info *i1, insn_info *i2, bool load_p)
-{
-  // Temporary storage for REG_FRAME_RELATED_EXPR notes.
-  rtx fr_expr[2] = {};
-
-  bool found_eh_region = false;
-  rtx result = NULL_RTX;
-  result = filter_notes (REG_NOTES (i2->rtl ()), result,
-			 &found_eh_region, fr_expr + 1);
-  result = filter_notes (REG_NOTES (i1->rtl ()), result,
-			 &found_eh_region, fr_expr);
-
-  if (!load_p)
-    {
-      // Simple frame-related sp-relative saves don't need CFI notes, but when
-      // we combine them into an stp we will need a CFI note as dwarf2cfi can't
-      // interpret the unspec pair representation directly.
-      if (RTX_FRAME_RELATED_P (i1->rtl ()) && !fr_expr[0])
-	fr_expr[0] = copy_rtx (PATTERN (i1->rtl ()));
-      if (RTX_FRAME_RELATED_P (i2->rtl ()) && !fr_expr[1])
-	fr_expr[1] = copy_rtx (PATTERN (i2->rtl ()));
-    }
-
-  rtx fr_pat = NULL_RTX;
-  if (fr_expr[0] && fr_expr[1])
-    {
-      // Combining two frame-related insns, need to construct
-      // a REG_FRAME_RELATED_EXPR note which represents the combined
-      // operation.
-      RTX_FRAME_RELATED_P (fr_expr[1]) = 1;
-      fr_pat = gen_rtx_PARALLEL (VOIDmode,
-				 gen_rtvec (2, fr_expr[0], fr_expr[1]));
-    }
-  else
-    fr_pat = fr_expr[0] ? fr_expr[0] : fr_expr[1];
-
-  if (fr_pat)
-    result = alloc_reg_note (REG_FRAME_RELATED_EXPR,
-			     fr_pat, result);
-
-  return result;
-}
-
-// Given two memory accesses in PATS, at least one of which is of a
-// writeback form, extract two non-writeback memory accesses addressed
-// relative to the initial value of the base register, and output these
-// in PATS.  Return an rtx that represents the overall change to the
-// base register.
-static rtx
-extract_writebacks (bool load_p, rtx pats[2], int changed)
-{
-  rtx base_reg = NULL_RTX;
-  poly_int64 current_offset = 0;
-
-  poly_int64 offsets[2];
-
-  for (int i = 0; i < 2; i++)
-    {
-      rtx mem = XEXP (pats[i], load_p);
-      rtx reg = XEXP (pats[i], !load_p);
-
-      rtx addr = XEXP (mem, 0);
-      const bool autoinc_p = GET_RTX_CLASS (GET_CODE (addr)) == RTX_AUTOINC;
-
-      poly_int64 offset;
-      rtx this_base = pair_mem_strip_offset (mem, &offset);
-      gcc_assert (REG_P (this_base));
-      if (base_reg)
-	gcc_assert (rtx_equal_p (base_reg, this_base));
-      else
-	base_reg = this_base;
-
-      // If we changed base for the current insn, then we already
-      // derived the correct mem for this insn from the effective
-      // address of the other access.
-      if (i == changed)
-	{
-	  gcc_checking_assert (!autoinc_p);
-	  offsets[i] = offset;
-	  continue;
-	}
-
-      if (autoinc_p && any_pre_modify_p (addr))
-	current_offset += offset;
-
-      poly_int64 this_off = current_offset;
-      if (!autoinc_p)
-	this_off += offset;
-
-      offsets[i] = this_off;
-      rtx new_mem = change_address (mem, GET_MODE (mem),
-				    plus_constant (GET_MODE (base_reg),
-						   base_reg, this_off));
-      pats[i] = load_p
-	? gen_rtx_SET (reg, new_mem)
-	: gen_rtx_SET (new_mem, reg);
-
-      if (autoinc_p && any_post_modify_p (addr))
-	current_offset += offset;
-    }
-
-  if (known_eq (current_offset, 0))
-    return NULL_RTX;
-
-  return gen_rtx_SET (base_reg, plus_constant (GET_MODE (base_reg),
-					       base_reg, current_offset));
-}
-
-// INSNS contains either {nullptr, pair insn} (when promoting an existing
-// non-writeback pair) or contains the candidate insns used to form the pair
-// (when fusing a new pair).
-//
-// PAIR_RANGE specifies where we want to form the final pair.
-// INITIAL_OFFSET gives the current base offset for the pair.
-// Bit I of INITIAL_WRITEBACK is set if INSNS[I] initially had writeback.
-// ACCESS_SIZE gives the access size for a single arm of the pair.
-// BASE_DEF gives the initial def of the base register consumed by the pair.
-//
-// Given the above, this function looks for a trailing destructive update of the
-// base register.  If there is one, we choose the first such update after
-// PAIR_DST that is still in the same BB as our pair.  We return the new def in
-// *ADD_DEF and the resulting writeback effect in *WRITEBACK_EFFECT.
-insn_info *
-pair_fusion::find_trailing_add (insn_info *insns[2],
-				const insn_range_info &pair_range,
-				int initial_writeback,
-				rtx *writeback_effect,
-				def_info **add_def,
-				def_info *base_def,
-				poly_int64 initial_offset,
-				unsigned access_size)
-{
-  // Punt on frame-related insns, it is better to be conservative and
-  // not try to form writeback pairs here, and means we don't have to
-  // worry about the writeback case in forming REG_FRAME_RELATED_EXPR
-  // notes (see combine_reg_notes).
-  if ((insns[0] && RTX_FRAME_RELATED_P (insns[0]->rtl ()))
-      || RTX_FRAME_RELATED_P (insns[1]->rtl ()))
-    return nullptr;
-
-  insn_info *pair_dst = pair_range.singleton ();
-  gcc_assert (pair_dst);
-
-  def_info *def = base_def->next_def ();
-
-  // In the case that either of the initial pair insns had writeback,
-  // then there will be intervening defs of the base register.
-  // Skip over these.
-  for (int i = 0; i < 2; i++)
-    if (initial_writeback & (1 << i))
-      {
-	gcc_assert (def->insn () == insns[i]);
-	def = def->next_def ();
-      }
-
-  if (!def || def->bb () != pair_dst->bb ())
-    return nullptr;
-
-  // DEF should now be the first def of the base register after PAIR_DST.
-  insn_info *cand = def->insn ();
-  gcc_assert (*cand > *pair_dst);
-
-  const auto base_regno = base_def->regno ();
-
-  // If CAND doesn't also use our base register,
-  // it can't destructively update it.
-  if (!find_access (cand->uses (), base_regno))
-    return nullptr;
-
-  auto rti = cand->rtl ();
-
-  if (!INSN_P (rti))
-    return nullptr;
-
-  auto pat = PATTERN (rti);
-  if (GET_CODE (pat) != SET)
-    return nullptr;
-
-  auto dest = XEXP (pat, 0);
-  if (!REG_P (dest) || REGNO (dest) != base_regno)
-    return nullptr;
-
-  poly_int64 offset;
-  rtx rhs_base = strip_offset (XEXP (pat, 1), &offset);
-  if (!REG_P (rhs_base)
-      || REGNO (rhs_base) != base_regno
-      || !offset.is_constant ())
-    return nullptr;
-
-  // If the initial base offset is zero, we can handle any add offset
-  // (post-inc).  Otherwise, we require the offsets to match (pre-inc).
-  if (!known_eq (initial_offset, 0) && !known_eq (offset, initial_offset))
-    return nullptr;
-
-  auto off_hwi = offset.to_constant ();
-
-  if (off_hwi % access_size != 0)
-    return nullptr;
-
-  off_hwi /= access_size;
-
-  if (!pair_mem_in_range_p (off_hwi))
-    return nullptr;
-
-  auto dump_prefix = [&]()
-    {
-      if (!insns[0])
-	fprintf (dump_file, "existing pair i%d: ", insns[1]->uid ());
-      else
-	fprintf (dump_file, "  (%d,%d)",
-		 insns[0]->uid (), insns[1]->uid ());
-    };
-
-  insn_info *hazard = latest_hazard_before (cand, nullptr, insns[1]);
-  if (!hazard || *hazard <= *pair_dst)
-    {
-      if (dump_file)
-	{
-	  dump_prefix ();
-	  fprintf (dump_file,
-		   "folding in trailing add (%d) to use writeback form\n",
-		   cand->uid ());
-	}
-
-      *add_def = def;
-      *writeback_effect = copy_rtx (pat);
-      return cand;
-    }
-
-  if (dump_file)
-    {
-      dump_prefix ();
-      fprintf (dump_file,
-	       "can't fold in trailing add (%d), hazard = %d\n",
-	       cand->uid (), hazard->uid ());
-    }
-
-  return nullptr;
-}
-
-// We just emitted a tombstone with uid UID, track it in a bitmap for
-// this BB so we can easily identify it later when cleaning up tombstones.
-void
-pair_fusion_bb_info::track_tombstone (int uid)
-{
-  if (!m_emitted_tombstone)
-    {
-      // Lazily initialize the bitmap for tracking tombstone insns.
-      bitmap_obstack_initialize (&m_bitmap_obstack);
-      bitmap_initialize (&m_tombstone_bitmap, &m_bitmap_obstack);
-      m_emitted_tombstone = true;
-    }
-
-  if (!bitmap_set_bit (&m_tombstone_bitmap, uid))
-    gcc_unreachable (); // Bit should have changed.
-}
-
-// Reset the debug insn containing USE (the debug insn has been
-// optimized away).
-static void
-reset_debug_use (use_info *use)
-{
-  auto use_insn = use->insn ();
-  auto use_rtl = use_insn->rtl ();
-  insn_change change (use_insn);
-  change.new_uses = {};
-  INSN_VAR_LOCATION_LOC (use_rtl) = gen_rtx_UNKNOWN_VAR_LOC ();
-  crtl->ssa->change_insn (change);
-}
-
-// USE is a debug use that needs updating because DEF (a def of the same
-// register) is being re-ordered over it.  If BASE is non-null, then DEF
-// is an update of the register BASE by a constant, given by WB_OFFSET,
-// and we can preserve debug info by accounting for the change in side
-// effects.
-static void
-fixup_debug_use (obstack_watermark &attempt,
-		 use_info *use,
-		 def_info *def,
-		 rtx base,
-		 poly_int64 wb_offset)
-{
-  auto use_insn = use->insn ();
-  if (base)
-    {
-      auto use_rtl = use_insn->rtl ();
-      insn_change change (use_insn);
-
-      gcc_checking_assert (REG_P (base) && use->regno () == REGNO (base));
-      change.new_uses = check_remove_regno_access (attempt,
-						   change.new_uses,
-						   use->regno ());
-
-      // The effect of the writeback is to add WB_OFFSET to BASE.  If
-      // we're re-ordering DEF below USE, then we update USE by adding
-      // WB_OFFSET to it.  Otherwise, if we're re-ordering DEF above
-      // USE, we update USE by undoing the effect of the writeback
-      // (subtracting WB_OFFSET).
-      use_info *new_use;
-      if (*def->insn () > *use_insn)
-	{
-	  // We now need USE_INSN to consume DEF.  Create a new use of DEF.
-	  //
-	  // N.B. this means until we call change_insns for the main change
-	  // group we will temporarily have a debug use consuming a def that
-	  // comes after it, but RTL-SSA doesn't currently support updating
-	  // debug insns as part of the main change group (together with
-	  // nondebug changes), so we will have to live with this update
-	  // leaving the IR being temporarily inconsistent.  It seems to
-	  // work out OK once the main change group is applied.
-	  wb_offset *= -1;
-	  new_use = crtl->ssa->create_use (attempt,
-					   use_insn,
-					   as_a<set_info *> (def));
-	}
-      else
-	new_use = find_access (def->insn ()->uses (), use->regno ());
-
-      change.new_uses = insert_access (attempt, new_use, change.new_uses);
-
-      if (dump_file)
-	{
-	  const char *dir = (*def->insn () < *use_insn) ? "down" : "up";
-	  pretty_printer pp;
-	  pp_string (&pp, "[");
-	  pp_access (&pp, use, 0);
-	  pp_string (&pp, "]");
-	  pp_string (&pp, " due to wb def ");
-	  pp_string (&pp, "[");
-	  pp_access (&pp, def, 0);
-	  pp_string (&pp, "]");
-	  fprintf (dump_file,
-		   "  i%d: fix up debug use %s re-ordered %s, "
-		   "sub r%u -> r%u + ",
-		   use_insn->uid (), pp_formatted_text (&pp),
-		   dir, REGNO (base), REGNO (base));
-	  print_dec (wb_offset, dump_file);
-	  fprintf (dump_file, "\n");
-	}
-
-      insn_propagation prop (use_rtl, base,
-			     plus_constant (GET_MODE (base), base, wb_offset));
-      if (prop.apply_to_pattern (&INSN_VAR_LOCATION_LOC (use_rtl)))
-	crtl->ssa->change_insn (change);
-      else
-	{
-	  if (dump_file)
-	    fprintf (dump_file, "  i%d: RTL substitution failed (%s)"
-		     ", resetting debug insn", use_insn->uid (),
-		     prop.failure_reason);
-	  reset_debug_use (use);
-	}
-    }
-  else
-    {
-      if (dump_file)
-	{
-	  pretty_printer pp;
-	  pp_string (&pp, "[");
-	  pp_access (&pp, use, 0);
-	  pp_string (&pp, "] due to re-ordered load def [");
-	  pp_access (&pp, def, 0);
-	  pp_string (&pp, "]");
-	  fprintf (dump_file, "  i%d: resetting debug use %s\n",
-		   use_insn->uid (), pp_formatted_text (&pp));
-	}
-      reset_debug_use (use);
-    }
-}
-
-// Update debug uses when folding in a trailing add insn to form a
-// writeback pair.
-//
-// ATTEMPT is used to allocate RTL-SSA temporaries for the changes,
-// the final pair is placed immediately after PAIR_DST, TRAILING_ADD
-// is a trailing add insn which is being folded into the pair to make it
-// use writeback addressing, and WRITEBACK_EFFECT is the pattern for
-// TRAILING_ADD.
-static void
-fixup_debug_uses_trailing_add (obstack_watermark &attempt,
-			       insn_info *pair_dst,
-			       insn_info *trailing_add,
-			       rtx writeback_effect)
-{
-  rtx base = SET_DEST (writeback_effect);
-
-  poly_int64 wb_offset;
-  rtx base2 = strip_offset (SET_SRC (writeback_effect), &wb_offset);
-  gcc_checking_assert (rtx_equal_p (base, base2));
-
-  auto defs = trailing_add->defs ();
-  gcc_checking_assert (defs.size () == 1);
-  def_info *def = defs[0];
-
-  if (auto set = safe_dyn_cast<set_info *> (def->prev_def ()))
-    for (auto use : iterate_safely (set->debug_insn_uses ()))
-      if (*use->insn () > *pair_dst)
-	// DEF is getting re-ordered above USE, fix up USE accordingly.
-	fixup_debug_use (attempt, use, def, base, wb_offset);
-}
-
-// Called from fuse_pair, fixes up any debug uses that will be affected
-// by the changes.
-//
-// ATTEMPT is the obstack watermark used to allocate RTL-SSA temporaries for
-// the changes, INSNS gives the candidate insns: at this point the use/def
-// information should still be as on entry to fuse_pair, but the patterns may
-// have changed, hence we pass ORIG_RTL which contains the original patterns
-// for the candidate insns.
-//
-// The final pair will be placed immediately after PAIR_DST, LOAD_P is true if
-// it is a load pair, bit I of WRITEBACK is set if INSNS[I] originally had
-// writeback, and WRITEBACK_EFFECT is an rtx describing the overall update to
-// the base register in the final pair (if any).  BASE_REGNO gives the register
-// number of the base register used in the final pair.
-static void
-fixup_debug_uses (obstack_watermark &attempt,
-		  insn_info *insns[2],
-		  rtx orig_rtl[2],
-		  insn_info *pair_dst,
-		  insn_info *trailing_add,
-		  bool load_p,
-		  int writeback,
-		  rtx writeback_effect,
-		  unsigned base_regno)
-{
-  // USE is a debug use that needs updating because DEF (a def of the
-  // resource) is being re-ordered over it.  If WRITEBACK_PAT is non-NULL,
-  // then it gives the original RTL pattern for DEF's insn, and DEF is a
-  // writeback update of the base register.
-  //
-  // This simply unpacks WRITEBACK_PAT if needed and calls fixup_debug_use.
-  auto update_debug_use = [&](use_info *use, def_info *def,
-			      rtx writeback_pat)
-    {
-      poly_int64 offset = 0;
-      rtx base = NULL_RTX;
-      if (writeback_pat)
-	{
-	  rtx mem = XEXP (writeback_pat, load_p);
-	  gcc_checking_assert (GET_RTX_CLASS (GET_CODE (XEXP (mem, 0)))
-			       == RTX_AUTOINC);
-
-	  base = pair_mem_strip_offset (mem, &offset);
-	  gcc_checking_assert (REG_P (base) && REGNO (base) == base_regno);
-	}
-      fixup_debug_use (attempt, use, def, base, offset);
-    };
-
-  // Reset any debug uses of mem over which we re-ordered a store.
-  //
-  // It would be nice to try and preserve debug info here, but it seems that
-  // would require doing alias analysis to see if the store aliases with the
-  // debug use, which seems a little extravagant just to preserve debug info.
-  if (!load_p)
-    {
-      auto def = memory_access (insns[0]->defs ());
-      auto last_def = memory_access (insns[1]->defs ());
-      for (; def != last_def; def = def->next_def ())
-	{
-	  auto set = as_a<set_info *> (def);
-	  for (auto use : iterate_safely (set->debug_insn_uses ()))
-	    {
-	      if (dump_file)
-		fprintf (dump_file, "  i%d: resetting debug use of mem\n",
-			 use->insn ()->uid ());
-	      reset_debug_use (use);
-	    }
-	}
-    }
-
-  // Now let's take care of register uses, starting with debug uses
-  // attached to defs from our first insn.
-  for (auto def : insns[0]->defs ())
-    {
-      auto set = dyn_cast<set_info *> (def);
-      if (!set || set->is_mem () || !set->first_debug_insn_use ())
-	continue;
-
-      def_info *defs[2] = {
-	def,
-	find_access (insns[1]->defs (), def->regno ())
-      };
-
-      rtx writeback_pats[2] = {};
-      if (def->regno () == base_regno)
-	for (int i = 0; i < 2; i++)
-	  if (writeback & (1 << i))
-	    {
-	      gcc_checking_assert (defs[i]);
-	      writeback_pats[i] = orig_rtl[i];
-	    }
-
-      // Now that we've characterized the defs involved, go through the
-      // debug uses and determine how to update them (if needed).
-      for (auto use : iterate_safely (set->debug_insn_uses ()))
-	{
-	  if (*pair_dst < *use->insn () && defs[1])
-	    // We're re-ordering defs[1] above a previous use of the
-	    // same resource.
-	    update_debug_use (use, defs[1], writeback_pats[1]);
-	  else if (*pair_dst >= *use->insn ())
-	    // We're re-ordering defs[0] below its use.
-	    update_debug_use (use, defs[0], writeback_pats[0]);
-	}
-    }
-
-  // Now let's look at registers which are def'd by the second insn
-  // but not by the first insn, there may still be debug uses of a
-  // previous def which can be affected by moving the second insn up.
-  for (auto def : insns[1]->defs ())
-    {
-      // This should be M log N where N is the number of defs in
-      // insns[0] and M is the number of defs in insns[1].
-      if (def->is_mem () || find_access (insns[0]->defs (), def->regno ()))
-	  continue;
-
-      auto prev_set = safe_dyn_cast<set_info *> (def->prev_def ());
-      if (!prev_set)
-	continue;
-
-      rtx writeback_pat = NULL_RTX;
-      if (def->regno () == base_regno && (writeback & 2))
-	writeback_pat = orig_rtl[1];
-
-      // We have a def in insns[1] which isn't def'd by the first insn.
-      // Look to the previous def and see if it has any debug uses.
-      for (auto use : iterate_safely (prev_set->debug_insn_uses ()))
-	if (*pair_dst < *use->insn ())
-	  // We're ordering DEF above a previous use of the same register.
-	  update_debug_use (use, def, writeback_pat);
-    }
-
-  if ((writeback & 2) && !writeback_effect)
-    {
-      // If the second insn initially had writeback but the final
-      // pair does not, then there may be trailing debug uses of the
-      // second writeback def which need re-parenting: do that.
-      auto def = find_access (insns[1]->defs (), base_regno);
-      gcc_assert (def);
-      auto set = as_a<set_info *> (def);
-      for (auto use : iterate_safely (set->debug_insn_uses ()))
-	{
-	  insn_change change (use->insn ());
-	  change.new_uses = check_remove_regno_access (attempt,
-						       change.new_uses,
-						       base_regno);
-	  auto new_use = find_access (insns[0]->uses (), base_regno);
-
-	  // N.B. insns must have already shared a common base due to writeback.
-	  gcc_assert (new_use);
-
-	  if (dump_file)
-	    fprintf (dump_file,
-		     "  i%d: cancelling wb, re-parenting trailing debug use\n",
-		     use->insn ()->uid ());
-
-	  change.new_uses = insert_access (attempt, new_use, change.new_uses);
-	  crtl->ssa->change_insn (change);
-	}
-    }
-  else if (trailing_add)
-    fixup_debug_uses_trailing_add (attempt, pair_dst, trailing_add,
-				   writeback_effect);
-}
-
-// Try and actually fuse the pair given by insns I1 and I2.
-//
-// Here we've done enough analysis to know this is safe, we only
-// reject the pair at this stage if either the tuning policy says to,
-// or recog fails on the final pair insn.
-//
-// LOAD_P is true for loads, ACCESS_SIZE gives the access size of each
-// candidate insn.  Bit i of WRITEBACK is set if the ith insn (in program
-// order) uses writeback.
-//
-// BASE gives the chosen base candidate for the pair and MOVE_RANGE is
-// a singleton range which says where to place the pair.
-bool
-pair_fusion_bb_info::fuse_pair (bool load_p,
-				unsigned access_size,
-				int writeback,
-				insn_info *i1, insn_info *i2,
-				base_cand &base,
-				const insn_range_info &move_range)
-{
-  auto attempt = crtl->ssa->new_change_attempt ();
-
-  auto make_change = [&attempt](insn_info *insn)
-    {
-      return crtl->ssa->change_alloc<insn_change> (attempt, insn);
-    };
-  auto make_delete = [&attempt](insn_info *insn)
-    {
-      return crtl->ssa->change_alloc<insn_change> (attempt,
-						   insn,
-						   insn_change::DELETE);
-    };
-
-  insn_info *first = (*i1 < *i2) ? i1 : i2;
-  insn_info *second = (first == i1) ? i2 : i1;
-
-  insn_info *pair_dst = move_range.singleton ();
-  gcc_assert (pair_dst);
-
-  insn_info *insns[2] = { first, second };
-
-  auto_vec<insn_change *> changes;
-  auto_vec<int, 2> tombstone_uids (2);
-
-  rtx pats[2] = {
-    PATTERN (first->rtl ()),
-    PATTERN (second->rtl ())
-  };
-
-  // Make copies of the patterns as we might need to refer to the original RTL
-  // later, for example when updating debug uses (which is after we've updated
-  // one or both of the patterns in the candidate insns).
-  rtx orig_rtl[2];
-  for (int i = 0; i < 2; i++)
-    orig_rtl[i] = copy_rtx (pats[i]);
-
-  use_array input_uses[2] = { first->uses (), second->uses () };
-  def_array input_defs[2] = { first->defs (), second->defs () };
-
-  int changed_insn = -1;
-  if (base.from_insn != -1)
-    {
-      // If we're not already using a shared base, we need
-      // to re-write one of the accesses to use the base from
-      // the other insn.
-      gcc_checking_assert (base.from_insn == 0 || base.from_insn == 1);
-      changed_insn = !base.from_insn;
-
-      rtx base_pat = pats[base.from_insn];
-      rtx change_pat = pats[changed_insn];
-      rtx base_mem = XEXP (base_pat, load_p);
-      rtx change_mem = XEXP (change_pat, load_p);
-
-      const bool lower_base_p = (insns[base.from_insn] == i1);
-      HOST_WIDE_INT adjust_amt = access_size;
-      if (!lower_base_p)
-	adjust_amt *= -1;
-
-      rtx change_reg = XEXP (change_pat, !load_p);
-      rtx effective_base = drop_writeback (base_mem);
-      rtx adjusted_addr = plus_constant (Pmode,
-					 XEXP (effective_base, 0),
-					 adjust_amt);
-      rtx new_mem = replace_equiv_address_nv (change_mem, adjusted_addr);
-      rtx new_set = load_p
-	? gen_rtx_SET (change_reg, new_mem)
-	: gen_rtx_SET (new_mem, change_reg);
-
-      pats[changed_insn] = new_set;
-
-      auto keep_use = [&](use_info *u)
-	{
-	  return refers_to_regno_p (u->regno (), u->regno () + 1,
-				    change_pat, &XEXP (change_pat, load_p));
-	};
-
-      // Drop any uses that only occur in the old address.
-      input_uses[changed_insn] = filter_accesses (attempt,
-						  input_uses[changed_insn],
-						  keep_use);
-    }
-
-  rtx writeback_effect = NULL_RTX;
-  if (writeback)
-    writeback_effect = extract_writebacks (load_p, pats, changed_insn);
-
-  const auto base_regno = base.def->regno ();
-
-  if (base.from_insn == -1 && (writeback & 1))
-    {
-      // If the first of the candidate insns had a writeback form, we'll need to
-      // drop the use of the updated base register from the second insn's uses.
-      //
-      // N.B. we needn't worry about the base register occurring as a store
-      // operand, as we checked that there was no non-address true dependence
-      // between the insns in try_fuse_pair.
-      gcc_checking_assert (find_access (input_uses[1], base_regno));
-      input_uses[1] = check_remove_regno_access (attempt,
-						 input_uses[1],
-						 base_regno);
-    }
-
-  // Go through and drop uses that only occur in register notes,
-  // as we won't be preserving those.
-  for (int i = 0; i < 2; i++)
-    {
-      auto rti = insns[i]->rtl ();
-      if (!REG_NOTES (rti))
-	continue;
-
-      input_uses[i] = remove_note_accesses (attempt, input_uses[i]);
-    }
-
-  // Edge case: if the first insn is a writeback load and the
-  // second insn is a non-writeback load which transfers into the base
-  // register, then we should drop the writeback altogether as the
-  // update of the base register from the second load should prevail.
-  //
-  // For example:
-  //   ldr x2, [x1], #8
-  //   ldr x1, [x1]
-  //   -->
-  //   ldp x2, x1, [x1]
-  if (writeback == 1
-      && load_p
-      && find_access (input_defs[1], base_regno))
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "  load pair: i%d has wb but subsequent i%d has non-wb "
-		 "update of base (r%d), dropping wb\n",
-		 insns[0]->uid (), insns[1]->uid (), base_regno);
-      gcc_assert (writeback_effect);
-      writeback_effect = NULL_RTX;
-    }
-
-  // So far the patterns have been in instruction order,
-  // now we want them in offset order.
-  if (i1 != first)
-    std::swap (pats[0], pats[1]);
-
-  poly_int64 offsets[2];
-  for (int i = 0; i < 2; i++)
-    {
-      rtx mem = XEXP (pats[i], load_p);
-      gcc_checking_assert (MEM_P (mem));
-      rtx base = strip_offset (XEXP (mem, 0), offsets + i);
-      gcc_checking_assert (REG_P (base));
-      gcc_checking_assert (base_regno == REGNO (base));
-    }
-
-  // If either of the original insns had writeback, but the resulting pair insn
-  // does not (can happen e.g. in the load pair edge case above, or if the
-  // writeback effects cancel out), then drop the def (s) of the base register
-  // as appropriate.
-  //
-  // Also drop the first def in the case that both of the original insns had
-  // writeback.  The second def could well have uses, but the first def should
-  // only be used by the second insn (and we dropped that use above).
-  for (int i = 0; i < 2; i++)
-    if ((!writeback_effect && (writeback & (1 << i)))
-	|| (i == 0 && writeback == 3))
-      input_defs[i] = check_remove_regno_access (attempt,
-						 input_defs[i],
-						 base_regno);
-
-  // If we don't currently have a writeback pair, and we don't have
-  // a load that clobbers the base register, look for a trailing destructive
-  // update of the base register and try and fold it in to make this into a
-  // writeback pair.
-  insn_info *trailing_add = nullptr;
-  if (m_pass->should_handle_writeback (writeback::ALL)
-      && !writeback_effect
-      && (!load_p || (!refers_to_regno_p (base_regno, base_regno + 1,
-					 XEXP (pats[0], 0), nullptr)
-		      && !refers_to_regno_p (base_regno, base_regno + 1,
-					     XEXP (pats[1], 0), nullptr))))
-    {
-      def_info *add_def;
-      trailing_add = m_pass->find_trailing_add (insns, move_range, writeback,
-						&writeback_effect,
-						&add_def, base.def, offsets[0],
-						access_size);
-      if (trailing_add)
-	{
-	  // The def of the base register from the trailing add should prevail.
-	  input_defs[0] = insert_access (attempt, add_def, input_defs[0]);
-	  gcc_assert (input_defs[0].is_valid ());
-	}
-    }
-
-  // Now that we know what base mem we're going to use, check if it's OK
-  // with the pair mem policy.
-  rtx first_mem = XEXP (pats[0], load_p);
-  if (!m_pass->pair_mem_ok_with_policy (first_mem, load_p))
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "punting on pair (%d,%d), pair mem policy says no\n",
-		 i1->uid (), i2->uid ());
-      return false;
-    }
-
-  rtx reg_notes = combine_reg_notes (first, second, load_p);
-
-  rtx pair_pat = m_pass->gen_pair (pats, writeback_effect, load_p);
-  insn_change *pair_change = nullptr;
-  auto set_pair_pat = [pair_pat,reg_notes](insn_change *change) {
-    rtx_insn *rti = change->insn ()->rtl ();
-    validate_unshare_change (rti, &PATTERN (rti), pair_pat, true);
-    validate_change (rti, &REG_NOTES (rti), reg_notes, true);
-  };
-
-  if (load_p)
-    {
-      changes.safe_push (make_delete (first));
-      pair_change = make_change (second);
-      changes.safe_push (pair_change);
-
-      pair_change->move_range = move_range;
-      pair_change->new_defs = merge_access_arrays (attempt,
-						   input_defs[0],
-						   input_defs[1]);
-      gcc_assert (pair_change->new_defs.is_valid ());
-
-      pair_change->new_uses
-	= merge_access_arrays (attempt,
-			       drop_memory_access (input_uses[0]),
-			       drop_memory_access (input_uses[1]));
-      gcc_assert (pair_change->new_uses.is_valid ());
-      set_pair_pat (pair_change);
-    }
-  else
-    {
-      using Action = store_change_builder::action;
-      insn_info *store_to_change = try_repurpose_store (first, second,
-							move_range);
-      store_change_builder builder (insns, store_to_change, pair_dst);
-      insn_change *change;
-      set_info *new_set = nullptr;
-      for (; !builder.done (); builder.advance ())
-	{
-	  auto action = builder.get_change ();
-	  change = (action.type == Action::INSERT)
-	    ? nullptr : make_change (action.insn);
-	  switch (action.type)
-	    {
-	    case Action::CHANGE:
-	    {
-	      set_pair_pat (change);
-	      change->new_uses = merge_access_arrays (attempt,
-						      input_uses[0],
-						      input_uses[1]);
-	      auto d1 = drop_memory_access (input_defs[0]);
-	      auto d2 = drop_memory_access (input_defs[1]);
-	      change->new_defs = merge_access_arrays (attempt, d1, d2);
-	      gcc_assert (change->new_defs.is_valid ());
-	      def_info *store_def = memory_access (change->insn ()->defs ());
-	      change->new_defs = insert_access (attempt,
-						store_def,
-						change->new_defs);
-	      gcc_assert (change->new_defs.is_valid ());
-	      change->move_range = move_range;
-	      pair_change = change;
-	      break;
-	    }
-	    case Action::TOMBSTONE:
-	    {
-	      tombstone_uids.quick_push (change->insn ()->uid ());
-	      rtx_insn *rti = change->insn ()->rtl ();
-	      validate_change (rti, &PATTERN (rti), gen_tombstone (), true);
-	      validate_change (rti, &REG_NOTES (rti), NULL_RTX, true);
-	      change->new_uses = use_array (nullptr, 0);
-	      break;
-	    }
-	    case Action::INSERT:
-	    {
-	      if (dump_file)
-		fprintf (dump_file,
-			 "  stp: cannot re-purpose candidate stores\n");
-
-	      auto new_insn = crtl->ssa->create_insn (attempt, INSN, pair_pat);
-	      change = make_change (new_insn);
-	      change->move_range = move_range;
-	      change->new_uses = merge_access_arrays (attempt,
-						      input_uses[0],
-						      input_uses[1]);
-	      gcc_assert (change->new_uses.is_valid ());
-
-	      auto d1 = drop_memory_access (input_defs[0]);
-	      auto d2 = drop_memory_access (input_defs[1]);
-	      change->new_defs = merge_access_arrays (attempt, d1, d2);
-	      gcc_assert (change->new_defs.is_valid ());
-
-	      new_set = crtl->ssa->create_set (attempt, new_insn, memory);
-	      change->new_defs = insert_access (attempt, new_set,
-						change->new_defs);
-	      gcc_assert (change->new_defs.is_valid ());
-	      pair_change = change;
-	      break;
-	    }
-	    case Action::FIXUP_USE:
-	    {
-	      // This use now needs to consume memory from our stp.
-	      if (dump_file)
-		fprintf (dump_file,
-			 "  stp: changing i%d to use mem from new stp "
-			 "(after i%d)\n",
-			 action.insn->uid (), pair_dst->uid ());
-	      change->new_uses = drop_memory_access (change->new_uses);
-	      gcc_assert (new_set);
-	      auto new_use = crtl->ssa->create_use (attempt, action.insn,
-						    new_set);
-	      change->new_uses = insert_access (attempt, new_use,
-						change->new_uses);
-	      break;
-	    }
-	    }
-	  changes.safe_push (change);
-	}
-    }
-
-  if (trailing_add)
-    changes.safe_push (make_delete (trailing_add));
-  else if ((writeback & 2) && !writeback_effect)
-    {
-      // The second insn initially had writeback but now the pair does not,
-      // need to update any nondebug uses of the base register def in the
-      // second insn.  We'll take care of debug uses later.
-      auto def = find_access (insns[1]->defs (), base_regno);
-      gcc_assert (def);
-      auto set = dyn_cast<set_info *> (def);
-      if (set && set->has_nondebug_uses ())
-	{
-	  auto orig_use = find_access (insns[0]->uses (), base_regno);
-	  for (auto use : set->nondebug_insn_uses ())
-	    {
-	      auto change = make_change (use->insn ());
-	      change->new_uses = check_remove_regno_access (attempt,
-							    change->new_uses,
-							    base_regno);
-	      change->new_uses = insert_access (attempt,
-						orig_use,
-						change->new_uses);
-	      changes.safe_push (change);
-	    }
-	}
-    }
-
-  auto is_changing = insn_is_changing (changes);
-  for (unsigned i = 0; i < changes.length (); i++)
-    gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
-
-  // Check the pair pattern is recog'd.
-  if (!rtl_ssa::recog_ignoring (attempt, *pair_change, is_changing))
-    {
-      if (dump_file)
-	fprintf (dump_file, "  failed to form pair, recog failed\n");
-
-      // Free any reg notes we allocated.
-      while (reg_notes)
-	{
-	  rtx next = XEXP (reg_notes, 1);
-	  free_EXPR_LIST_node (reg_notes);
-	  reg_notes = next;
-	}
-      cancel_changes (0);
-      return false;
-    }
-
-  gcc_assert (crtl->ssa->verify_insn_changes (changes));
-
-  // Fix up any debug uses that will be affected by the changes.
-  if (MAY_HAVE_DEBUG_INSNS)
-    fixup_debug_uses (attempt, insns, orig_rtl, pair_dst, trailing_add,
-		      load_p, writeback, writeback_effect, base_regno);
-
-  confirm_change_group ();
-  crtl->ssa->change_insns (changes);
-
-  gcc_checking_assert (tombstone_uids.length () <= 2);
-  for (auto uid : tombstone_uids)
-    track_tombstone (uid);
-
-  return true;
-}
-
-// Return true if STORE_INSN may modify mem rtx MEM.  Make sure we keep
-// within our BUDGET for alias analysis.
-static bool
-store_modifies_mem_p (rtx mem, insn_info *store_insn, int &budget)
-{
-  if (!budget)
-    {
-      if (dump_file)
-	{
-	  fprintf (dump_file,
-		   "exceeded budget, assuming store %d aliases with mem ",
-		   store_insn->uid ());
-	  print_simple_rtl (dump_file, mem);
-	  fprintf (dump_file, "\n");
-	}
-
-      return true;
-    }
-
-  budget--;
-  return memory_modified_in_insn_p (mem, store_insn->rtl ());
-}
-
-// Return true if LOAD may be modified by STORE.  Make sure we keep
-// within our BUDGET for alias analysis.
-static bool
-load_modified_by_store_p (insn_info *load,
-			  insn_info *store,
-			  int &budget)
-{
-  gcc_checking_assert (budget >= 0);
-
-  if (!budget)
-    {
-      if (dump_file)
-	{
-	  fprintf (dump_file,
-		   "exceeded budget, assuming load %d aliases with store %d\n",
-		   load->uid (), store->uid ());
-	}
-      return true;
-    }
-
-  // It isn't safe to re-order stores over calls.
-  if (CALL_P (load->rtl ()))
-    return true;
-
-  budget--;
-
-  // Iterate over all MEMs in the load, seeing if any alias with
-  // our store.
-  subrtx_var_iterator::array_type array;
-  rtx pat = PATTERN (load->rtl ());
-  FOR_EACH_SUBRTX_VAR (iter, array, pat, NONCONST)
-    if (MEM_P (*iter) && memory_modified_in_insn_p (*iter, store->rtl ()))
-      return true;
-
-  return false;
-}
-
-// Implement some common functionality used by both store_walker
-// and load_walker.
-template<bool reverse>
-class def_walker : public alias_walker
-{
-protected:
-  using def_iter_t = typename std::conditional<reverse,
-	reverse_def_iterator, def_iterator>::type;
-
-  static use_info *start_use_chain (def_iter_t &def_iter)
-  {
-    set_info *set = nullptr;
-    for (; *def_iter; def_iter++)
-      {
-	set = dyn_cast<set_info *> (*def_iter);
-	if (!set)
-	  continue;
-
-	use_info *use = reverse
-	  ? set->last_nondebug_insn_use ()
-	  : set->first_nondebug_insn_use ();
-
-	if (use)
-	  return use;
-      }
-
-    return nullptr;
-  }
-
-  def_iter_t def_iter;
-  insn_info *limit;
-  def_walker (def_info *def, insn_info *limit) :
-    def_iter (def), limit (limit) {}
-
-  virtual bool iter_valid () const { return *def_iter; }
-
-public:
-  insn_info *insn () const override { return (*def_iter)->insn (); }
-  void advance () override { def_iter++; }
-  bool valid () const override final
-  {
-    if (!iter_valid ())
-      return false;
-
-    if (reverse)
-      return *(insn ()) > *limit;
-    else
-      return *(insn ()) < *limit;
-  }
-};
-
-// alias_walker that iterates over stores.
-template<bool reverse, typename InsnPredicate>
-class store_walker : public def_walker<reverse>
-{
-  rtx cand_mem;
-  InsnPredicate tombstone_p;
-
-public:
-  store_walker (def_info *mem_def, rtx mem, insn_info *limit_insn,
-		InsnPredicate tombstone_fn) :
-    def_walker<reverse> (mem_def, limit_insn),
-    cand_mem (mem), tombstone_p (tombstone_fn) {}
-
-  bool conflict_p (int &budget) const override final
-  {
-    if (tombstone_p (this->insn ()))
-      return false;
-
-    return store_modifies_mem_p (cand_mem, this->insn (), budget);
-  }
-};
-
-// alias_walker that iterates over loads.
-template<bool reverse>
-class load_walker : public def_walker<reverse>
-{
-  using Base = def_walker<reverse>;
-  using use_iter_t = typename std::conditional<reverse,
-	reverse_use_iterator, nondebug_insn_use_iterator>::type;
-
-  use_iter_t use_iter;
-  insn_info *cand_store;
-
-  bool iter_valid () const override final { return *use_iter; }
-
-public:
-  void advance () override final
-  {
-    use_iter++;
-    if (*use_iter)
-      return;
-    this->def_iter++;
-    use_iter = Base::start_use_chain (this->def_iter);
-  }
-
-  insn_info *insn () const override final
-  {
-    return (*use_iter)->insn ();
-  }
-
-  bool conflict_p (int &budget) const override final
-  {
-    return load_modified_by_store_p (insn (), cand_store, budget);
-  }
-
-  load_walker (def_info *def, insn_info *store, insn_info *limit_insn)
-    : Base (def, limit_insn),
-      use_iter (Base::start_use_chain (this->def_iter)),
-      cand_store (store) {}
-};
-
-// Process our alias_walkers in a round-robin fashion, proceeding until
-// nothing more can be learned from alias analysis.
-//
-// We try to maintain the invariant that if a walker becomes invalid, we
-// set its pointer to null.
-void
-pair_fusion::do_alias_analysis (insn_info *alias_hazards[4],
-				alias_walker *walkers[4],
-				bool load_p)
-{
-  const int n_walkers = 2 + (2 * !load_p);
-  int budget = pair_mem_alias_check_limit ();
-
-  auto next_walker = [walkers,n_walkers](int current) -> int {
-    for (int j = 1; j <= n_walkers; j++)
-      {
-	int idx = (current + j) % n_walkers;
-	if (walkers[idx])
-	  return idx;
-      }
-    return -1;
-  };
-
-  int i = -1;
-  for (int j = 0; j < n_walkers; j++)
-    {
-      alias_hazards[j] = nullptr;
-      if (!walkers[j])
-	continue;
-
-      if (!walkers[j]->valid ())
-	walkers[j] = nullptr;
-      else if (i == -1)
-	i = j;
-    }
-
-  while (i >= 0)
-    {
-      int insn_i = i % 2;
-      int paired_i = (i & 2) + !insn_i;
-      int pair_fst = (i & 2);
-      int pair_snd = (i & 2) + 1;
-
-      if (walkers[i]->conflict_p (budget))
-	{
-	  alias_hazards[i] = walkers[i]->insn ();
-
-	  // We got an aliasing conflict for this {load,store} walker,
-	  // so we don't need to walk any further.
-	  walkers[i] = nullptr;
-
-	  // If we have a pair of alias conflicts that prevent
-	  // forming the pair, stop.  There's no need to do further
-	  // analysis.
-	  if (alias_hazards[paired_i]
-	      && (*alias_hazards[pair_fst] <= *alias_hazards[pair_snd]))
-	    return;
-
-	  if (!load_p)
-	    {
-	      int other_pair_fst = (pair_fst ? 0 : 2);
-	      int other_paired_i = other_pair_fst + !insn_i;
-
-	      int x_pair_fst = (i == pair_fst) ? i : other_paired_i;
-	      int x_pair_snd = (i == pair_fst) ? other_paired_i : i;
-
-	      // Similarly, handle the case where we have a {load,store}
-	      // or {store,load} alias hazard pair that prevents forming
-	      // the pair.
-	      if (alias_hazards[other_paired_i]
-		  && *alias_hazards[x_pair_fst] <= *alias_hazards[x_pair_snd])
-		return;
-	    }
-	}
-
-      if (walkers[i])
-	{
-	  walkers[i]->advance ();
-
-	  if (!walkers[i]->valid ())
-	    walkers[i] = nullptr;
-	}
-
-      i = next_walker (i);
-    }
-}
-
-// Given INSNS (in program order) which are known to be adjacent, look
-// to see if either insn has a suitable RTL (register) base that we can
-// use to form a pair.  Push these to BASE_CANDS if we find any.  CAND_MEMs
-// gives the relevant mems from the candidate insns, ACCESS_SIZE gives the
-// size of a single candidate access, and REVERSED says whether the accesses
-// are inverted in offset order.
-//
-// Returns an integer where bit (1 << i) is set if INSNS[i] uses writeback
-// addressing.
-int
-pair_fusion::get_viable_bases (insn_info *insns[2],
-			       vec<base_cand> &base_cands,
-			       rtx cand_mems[2],
-			       unsigned access_size,
-			       bool reversed)
-{
-  // We discovered this pair through a common base.  Need to ensure that
-  // we have a common base register that is live at both locations.
-  def_info *base_defs[2] = {};
-  int writeback = 0;
-  for (int i = 0; i < 2; i++)
-    {
-      const bool is_lower = (i == reversed);
-      poly_int64 poly_off;
-      rtx base = pair_mem_strip_offset (cand_mems[i], &poly_off);
-      if (GET_RTX_CLASS (GET_CODE (XEXP (cand_mems[i], 0))) == RTX_AUTOINC)
-	writeback |= (1 << i);
-
-      if (!REG_P (base) || !poly_off.is_constant ())
-	continue;
-
-      // Punt on accesses relative to eliminable regs.  See the comment in
-      // pair_fusion_bb_info::track_access for a detailed explanation of this.
-      if (!reload_completed
-	  && (REGNO (base) == FRAME_POINTER_REGNUM
-	      || REGNO (base) == ARG_POINTER_REGNUM))
-	continue;
-
-      HOST_WIDE_INT base_off = poly_off.to_constant ();
-
-      // It should be unlikely that we ever punt here, since MEM_EXPR offset
-      // alignment should be a good proxy for register offset alignment.
-      if (base_off % access_size != 0)
-	{
-	  if (dump_file)
-	    fprintf (dump_file,
-		     "base not viable, offset misaligned (insn %d)\n",
-		     insns[i]->uid ());
-	  continue;
-	}
-
-      base_off /= access_size;
-
-      if (!is_lower)
-	base_off--;
-
-      if (!pair_mem_in_range_p (base_off))
-	continue;
-
-      use_info *use = find_access (insns[i]->uses (), REGNO (base));
-      gcc_assert (use);
-      base_defs[i] = use->def ();
-    }
-
-  if (!base_defs[0] && !base_defs[1])
-    {
-      if (dump_file)
-	fprintf (dump_file, "no viable base register for pair (%d,%d)\n",
-		 insns[0]->uid (), insns[1]->uid ());
-      return writeback;
-    }
-
-  for (int i = 0; i < 2; i++)
-    if ((writeback & (1 << i)) && !base_defs[i])
-      {
-	if (dump_file)
-	  fprintf (dump_file, "insn %d has writeback but base isn't viable\n",
-		   insns[i]->uid ());
-	return writeback;
-      }
-
-  if (writeback == 3
-      && base_defs[0]->regno () != base_defs[1]->regno ())
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "pair (%d,%d): double writeback with distinct regs (%d,%d): "
-		 "punting\n",
-		 insns[0]->uid (), insns[1]->uid (),
-		 base_defs[0]->regno (), base_defs[1]->regno ());
-      return writeback;
-    }
-
-  if (base_defs[0] && base_defs[1]
-      && base_defs[0]->regno () == base_defs[1]->regno ())
-    {
-      // Easy case: insns already share the same base reg.
-      base_cands.quick_push (base_defs[0]);
-      return writeback;
-    }
-
-  // Otherwise, we know that one of the bases must change.
-  //
-  // Note that if there is writeback we must use the writeback base
-  // (we know now there is exactly one).
-  for (int i = 0; i < 2; i++)
-    if (base_defs[i] && (!writeback || (writeback & (1 << i))))
-      base_cands.quick_push (base_cand { base_defs[i], i });
-
-  return writeback;
-}
-
-// Given two adjacent memory accesses of the same size, I1 and I2, try
-// and see if we can merge them into a paired access.
-//
-// ACCESS_SIZE gives the (common) size of a single access, LOAD_P is true
-// if the accesses are both loads, otherwise they are both stores.
-bool
-pair_fusion_bb_info::try_fuse_pair (bool load_p, unsigned access_size,
-				    insn_info *i1, insn_info *i2)
-{
-  if (dump_file)
-    fprintf (dump_file, "analyzing pair (load=%d): (%d,%d)\n",
-	     load_p, i1->uid (), i2->uid ());
-
-  insn_info *insns[2];
-  bool reversed = false;
-  if (*i1 < *i2)
-    {
-      insns[0] = i1;
-      insns[1] = i2;
-    }
-  else
-    {
-      insns[0] = i2;
-      insns[1] = i1;
-      reversed = true;
-    }
-
-  rtx cand_mems[2];
-  rtx reg_ops[2];
-  rtx pats[2];
-  for (int i = 0; i < 2; i++)
-    {
-      pats[i] = PATTERN (insns[i]->rtl ());
-      cand_mems[i] = XEXP (pats[i], load_p);
-      reg_ops[i] = XEXP (pats[i], !load_p);
-    }
-
-  if (load_p && reg_overlap_mentioned_p (reg_ops[0], reg_ops[1]))
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "punting on load pair due to reg conflcits (%d,%d)\n",
-		 insns[0]->uid (), insns[1]->uid ());
-      return false;
-    }
-
-  if (cfun->can_throw_non_call_exceptions
-      && find_reg_note (insns[0]->rtl (), REG_EH_REGION, NULL_RTX)
-      && find_reg_note (insns[1]->rtl (), REG_EH_REGION, NULL_RTX))
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "can't combine insns with EH side effects (%d,%d)\n",
-		 insns[0]->uid (), insns[1]->uid ());
-      return false;
-    }
-
-  auto_vec<base_cand, 2> base_cands (2);
-
-  int writeback = m_pass->get_viable_bases (insns, base_cands, cand_mems,
-					    access_size, reversed);
-  if (base_cands.is_empty ())
-    {
-      if (dump_file)
-	fprintf (dump_file, "no viable base for pair (%d,%d)\n",
-		 insns[0]->uid (), insns[1]->uid ());
-      return false;
-    }
-
-  // Punt on frame-related insns with writeback.  We probably won't see
-  // these in practice, but this is conservative and ensures we don't
-  // have to worry about these later on.
-  if (writeback && (RTX_FRAME_RELATED_P (i1->rtl ())
-		    || RTX_FRAME_RELATED_P (i2->rtl ())))
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "rejecting pair (%d,%d): frame-related insn with writeback\n",
-		 i1->uid (), i2->uid ());
-      return false;
-    }
-
-  rtx *ignore = &XEXP (pats[1], load_p);
-  for (auto use : insns[1]->uses ())
-    if (!use->is_mem ()
-	&& refers_to_regno_p (use->regno (), use->regno () + 1, pats[1], ignore)
-	&& use->def () && use->def ()->insn () == insns[0])
-      {
-	// N.B. we allow a true dependence on the base address, as this
-	// happens in the case of auto-inc accesses.  Consider a post-increment
-	// load followed by a regular indexed load, for example.
-	if (dump_file)
-	  fprintf (dump_file,
-		   "%d has non-address true dependence on %d, rejecting pair\n",
-		   insns[1]->uid (), insns[0]->uid ());
-	return false;
-      }
-
-  unsigned i = 0;
-  while (i < base_cands.length ())
-    {
-      base_cand &cand = base_cands[i];
-
-      rtx *ignore[2] = {};
-      for (int j = 0; j < 2; j++)
-	if (cand.from_insn == !j)
-	  ignore[j] = &XEXP (cand_mems[j], 0);
-
-      insn_info *h = first_hazard_after (insns[0], ignore[0]);
-      if (h && *h < *insns[1])
-	cand.hazards[0] = h;
-
-      h = latest_hazard_before (insns[1], ignore[1]);
-      if (h && *h > *insns[0])
-	cand.hazards[1] = h;
-
-      if (!cand.viable ())
-	{
-	  if (dump_file)
-	    fprintf (dump_file,
-		     "pair (%d,%d): rejecting base %d due to dataflow "
-		     "hazards (%d,%d)\n",
-		     insns[0]->uid (),
-		     insns[1]->uid (),
-		     cand.def->regno (),
-		     cand.hazards[0]->uid (),
-		     cand.hazards[1]->uid ());
-
-	  base_cands.ordered_remove (i);
-	}
-      else
-	i++;
-    }
-
-  if (base_cands.is_empty ())
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "can't form pair (%d,%d) due to dataflow hazards\n",
-		 insns[0]->uid (), insns[1]->uid ());
-      return false;
-    }
-
-  insn_info *alias_hazards[4] = {};
-
-  // First def of memory after the first insn, and last def of memory
-  // before the second insn, respectively.
-  def_info *mem_defs[2] = {};
-  if (load_p)
-    {
-      if (!MEM_READONLY_P (cand_mems[0]))
-	{
-	  mem_defs[0] = memory_access (insns[0]->uses ())->def ();
-	  gcc_checking_assert (mem_defs[0]);
-	  mem_defs[0] = mem_defs[0]->next_def ();
-	}
-      if (!MEM_READONLY_P (cand_mems[1]))
-	{
-	  mem_defs[1] = memory_access (insns[1]->uses ())->def ();
-	  gcc_checking_assert (mem_defs[1]);
-	}
-    }
-  else
-    {
-      mem_defs[0] = memory_access (insns[0]->defs ())->next_def ();
-      mem_defs[1] = memory_access (insns[1]->defs ())->prev_def ();
-      gcc_checking_assert (mem_defs[0]);
-      gcc_checking_assert (mem_defs[1]);
-    }
-
-  auto tombstone_p = [&](insn_info *insn) -> bool {
-    return m_emitted_tombstone
-	   && bitmap_bit_p (&m_tombstone_bitmap, insn->uid ());
-  };
-
-  store_walker<false, decltype(tombstone_p)>
-    forward_store_walker (mem_defs[0], cand_mems[0], insns[1], tombstone_p);
-
-  store_walker<true, decltype(tombstone_p)>
-    backward_store_walker (mem_defs[1], cand_mems[1], insns[0], tombstone_p);
-
-  alias_walker *walkers[4] = {};
-  if (mem_defs[0])
-    walkers[0] = &forward_store_walker;
-  if (mem_defs[1])
-    walkers[1] = &backward_store_walker;
-
-  if (load_p && (mem_defs[0] || mem_defs[1]))
-    m_pass->do_alias_analysis (alias_hazards, walkers, load_p);
-  else
-    {
-      // We want to find any loads hanging off the first store.
-      mem_defs[0] = memory_access (insns[0]->defs ());
-      load_walker<false> forward_load_walker (mem_defs[0], insns[0], insns[1]);
-      load_walker<true> backward_load_walker (mem_defs[1], insns[1], insns[0]);
-      walkers[2] = &forward_load_walker;
-      walkers[3] = &backward_load_walker;
-      m_pass->do_alias_analysis (alias_hazards, walkers, load_p);
-      // Now consolidate hazards back down.
-      if (alias_hazards[2]
-	  && (!alias_hazards[0] || (*alias_hazards[2] < *alias_hazards[0])))
-	alias_hazards[0] = alias_hazards[2];
-
-      if (alias_hazards[3]
-	  && (!alias_hazards[1] || (*alias_hazards[3] > *alias_hazards[1])))
-	alias_hazards[1] = alias_hazards[3];
-    }
-
-  if (alias_hazards[0] && alias_hazards[1]
-      && *alias_hazards[0] <= *alias_hazards[1])
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "cannot form pair (%d,%d) due to alias conflicts (%d,%d)\n",
-		 i1->uid (), i2->uid (),
-		 alias_hazards[0]->uid (), alias_hazards[1]->uid ());
-      return false;
-    }
-
-  // Now narrow the hazards on each base candidate using
-  // the alias hazards.
-  i = 0;
-  while (i < base_cands.length ())
-    {
-      base_cand &cand = base_cands[i];
-      if (alias_hazards[0] && (!cand.hazards[0]
-			       || *alias_hazards[0] < *cand.hazards[0]))
-	cand.hazards[0] = alias_hazards[0];
-      if (alias_hazards[1] && (!cand.hazards[1]
-			       || *alias_hazards[1] > *cand.hazards[1]))
-	cand.hazards[1] = alias_hazards[1];
-
-      if (cand.viable ())
-	i++;
-      else
-	{
-	  if (dump_file)
-	    fprintf (dump_file, "pair (%d,%d): rejecting base %d due to "
-				"alias/dataflow hazards (%d,%d)",
-				insns[0]->uid (), insns[1]->uid (),
-				cand.def->regno (),
-				cand.hazards[0]->uid (),
-				cand.hazards[1]->uid ());
-
-	  base_cands.ordered_remove (i);
-	}
-    }
-
-  if (base_cands.is_empty ())
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "cannot form pair (%d,%d) due to alias/dataflow hazards",
-		 insns[0]->uid (), insns[1]->uid ());
-
-      return false;
-    }
-
-  base_cand *base = &base_cands[0];
-  if (base_cands.length () > 1)
-    {
-      // If there are still multiple viable bases, it makes sense
-      // to choose one that allows us to reduce register pressure,
-      // for loads this means moving further down, for stores this
-      // means moving further up.
-      gcc_checking_assert (base_cands.length () == 2);
-      const int hazard_i = !load_p;
-      if (base->hazards[hazard_i])
-	{
-	  if (!base_cands[1].hazards[hazard_i])
-	    base = &base_cands[1];
-	  else if (load_p
-		   && *base_cands[1].hazards[hazard_i]
-		      > *(base->hazards[hazard_i]))
-	    base = &base_cands[1];
-	  else if (!load_p
-		   && *base_cands[1].hazards[hazard_i]
-		      < *(base->hazards[hazard_i]))
-	    base = &base_cands[1];
-	}
-    }
-
-  // Otherwise, hazards[0] > hazards[1].
-  // Pair can be formed anywhere in (hazards[1], hazards[0]).
-  insn_range_info range (insns[0], insns[1]);
-  if (base->hazards[1])
-    range.first = base->hazards[1];
-  if (base->hazards[0])
-    range.last = base->hazards[0]->prev_nondebug_insn ();
-
-  // If the second insn can throw, narrow the move range to exactly that insn.
-  // This prevents us trying to move the second insn from the end of the BB.
-  if (cfun->can_throw_non_call_exceptions
-      && find_reg_note (insns[1]->rtl (), REG_EH_REGION, NULL_RTX))
-    {
-      gcc_assert (range.includes (insns[1]));
-      range = insn_range_info (insns[1]);
-    }
-
-  // Placement strategy: push loads down and pull stores up, this should
-  // help register pressure by reducing live ranges.
-  if (load_p)
-    range.first = range.last;
-  else
-    range.last = range.first;
-
-  if (dump_file)
-    {
-      auto print_hazard = [](insn_info *i)
-	{
-	  if (i)
-	    fprintf (dump_file, "%d", i->uid ());
-	  else
-	    fprintf (dump_file, "-");
-	};
-      auto print_pair = [print_hazard](insn_info **i)
-	{
-	  print_hazard (i[0]);
-	  fprintf (dump_file, ",");
-	  print_hazard (i[1]);
-	};
-
-      fprintf (dump_file, "fusing pair [L=%d] (%d,%d), base=%d, hazards: (",
-	      load_p, insns[0]->uid (), insns[1]->uid (),
-	      base->def->regno ());
-      print_pair (base->hazards);
-      fprintf (dump_file, "), move_range: (%d,%d)\n",
-	       range.first->uid (), range.last->uid ());
-    }
-
-  return fuse_pair (load_p, access_size, writeback,
-		    i1, i2, *base, range);
-}
-
-static void
-dump_insn_list (FILE *f, const insn_list_t &l)
-{
-  fprintf (f, "(");
-
-  auto i = l.begin ();
-  auto end = l.end ();
-
-  if (i != end)
-    fprintf (f, "%d", (*i)->uid ());
-  i++;
-
-  for (; i != end; i++)
-    fprintf (f, ", %d", (*i)->uid ());
-
-  fprintf (f, ")");
-}
-
-DEBUG_FUNCTION void
-debug (const insn_list_t &l)
-{
-  dump_insn_list (stderr, l);
-  fprintf (stderr, "\n");
-}
-
-// LEFT_LIST and RIGHT_LIST are lists of candidate instructions where all insns
-// in LEFT_LIST are known to be adjacent to those in RIGHT_LIST.
-//
-// This function traverses the resulting 2D matrix of possible pair candidates
-// and attempts to merge them into pairs.
-//
-// The algorithm is straightforward: if we consider a combined list of
-// candidates X obtained by merging LEFT_LIST and RIGHT_LIST in program order,
-// then we advance through X until we reach a crossing point (where X[i] and
-// X[i+1] come from different source lists).
-//
-// At this point we know X[i] and X[i+1] are adjacent accesses, and we try to
-// fuse them into a pair.  If this succeeds, we remove X[i] and X[i+1] from
-// their original lists and continue as above.
-//
-// In the failure case, we advance through the source list containing X[i] and
-// continue as above (proceeding to the next crossing point).
-//
-// The rationale for skipping over groups of consecutive candidates from the
-// same source list is as follows:
-//
-// In the store case, the insns in the group can't be re-ordered over each
-// other as they are guaranteed to store to the same location, so we're
-// guaranteed not to lose opportunities by doing this.
-//
-// In the load case, subsequent loads from the same location are either
-// redundant (in which case they should have been cleaned up by an earlier
-// optimization pass) or there is an intervening aliasing hazard, in which case
-// we can't re-order them anyway, so provided earlier passes have cleaned up
-// redundant loads, we shouldn't miss opportunities by doing this.
-void
-pair_fusion_bb_info::merge_pairs (insn_list_t &left_list,
-			  insn_list_t &right_list,
-			  bool load_p,
-			  unsigned access_size)
-{
-  if (dump_file)
-    {
-      fprintf (dump_file, "merge_pairs [L=%d], cand vecs ", load_p);
-      dump_insn_list (dump_file, left_list);
-      fprintf (dump_file, " x ");
-      dump_insn_list (dump_file, right_list);
-      fprintf (dump_file, "\n");
-    }
-
-  auto iter_l = left_list.begin ();
-  auto iter_r = right_list.begin ();
-
-  while (iter_l != left_list.end () && iter_r != right_list.end ())
-    {
-      auto next_l = std::next (iter_l);
-      auto next_r = std::next (iter_r);
-      if (**iter_l < **iter_r
-	  && next_l != left_list.end ()
-	  && **next_l < **iter_r)
-	iter_l = next_l;
-      else if (**iter_r < **iter_l
-	       && next_r != right_list.end ()
-	       && **next_r < **iter_l)
-	iter_r = next_r;
-      else if (try_fuse_pair (load_p, access_size, *iter_l, *iter_r))
-	{
-	  left_list.erase (iter_l);
-	  iter_l = next_l;
-	  right_list.erase (iter_r);
-	  iter_r = next_r;
-	}
-      else if (**iter_l < **iter_r)
-	iter_l = next_l;
-      else
-	iter_r = next_r;
-    }
-}
-
-// Iterate over the accesses in GROUP, looking for adjacent sets
-// of accesses.  If we find two sets of adjacent accesses, call
-// merge_pairs.
-void
-pair_fusion_bb_info::transform_for_base (int encoded_lfs,
-				 access_group &group)
-{
-  const auto lfs = decode_lfs (encoded_lfs);
-  const unsigned access_size = lfs.size;
-
-  bool skip_next = true;
-  access_record *prev_access = nullptr;
-
-  for (auto &access : group.list)
-    {
-      if (skip_next)
-	skip_next = false;
-      else if (known_eq (access.offset, prev_access->offset + access_size))
-	{
-	  merge_pairs (prev_access->cand_insns,
-		       access.cand_insns,
-		       lfs.load_p,
-		       access_size);
-	  skip_next = access.cand_insns.empty ();
-	}
-      prev_access = &access;
-    }
-}
-
-// If we emitted tombstone insns for this BB, iterate through the BB
-// and remove all the tombstone insns, being sure to reparent any uses
-// of mem to previous defs when we do this.
-void
-pair_fusion_bb_info::cleanup_tombstones ()
-{
-  // No need to do anything if we didn't emit a tombstone insn for this BB.
-  if (!m_emitted_tombstone)
-    return;
-
-  for (auto insn : iterate_safely (m_bb->nondebug_insns ()))
-    {
-      if (!insn->is_real ()
-	  || !bitmap_bit_p (&m_tombstone_bitmap, insn->uid ()))
-	continue;
-
-      auto set = as_a<set_info *> (memory_access (insn->defs ()));
-      if (set->has_any_uses ())
-	{
-	  auto prev_set = as_a<set_info *> (set->prev_def ());
-	  while (set->first_use ())
-	    crtl->ssa->reparent_use (set->first_use (), prev_set);
-	}
-
-      // Now set has no uses, we can delete it.
-      insn_change change (insn, insn_change::DELETE);
-      crtl->ssa->change_insn (change);
-    }
-}
-
-template<typename Map>
-void
-pair_fusion_bb_info::traverse_base_map (Map &map)
-{
-  for (auto kv : map)
-    {
-      const auto &key = kv.first;
-      auto &value = kv.second;
-      transform_for_base (key.second, value);
-    }
-}
-
-void
-pair_fusion_bb_info::transform ()
-{
-  traverse_base_map (expr_map);
-  traverse_base_map (def_map);
-}
-
-// Given a load pair insn in PATTERN, unpack the insn, storing
-// the registers in REGS and returning the mem.
-static rtx
-aarch64_destructure_load_pair (rtx regs[2], rtx pattern)
-{
-  rtx mem = NULL_RTX;
-
-  for (int i = 0; i < 2; i++)
-    {
-      rtx pat = XVECEXP (pattern, 0, i);
-      regs[i] = XEXP (pat, 0);
-      rtx unspec = XEXP (pat, 1);
-      gcc_checking_assert (GET_CODE (unspec) == UNSPEC);
-      rtx this_mem = XVECEXP (unspec, 0, 0);
-      if (mem)
-	gcc_checking_assert (rtx_equal_p (mem, this_mem));
-      else
-	{
-	  gcc_checking_assert (MEM_P (this_mem));
-	  mem = this_mem;
-	}
-    }
-
-  return mem;
-}
-
-// Given a store pair insn in PATTERN, unpack the insn, storing
-// the register operands in REGS, and returning the mem.
-static rtx
-aarch64_destructure_store_pair (rtx regs[2], rtx pattern)
-{
-  rtx mem = XEXP (pattern, 0);
-  rtx unspec = XEXP (pattern, 1);
-  gcc_checking_assert (GET_CODE (unspec) == UNSPEC);
-  for (int i = 0; i < 2; i++)
-    regs[i] = XVECEXP (unspec, 0, i);
-  return mem;
-}
-
-rtx
-aarch64_pair_fusion::destructure_pair (rtx regs[2], rtx pattern, bool load_p)
-{
-  if (load_p)
-    return aarch64_destructure_load_pair (regs, pattern);
-  else
-    return aarch64_destructure_store_pair (regs, pattern);
-}
-
-rtx
-aarch64_pair_fusion::gen_promote_writeback_pair (rtx wb_effect, rtx pair_mem,
-						 rtx regs[2],
-						 bool load_p)
-{
-  auto op_mode = aarch64_operand_mode_for_pair_mode (GET_MODE (pair_mem));
-
-  machine_mode modes[2];
-  for (int i = 0; i < 2; i++)
-    {
-      machine_mode mode = GET_MODE (regs[i]);
-      if (load_p)
-	gcc_checking_assert (mode != VOIDmode);
-      else if (mode == VOIDmode)
-	mode = op_mode;
-
-      modes[i] = mode;
-    }
-
-  const auto op_size = GET_MODE_SIZE (modes[0]);
-  gcc_checking_assert (known_eq (op_size, GET_MODE_SIZE (modes[1])));
-
-  rtx pats[2];
-  for (int i = 0; i < 2; i++)
-    {
-      rtx mem = adjust_address_nv (pair_mem, modes[i], op_size * i);
-      pats[i] = load_p
-	? gen_rtx_SET (regs[i], mem)
-	: gen_rtx_SET (mem, regs[i]);
-    }
+      rtx mem = adjust_address_nv (pair_mem, modes[i], op_size * i);
+      pats[i] = load_p
+	? gen_rtx_SET (regs[i], mem)
+	: gen_rtx_SET (mem, regs[i]);
+    }
 
   return gen_rtx_PARALLEL (VOIDmode,
 			   gen_rtvec (3, wb_effect, pats[0], pats[1]));
 }
 
-// Given an existing pair insn INSN, look for a trailing update of
-// the base register which we can fold in to make this pair use
-// a writeback addressing mode.
-void
-pair_fusion::try_promote_writeback (insn_info *insn, bool load_p)
-{
-  rtx regs[2];
-
-  rtx mem = destructure_pair (regs, PATTERN (insn->rtl ()), load_p);
-  gcc_checking_assert (MEM_P (mem));
-
-  poly_int64 offset;
-  rtx base = strip_offset (XEXP (mem, 0), &offset);
-  gcc_assert (REG_P (base));
-
-  const auto access_size = GET_MODE_SIZE (GET_MODE (mem)).to_constant () / 2;
-
-  if (find_access (insn->defs (), REGNO (base)))
-    {
-      gcc_assert (load_p);
-      if (dump_file)
-	fprintf (dump_file,
-		 "ldp %d clobbers base r%d, can't promote to writeback\n",
-		 insn->uid (), REGNO (base));
-      return;
-    }
-
-  auto base_use = find_access (insn->uses (), REGNO (base));
-  gcc_assert (base_use);
-
-  if (!base_use->def ())
-    {
-      if (dump_file)
-	fprintf (dump_file,
-		 "found pair (i%d, L=%d): but base r%d is upwards exposed\n",
-		 insn->uid (), load_p, REGNO (base));
-      return;
-    }
-
-  auto base_def = base_use->def ();
-
-  rtx wb_effect = NULL_RTX;
-  def_info *add_def;
-  const insn_range_info pair_range (insn);
-  insn_info *insns[2] = { nullptr, insn };
-  insn_info *trailing_add
-    = find_trailing_add (insns, pair_range, 0, &wb_effect,
-			 &add_def, base_def, offset,
-			 access_size);
-  if (!trailing_add)
-    return;
-
-  auto attempt = crtl->ssa->new_change_attempt ();
-
-  insn_change pair_change (insn);
-  insn_change del_change (trailing_add, insn_change::DELETE);
-  insn_change *changes[] = { &pair_change, &del_change };
-
-  rtx pair_pat = gen_promote_writeback_pair (wb_effect, mem, regs, load_p);
-  validate_unshare_change (insn->rtl (), &PATTERN (insn->rtl ()), pair_pat,
-			   true);
-
-  // The pair must gain the def of the base register from the add.
-  pair_change.new_defs = insert_access (attempt,
-					add_def,
-					pair_change.new_defs);
-  gcc_assert (pair_change.new_defs.is_valid ());
-
-  auto is_changing = insn_is_changing (changes);
-  for (unsigned i = 0; i < ARRAY_SIZE (changes); i++)
-    gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
-
-  if (!rtl_ssa::recog_ignoring (attempt, pair_change, is_changing))
-    {
-      if (dump_file)
-	fprintf (dump_file, "i%d: recog failed on wb pair, bailing out\n",
-		 insn->uid ());
-      cancel_changes (0);
-      return;
-    }
-
-  gcc_assert (crtl->ssa->verify_insn_changes (changes));
-
-  if (MAY_HAVE_DEBUG_INSNS)
-    fixup_debug_uses_trailing_add (attempt, insn, trailing_add, wb_effect);
-
-  confirm_change_group ();
-  crtl->ssa->change_insns (changes);
-}
-
-// Main function for the pass.  Iterate over the insns in BB looking
-// for load/store candidates.  If running after RA, also try and promote
-// non-writeback pairs to use writeback addressing.  Then try to fuse
-// candidates into pairs.
-void pair_fusion::process_block (bb_info *bb)
-{
-  const bool track_loads = track_loads_p ();
-  const bool track_stores = track_stores_p ();
-
-  pair_fusion_bb_info bb_state (bb, this);
-
-  for (auto insn : bb->nondebug_insns ())
-    {
-      rtx_insn *rti = insn->rtl ();
-
-      if (!rti || !INSN_P (rti))
-	continue;
-
-      rtx pat = PATTERN (rti);
-      bool load_p;
-      if (reload_completed
-	  && should_handle_writeback (writeback::ALL)
-	  && pair_mem_insn_p (rti, load_p))
-	try_promote_writeback (insn, load_p);
-
-      if (GET_CODE (pat) != SET)
-	continue;
-
-      if (track_stores && MEM_P (XEXP (pat, 0)))
-	bb_state.track_access (insn, false, XEXP (pat, 0));
-      else if (track_loads && MEM_P (XEXP (pat, 1)))
-	bb_state.track_access (insn, true, XEXP (pat, 1));
-    }
-
-  bb_state.transform ();
-  bb_state.cleanup_tombstones ();
-}
-
 namespace {
 
 const pass_data pass_data_ldp_fusion =
diff --git a/gcc/pair-fusion.cc b/gcc/pair-fusion.cc
new file mode 100644
index 00000000000..20d741079ca
--- /dev/null
+++ b/gcc/pair-fusion.cc
@@ -0,0 +1,2992 @@ 
+// Pair Mem fusion generic class implementation.
+// Copyright (C) 2024 Free Software Foundation, Inc.
+//
+// This file is part of GCC.
+//
+// GCC is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// GCC is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include "pair-fusion.h"
+#include "cfgcleanup.h"
+#include "tree-pass.h"
+#include "ordered-hash-map.h"
+#include "tree-dfa.h"
+#include "fold-const.h"
+#include "tree-hash-traits.h"
+#include "print-tree.h"
+#include "insn-attr.h"
+
+// We pack these fields (load_p, fpsimd_p, and size) into an integer
+// (LFS) which we use as part of the key into the main hash tables.
+//
+// The idea is that we group candidates together only if they agree on
+// the fields below.  Candidates that disagree on any of these
+// properties shouldn't be merged together.
+struct lfs_fields
+{
+  bool load_p;
+  bool fpsimd_p;
+  unsigned size;
+};
+
+using insn_list_t = std::list<insn_info *>;
+
+// Information about the accesses at a given offset from a particular
+// base.  Stored in an access_group, see below.
+struct access_record
+{
+  poly_int64 offset;
+  std::list<insn_info *> cand_insns;
+  std::list<access_record>::iterator place;
+
+  access_record (poly_int64 off) : offset (off) {}
+};
+
+// A group of accesses where adjacent accesses could be ldp/stp
+// candidates.  The splay tree supports efficient insertion,
+// while the list supports efficient iteration.
+struct access_group
+{
+  splay_tree<access_record *> tree;
+  std::list<access_record> list;
+
+  template<typename Alloc>
+  inline void track (Alloc node_alloc, poly_int64 offset, insn_info *insn);
+};
+
+// Information about an alternate base.  For a def_info D, it may
+// instead be expressed as D = BASE + OFFSET.
+struct alt_base
+{
+  def_info *base;
+  poly_int64 offset;
+};
+
+// Virtual base class for load/store walkers used in alias analysis.
+struct alias_walker
+{
+  virtual bool conflict_p (int &budget) const = 0;
+  virtual insn_info *insn () const = 0;
+  virtual bool valid () const = 0;
+  virtual void advance () = 0;
+};
+
+
+pair_fusion::pair_fusion ()
+{
+  calculate_dominance_info (CDI_DOMINATORS);
+  df_analyze ();
+  crtl->ssa = new rtl_ssa::function_info (cfun);
+}
+
+pair_fusion::~pair_fusion ()
+{
+  if (crtl->ssa->perform_pending_updates ())
+    cleanup_cfg (0);
+
+  free_dominance_info (CDI_DOMINATORS);
+
+  delete crtl->ssa;
+  crtl->ssa = nullptr;
+}
+
+// This is the main function to start the pass.
+void
+pair_fusion::run ()
+{
+  if (!track_loads_p () && !track_stores_p ())
+    return;
+
+  for (auto bb : crtl->ssa->bbs ())
+    process_block (bb);
+}
+
+// State used by the pass for a given basic block.
+struct pair_fusion_bb_info
+{
+  using def_hash = nofree_ptr_hash<def_info>;
+  using expr_key_t = pair_hash<tree_operand_hash, int_hash<int, -1, -2>>;
+  using def_key_t = pair_hash<def_hash, int_hash<int, -1, -2>>;
+
+  // Map of <tree base, LFS> -> access_group.
+  ordered_hash_map<expr_key_t, access_group> expr_map;
+
+  // Map of <RTL-SSA def_info *, LFS> -> access_group.
+  ordered_hash_map<def_key_t, access_group> def_map;
+
+  // Given the def_info for an RTL base register, express it as an offset from
+  // some canonical base instead.
+  //
+  // Canonicalizing bases in this way allows us to identify adjacent accesses
+  // even if they see different base register defs.
+  hash_map<def_hash, alt_base> canon_base_map;
+
+  static const size_t obstack_alignment = sizeof (void *);
+
+  pair_fusion_bb_info (bb_info *bb, pair_fusion *d)
+    : m_bb (bb), m_pass (d), m_emitted_tombstone (false)
+  {
+    obstack_specify_allocation (&m_obstack, OBSTACK_CHUNK_SIZE,
+				obstack_alignment, obstack_chunk_alloc,
+				obstack_chunk_free);
+  }
+  ~pair_fusion_bb_info ()
+  {
+    obstack_free (&m_obstack, nullptr);
+
+    if (m_emitted_tombstone)
+      {
+	bitmap_release (&m_tombstone_bitmap);
+	bitmap_obstack_release (&m_bitmap_obstack);
+      }
+  }
+
+  inline void track_access (insn_info *, bool load, rtx mem);
+  inline void transform ();
+  inline void cleanup_tombstones ();
+
+private:
+  obstack m_obstack;
+  bb_info *m_bb;
+  pair_fusion *m_pass;
+
+  // State for keeping track of tombstone insns emitted for this BB.
+  bitmap_obstack m_bitmap_obstack;
+  bitmap_head m_tombstone_bitmap;
+  bool m_emitted_tombstone;
+
+  inline splay_tree_node<access_record *> *node_alloc (access_record *);
+
+  template<typename Map>
+  inline void traverse_base_map (Map &map);
+  inline void transform_for_base (int load_size, access_group &group);
+
+  inline void merge_pairs (insn_list_t &, insn_list_t &,
+			   bool load_p, unsigned access_size);
+
+  inline bool try_fuse_pair (bool load_p, unsigned access_size,
+			     insn_info *i1, insn_info *i2);
+
+  inline bool fuse_pair (bool load_p, unsigned access_size,
+			 int writeback,
+			 insn_info *i1, insn_info *i2,
+			 base_cand &base,
+			 const insn_range_info &move_range);
+
+  inline void track_tombstone (int uid);
+
+  inline bool track_via_mem_expr (insn_info *, rtx mem, lfs_fields lfs);
+};
+splay_tree_node<access_record *> *
+pair_fusion_bb_info::node_alloc (access_record *access)
+{
+  using T = splay_tree_node<access_record *>;
+  void *addr = obstack_alloc (&m_obstack, sizeof (T));
+  return new (addr) T (access);
+}
+
+// Given a mem MEM, if the address has side effects, return a MEM that accesses
+// the same address but without the side effects.  Otherwise, return
+// MEM unchanged.
+static rtx
+drop_writeback (rtx mem)
+{
+  rtx addr = XEXP (mem, 0);
+
+  if (!side_effects_p (addr))
+    return mem;
+
+  switch (GET_CODE (addr))
+    {
+    case PRE_MODIFY:
+      addr = XEXP (addr, 1);
+      break;
+    case POST_MODIFY:
+    case POST_INC:
+    case POST_DEC:
+      addr = XEXP (addr, 0);
+      break;
+    case PRE_INC:
+    case PRE_DEC:
+    {
+      poly_int64 adjustment = GET_MODE_SIZE (GET_MODE (mem));
+      if (GET_CODE (addr) == PRE_DEC)
+	adjustment *= -1;
+      addr = plus_constant (GET_MODE (addr), XEXP (addr, 0), adjustment);
+      break;
+    }
+    default:
+      gcc_unreachable ();
+    }
+
+  return change_address (mem, GET_MODE (mem), addr);
+}
+
+// Convenience wrapper around strip_offset that can also look through
+// RTX_AUTOINC addresses.  The interface is like strip_offset except we take a
+// MEM so that we know the mode of the access.
+static rtx
+pair_mem_strip_offset (rtx mem, poly_int64 *offset)
+{
+  rtx addr = XEXP (mem, 0);
+
+  switch (GET_CODE (addr))
+    {
+    case PRE_MODIFY:
+    case POST_MODIFY:
+      addr = strip_offset (XEXP (addr, 1), offset);
+      gcc_checking_assert (REG_P (addr));
+      gcc_checking_assert (rtx_equal_p (XEXP (XEXP (mem, 0), 0), addr));
+      break;
+    case PRE_INC:
+    case POST_INC:
+      addr = XEXP (addr, 0);
+      *offset = GET_MODE_SIZE (GET_MODE (mem));
+      gcc_checking_assert (REG_P (addr));
+      break;
+    case PRE_DEC:
+    case POST_DEC:
+      addr = XEXP (addr, 0);
+      *offset = -GET_MODE_SIZE (GET_MODE (mem));
+      gcc_checking_assert (REG_P (addr));
+      break;
+
+    default:
+      addr = strip_offset (addr, offset);
+    }
+
+  return addr;
+}
+
+// Return true if X is a PRE_{INC,DEC,MODIFY} rtx.
+static bool
+any_pre_modify_p (rtx x)
+{
+  const auto code = GET_CODE (x);
+  return code == PRE_INC || code == PRE_DEC || code == PRE_MODIFY;
+}
+
+// Return true if X is a POST_{INC,DEC,MODIFY} rtx.
+static bool
+any_post_modify_p (rtx x)
+{
+  const auto code = GET_CODE (x);
+  return code == POST_INC || code == POST_DEC || code == POST_MODIFY;
+}
+
+// Given LFS (load_p, fpsimd_p, size) fields in FIELDS, encode these
+// into an integer for use as a hash table key.
+static int
+encode_lfs (lfs_fields fields)
+{
+  int size_log2 = exact_log2 (fields.size);
+  gcc_checking_assert (size_log2 >= 2 && size_log2 <= 4);
+  return ((int)fields.load_p << 3)
+    | ((int)fields.fpsimd_p << 2)
+    | (size_log2 - 2);
+}
+
+// Inverse of encode_lfs.
+static lfs_fields
+decode_lfs (int lfs)
+{
+  bool load_p = (lfs & (1 << 3));
+  bool fpsimd_p = (lfs & (1 << 2));
+  unsigned size = 1U << ((lfs & 3) + 2);
+  return { load_p, fpsimd_p, size };
+}
+
+// Track the access INSN at offset OFFSET in this access group.
+// ALLOC_NODE is used to allocate splay tree nodes.
+template<typename Alloc>
+void
+access_group::track (Alloc alloc_node, poly_int64 offset, insn_info *insn)
+{
+  auto insert_before = [&](std::list<access_record>::iterator after)
+    {
+      auto it = list.emplace (after, offset);
+      it->cand_insns.push_back (insn);
+      it->place = it;
+      return &*it;
+    };
+
+  if (!list.size ())
+    {
+      auto access = insert_before (list.end ());
+      tree.insert_max_node (alloc_node (access));
+      return;
+    }
+
+  auto compare = [&](splay_tree_node<access_record *> *node)
+    {
+      return compare_sizes_for_sort (offset, node->value ()->offset);
+    };
+  auto result = tree.lookup (compare);
+  splay_tree_node<access_record *> *node = tree.root ();
+  if (result == 0)
+    node->value ()->cand_insns.push_back (insn);
+  else
+    {
+      auto it = node->value ()->place;
+      auto after = (result > 0) ? std::next (it) : it;
+      auto access = insert_before (after);
+      tree.insert_child (node, result > 0, alloc_node (access));
+    }
+}
+
+// Given a candidate access INSN (with mem MEM), see if it has a suitable
+// MEM_EXPR base (i.e. a tree decl) relative to which we can track the access.
+// LFS is used as part of the key to the hash table, see track_access.
+bool
+pair_fusion_bb_info::track_via_mem_expr (insn_info *insn, rtx mem,
+					 lfs_fields lfs)
+{
+  if (!MEM_EXPR (mem) || !MEM_OFFSET_KNOWN_P (mem))
+    return false;
+
+  poly_int64 offset;
+  tree base_expr = get_addr_base_and_unit_offset (MEM_EXPR (mem),
+						  &offset);
+  if (!base_expr || !DECL_P (base_expr))
+    return false;
+
+  offset += MEM_OFFSET (mem);
+
+  const machine_mode mem_mode = GET_MODE (mem);
+  const HOST_WIDE_INT mem_size = GET_MODE_SIZE (mem_mode).to_constant ();
+
+  // Punt on misaligned offsets.  Paired memory access instructions require
+  // offsets to be a multiple of the access size, and we believe that
+  // misaligned offsets on MEM_EXPR bases are likely to lead to misaligned
+  // offsets w.r.t. RTL bases.
+  if (!multiple_p (offset, mem_size))
+    return false;
+
+  const auto key = std::make_pair (base_expr, encode_lfs (lfs));
+  access_group &group = expr_map.get_or_insert (key, NULL);
+  auto alloc = [&](access_record *access) { return node_alloc (access); };
+  group.track (alloc, offset, insn);
+
+  if (dump_file)
+    {
+      fprintf (dump_file, "[bb %u] tracking insn %d via ",
+	       m_bb->index (), insn->uid ());
+      print_node_brief (dump_file, "mem expr", base_expr, 0);
+      fprintf (dump_file, " [L=%d FP=%d, %smode, off=",
+	       lfs.load_p, lfs.fpsimd_p, mode_name[mem_mode]);
+      print_dec (offset, dump_file);
+      fprintf (dump_file, "]\n");
+    }
+
+  return true;
+}
+
+// Main function to begin pair discovery.  Given a memory access INSN,
+// determine whether it could be a candidate for fusing into a paired
+// access, and if so, track it in the appropriate data structure for
+// this basic block.  LOAD_P is true if the access is a load, and MEM
+// is the mem rtx that occurs in INSN.
+void
+pair_fusion_bb_info::track_access (insn_info *insn, bool load_p, rtx mem)
+{
+  // We can't combine volatile MEMs, so punt on these.
+  if (MEM_VOLATILE_P (mem))
+    return;
+
+  // Ignore writeback accesses if the hook says to do so.
+  if (!m_pass->should_handle_writeback (writeback::EXISTING)
+      && GET_RTX_CLASS (GET_CODE (XEXP (mem, 0))) == RTX_AUTOINC)
+    return;
+
+  const machine_mode mem_mode = GET_MODE (mem);
+  if (!m_pass->pair_operand_mode_ok_p (mem_mode))
+    return;
+
+  rtx reg_op = XEXP (PATTERN (insn->rtl ()), !load_p);
+
+  if (!m_pass->pair_reg_operand_ok_p (load_p, reg_op, mem_mode))
+    return;
+
+  // We want to segregate FP/SIMD accesses from GPR accesses.
+  const bool fpsimd_op_p = m_pass->fpsimd_op_p (reg_op, mem_mode, load_p);
+
+  // Note pair_operand_mode_ok_p already rejected VL modes.
+  const HOST_WIDE_INT mem_size = GET_MODE_SIZE (mem_mode).to_constant ();
+  const lfs_fields lfs = { load_p, fpsimd_op_p, mem_size };
+
+  if (track_via_mem_expr (insn, mem, lfs))
+    return;
+
+  poly_int64 mem_off;
+  rtx addr = XEXP (mem, 0);
+  const bool autoinc_p = GET_RTX_CLASS (GET_CODE (addr)) == RTX_AUTOINC;
+  rtx base = pair_mem_strip_offset (mem, &mem_off);
+  if (!REG_P (base))
+    return;
+
+  // Need to calculate two (possibly different) offsets:
+  //  - Offset at which the access occurs.
+  //  - Offset of the new base def.
+  poly_int64 access_off;
+  if (autoinc_p && any_post_modify_p (addr))
+    access_off = 0;
+  else
+    access_off = mem_off;
+
+  poly_int64 new_def_off = mem_off;
+
+  // Punt on accesses relative to eliminable regs.  Since we don't know the
+  // elimination offset pre-RA, we should postpone forming pairs on such
+  // accesses until after RA.
+  //
+  // As it stands, addresses in range for an individual load/store but not
+  // for a paired access are currently reloaded inefficiently,
+  // ending up with a separate base register for each pair.
+  //
+  // In theory LRA should make use of
+  // targetm.legitimize_address_displacement to promote sharing of
+  // bases among multiple (nearby) address reloads, but the current
+  // LRA code returns early from process_address_1 for operands that
+  // satisfy "m", even if they don't satisfy the real (relaxed) address
+  // constraint; this early return means we never get to the code
+  // that calls targetm.legitimize_address_displacement.
+  //
+  // So for now, it's better to punt when we can't be sure that the
+  // offset is in range for paired access.  On aarch64, out-of-range cases
+  // can then be handled after RA by the out-of-range LDP/STP peepholes.
+  // Eventually, it would be nice to handle known out-of-range opportunities
+  // in the pass itself (for stack accesses, this would be in the post-RA pass).
+  if (!reload_completed
+      && (REGNO (base) == FRAME_POINTER_REGNUM
+	  || REGNO (base) == ARG_POINTER_REGNUM))
+    return;
+
+  // Now need to find def of base register.
+  use_info *base_use = find_access (insn->uses (), REGNO (base));
+  gcc_assert (base_use);
+  def_info *base_def = base_use->def ();
+  if (!base_def)
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "base register (regno %d) of insn %d is undefined",
+		 REGNO (base), insn->uid ());
+      return;
+    }
+
+  alt_base *canon_base = canon_base_map.get (base_def);
+  if (canon_base)
+    {
+      // Express this as the combined offset from the canonical base.
+      base_def = canon_base->base;
+      new_def_off += canon_base->offset;
+      access_off += canon_base->offset;
+    }
+
+  if (autoinc_p)
+    {
+      auto def = find_access (insn->defs (), REGNO (base));
+      gcc_assert (def);
+
+      // Record that DEF = BASE_DEF + MEM_OFF.
+      if (dump_file)
+	{
+	  pretty_printer pp;
+	  pp_access (&pp, def, 0);
+	  pp_string (&pp, " = ");
+	  pp_access (&pp, base_def, 0);
+	  fprintf (dump_file, "[bb %u] recording %s + ",
+		   m_bb->index (), pp_formatted_text (&pp));
+	  print_dec (new_def_off, dump_file);
+	  fprintf (dump_file, "\n");
+	}
+
+      alt_base base_rec { base_def, new_def_off };
+      if (canon_base_map.put (def, base_rec))
+	gcc_unreachable (); // Base defs should be unique.
+    }
+
+  // Punt on misaligned offsets.  Paired memory accesses require offsets
+  // to be a multiple of the access size.
+  if (!multiple_p (mem_off, mem_size))
+    return;
+
+  const auto key = std::make_pair (base_def, encode_lfs (lfs));
+  access_group &group = def_map.get_or_insert (key, NULL);
+  auto alloc = [&](access_record *access) { return node_alloc (access); };
+  group.track (alloc, access_off, insn);
+
+  if (dump_file)
+    {
+      pretty_printer pp;
+      pp_access (&pp, base_def, 0);
+
+      fprintf (dump_file, "[bb %u] tracking insn %d via %s",
+	       m_bb->index (), insn->uid (), pp_formatted_text (&pp));
+      fprintf (dump_file,
+	       " [L=%d, WB=%d, FP=%d, %smode, off=",
+	       lfs.load_p, autoinc_p, lfs.fpsimd_p, mode_name[mem_mode]);
+      print_dec (access_off, dump_file);
+      fprintf (dump_file, "]\n");
+    }
+}
+
+// Dummy predicate that never ignores any insns.
+static bool no_ignore (insn_info *) { return false; }
+
+// Return the latest dataflow hazard before INSN.
+//
+// If IGNORE is non-NULL, this points to a sub-rtx which we should ignore for
+// dataflow purposes.  This is needed when considering changing the RTL base of
+// an access discovered through a MEM_EXPR base.
+//
+// If IGNORE_INSN is non-NULL, we should further ignore any hazards arising
+// from that insn.
+//
+// N.B. we ignore any defs/uses of memory here as we deal with that separately,
+// making use of alias disambiguation.
+static insn_info *
+latest_hazard_before (insn_info *insn, rtx *ignore,
+		      insn_info *ignore_insn = nullptr)
+{
+  insn_info *result = nullptr;
+
+  // If the insn can throw then it is at the end of a BB and we can't
+  // move it, model this by recording a hazard in the previous insn
+  // which will prevent moving the insn up.
+  if (cfun->can_throw_non_call_exceptions
+      && find_reg_note (insn->rtl (), REG_EH_REGION, NULL_RTX))
+    return insn->prev_nondebug_insn ();
+
+  // Return true if we registered the hazard.
+  auto hazard = [&](insn_info *h) -> bool
+    {
+      gcc_checking_assert (*h < *insn);
+      if (h == ignore_insn)
+	return false;
+
+      if (!result || *h > *result)
+	result = h;
+
+      return true;
+    };
+
+  rtx pat = PATTERN (insn->rtl ());
+  auto ignore_use = [&](use_info *u)
+    {
+      if (u->is_mem ())
+	return true;
+
+      return !refers_to_regno_p (u->regno (), u->regno () + 1, pat, ignore);
+    };
+
+  // Find defs of uses in INSN (RaW).
+  for (auto use : insn->uses ())
+    if (!ignore_use (use) && use->def ())
+      hazard (use->def ()->insn ());
+
+  // Find previous defs (WaW) or previous uses (WaR) of defs in INSN.
+  for (auto def : insn->defs ())
+    {
+      if (def->is_mem ())
+	continue;
+
+      if (def->prev_def ())
+	{
+	  hazard (def->prev_def ()->insn ()); // WaW
+
+	  auto set = dyn_cast<set_info *> (def->prev_def ());
+	  if (set && set->has_nondebug_insn_uses ())
+	    for (auto use : set->reverse_nondebug_insn_uses ())
+	      if (use->insn () != insn && hazard (use->insn ())) // WaR
+		break;
+	}
+
+      if (!HARD_REGISTER_NUM_P (def->regno ()))
+	continue;
+
+      // Also need to check backwards for call clobbers (WaW).
+      for (auto call_group : def->ebb ()->call_clobbers ())
+	{
+	  if (!call_group->clobbers (def->resource ()))
+	    continue;
+
+	  auto clobber_insn = prev_call_clobbers_ignoring (*call_group,
+							   def->insn (),
+							   no_ignore);
+	  if (clobber_insn)
+	    hazard (clobber_insn);
+	}
+
+    }
+
+  return result;
+}
+
+// Return the first dataflow hazard after INSN.
+//
+// If IGNORE is non-NULL, this points to a sub-rtx which we should ignore for
+// dataflow purposes.  This is needed when considering changing the RTL base of
+// an access discovered through a MEM_EXPR base.
+//
+// N.B. we ignore any defs/uses of memory here as we deal with that separately,
+// making use of alias disambiguation.
+static insn_info *
+first_hazard_after (insn_info *insn, rtx *ignore)
+{
+  insn_info *result = nullptr;
+  auto hazard = [insn, &result](insn_info *h)
+    {
+      gcc_checking_assert (*h > *insn);
+      if (!result || *h < *result)
+	result = h;
+    };
+
+  rtx pat = PATTERN (insn->rtl ());
+  auto ignore_use = [&](use_info *u)
+    {
+      if (u->is_mem ())
+	return true;
+
+      return !refers_to_regno_p (u->regno (), u->regno () + 1, pat, ignore);
+    };
+
+  for (auto def : insn->defs ())
+    {
+      if (def->is_mem ())
+	continue;
+
+      if (def->next_def ())
+	hazard (def->next_def ()->insn ()); // WaW
+
+      auto set = dyn_cast<set_info *> (def);
+      if (set && set->has_nondebug_insn_uses ())
+	hazard (set->first_nondebug_insn_use ()->insn ()); // RaW
+
+      if (!HARD_REGISTER_NUM_P (def->regno ()))
+	continue;
+
+      // Also check for call clobbers of this def (WaW).
+      for (auto call_group : def->ebb ()->call_clobbers ())
+	{
+	  if (!call_group->clobbers (def->resource ()))
+	    continue;
+
+	  auto clobber_insn = next_call_clobbers_ignoring (*call_group,
+							   def->insn (),
+							   no_ignore);
+	  if (clobber_insn)
+	    hazard (clobber_insn);
+	}
+    }
+
+  // Find any subsequent defs of uses in INSN (WaR).
+  for (auto use : insn->uses ())
+    {
+      if (ignore_use (use))
+	continue;
+
+      if (use->def ())
+	{
+	  auto def = use->def ()->next_def ();
+	  if (def && def->insn () == insn)
+	    def = def->next_def ();
+
+	  if (def)
+	    hazard (def->insn ());
+	}
+
+      if (!HARD_REGISTER_NUM_P (use->regno ()))
+	continue;
+
+      // Also need to handle call clobbers of our uses (again WaR).
+      //
+      // See restrict_movement_for_uses_ignoring for why we don't
+      // need to check backwards for call clobbers.
+      for (auto call_group : use->ebb ()->call_clobbers ())
+	{
+	  if (!call_group->clobbers (use->resource ()))
+	    continue;
+
+	  auto clobber_insn = next_call_clobbers_ignoring (*call_group,
+							   use->insn (),
+							   no_ignore);
+	  if (clobber_insn)
+	    hazard (clobber_insn);
+	}
+    }
+
+  return result;
+}
+
+// Return true iff R1 and R2 overlap.
+static bool
+ranges_overlap_p (const insn_range_info &r1, const insn_range_info &r2)
+{
+  // If either range is empty, then their intersection is empty.
+  if (!r1 || !r2)
+    return false;
+
+  // When do they not overlap? When one range finishes before the other
+  // starts, i.e. (*r1.last < *r2.first || *r2.last < *r1.first).
+  // Inverting this, we get the below.
+  return *r1.last >= *r2.first && *r2.last >= *r1.first;
+}
+
+// Get the range of insns that def feeds.
+static insn_range_info get_def_range (def_info *def)
+{
+  insn_info *last = def->next_def ()->insn ()->prev_nondebug_insn ();
+  return { def->insn (), last };
+}
+
+// Given a def (of memory), return the downwards range within which we
+// can safely move this def.
+static insn_range_info
+def_downwards_move_range (def_info *def)
+{
+  auto range = get_def_range (def);
+
+  auto set = dyn_cast<set_info *> (def);
+  if (!set || !set->has_any_uses ())
+    return range;
+
+  auto use = set->first_nondebug_insn_use ();
+  if (use)
+    range = move_earlier_than (range, use->insn ());
+
+  return range;
+}
+
+// Given a def (of memory), return the upwards range within which we can
+// safely move this def.
+static insn_range_info
+def_upwards_move_range (def_info *def)
+{
+  def_info *prev = def->prev_def ();
+  insn_range_info range { prev->insn (), def->insn () };
+
+  auto set = dyn_cast<set_info *> (prev);
+  if (!set || !set->has_any_uses ())
+    return range;
+
+  auto use = set->last_nondebug_insn_use ();
+  if (use)
+    range = move_later_than (range, use->insn ());
+
+  return range;
+}
+
+// Class that implements a state machine for building the changes needed to form
+// a store pair instruction.  This allows us to easily build the changes in
+// program order, as required by rtl-ssa.
+struct store_change_builder
+{
+  enum class state
+  {
+    FIRST,
+    INSERT,
+    FIXUP_USE,
+    LAST,
+    DONE
+  };
+
+  enum class action
+  {
+    TOMBSTONE,
+    CHANGE,
+    INSERT,
+    FIXUP_USE
+  };
+
+  struct change
+  {
+    action type;
+    insn_info *insn;
+  };
+
+  bool done () const { return m_state == state::DONE; }
+
+  store_change_builder (insn_info *insns[2],
+			insn_info *repurpose,
+			insn_info *dest)
+    : m_state (state::FIRST), m_insns { insns[0], insns[1] },
+      m_repurpose (repurpose), m_dest (dest), m_use (nullptr) {}
+
+  change get_change () const
+  {
+    switch (m_state)
+      {
+      case state::FIRST:
+	return {
+	  m_insns[0] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
+	  m_insns[0]
+	};
+      case state::LAST:
+	return {
+	  m_insns[1] == m_repurpose ? action::CHANGE : action::TOMBSTONE,
+	  m_insns[1]
+	};
+      case state::INSERT:
+	return { action::INSERT, m_dest };
+      case state::FIXUP_USE:
+	return { action::FIXUP_USE, m_use->insn () };
+      case state::DONE:
+	break;
+      }
+
+    gcc_unreachable ();
+  }
+
+  // Transition to the next state.
+  void advance ()
+  {
+    switch (m_state)
+      {
+      case state::FIRST:
+	if (m_repurpose)
+	  m_state = state::LAST;
+	else
+	  m_state = state::INSERT;
+	break;
+      case state::INSERT:
+      {
+	def_info *def = memory_access (m_insns[0]->defs ());
+	while (*def->next_def ()->insn () <= *m_dest)
+	  def = def->next_def ();
+
+	// Now we know DEF feeds the insertion point for the new stp.
+	// Look for any uses of DEF that will consume the new stp.
+	gcc_assert (*def->insn () <= *m_dest
+		    && *def->next_def ()->insn () > *m_dest);
+
+	auto set = as_a<set_info *> (def);
+	for (auto use : set->nondebug_insn_uses ())
+	  if (*use->insn () > *m_dest)
+	    {
+	      m_use = use;
+	      break;
+	    }
+
+	if (m_use)
+	  m_state = state::FIXUP_USE;
+	else
+	  m_state = state::LAST;
+	break;
+      }
+      case state::FIXUP_USE:
+	m_use = m_use->next_nondebug_insn_use ();
+	if (!m_use)
+	  m_state = state::LAST;
+	break;
+      case state::LAST:
+	m_state = state::DONE;
+	break;
+      case state::DONE:
+	gcc_unreachable ();
+      }
+  }
+
+private:
+  state m_state;
+
+  // Original candidate stores.
+  insn_info *m_insns[2];
+
+  // If non-null, this is a candidate insn to change into an stp.  Otherwise we
+  // are deleting both original insns and inserting a new insn for the stp.
+  insn_info *m_repurpose;
+
+  // Destionation of the stp, it will be placed immediately after m_dest.
+  insn_info *m_dest;
+
+  // Current nondebug use that needs updating due to stp insertion.
+  use_info *m_use;
+};
+
+// Given candidate store insns FIRST and SECOND, see if we can re-purpose one
+// of them (together with its def of memory) for the stp insn.  If so, return
+// that insn.  Otherwise, return null.
+static insn_info *
+try_repurpose_store (insn_info *first,
+		     insn_info *second,
+		     const insn_range_info &move_range)
+{
+  def_info * const defs[2] = {
+    memory_access (first->defs ()),
+    memory_access (second->defs ())
+  };
+
+  if (move_range.includes (first)
+      || ranges_overlap_p (move_range, def_downwards_move_range (defs[0])))
+    return first;
+
+  if (move_range.includes (second)
+      || ranges_overlap_p (move_range, def_upwards_move_range (defs[1])))
+    return second;
+
+  return nullptr;
+}
+
+// Generate the RTL pattern for a "tombstone"; used temporarily during this pass
+// to replace stores that are marked for deletion where we can't immediately
+// delete the store (since there are uses of mem hanging off the store).
+//
+// These are deleted at the end of the pass and uses re-parented appropriately
+// at this point.
+static rtx
+gen_tombstone (void)
+{
+  return gen_rtx_CLOBBER (VOIDmode,
+			  gen_rtx_MEM (BLKmode, gen_rtx_SCRATCH (Pmode)));
+}
+
+// Go through the reg notes rooted at NOTE, dropping those that we should drop,
+// and preserving those that we want to keep by prepending them to (and
+// returning) RESULT.  EH_REGION is used to make sure we have at most one
+// REG_EH_REGION note in the resulting list.  FR_EXPR is used to return any
+// REG_FRAME_RELATED_EXPR note we find, as these can need special handling in
+// combine_reg_notes.
+static rtx
+filter_notes (rtx note, rtx result, bool *eh_region, rtx *fr_expr)
+{
+  for (; note; note = XEXP (note, 1))
+    {
+      switch (REG_NOTE_KIND (note))
+	{
+	case REG_DEAD:
+	  // REG_DEAD notes aren't required to be maintained.
+	case REG_EQUAL:
+	case REG_EQUIV:
+	case REG_UNUSED:
+	case REG_NOALIAS:
+	  // These can all be dropped.  For REG_EQU{AL,IV} they cannot apply to
+	  // non-single_set insns, and REG_UNUSED is re-computed by RTl-SSA, see
+	  // rtl-ssa/changes.cc:update_notes.
+	  //
+	  // Similarly, REG_NOALIAS cannot apply to a parallel.
+	case REG_INC:
+	  // When we form the pair insn, the reg update is implemented
+	  // as just another SET in the parallel, so isn't really an
+	  // auto-increment in the RTL sense, hence we drop the note.
+	  break;
+	case REG_EH_REGION:
+	  gcc_assert (!*eh_region);
+	  *eh_region = true;
+	  result = alloc_reg_note (REG_EH_REGION, XEXP (note, 0), result);
+	  break;
+	case REG_CFA_DEF_CFA:
+	case REG_CFA_OFFSET:
+	case REG_CFA_RESTORE:
+	  result = alloc_reg_note (REG_NOTE_KIND (note),
+				   copy_rtx (XEXP (note, 0)),
+				   result);
+	  break;
+	case REG_FRAME_RELATED_EXPR:
+	  gcc_assert (!*fr_expr);
+	  *fr_expr = copy_rtx (XEXP (note, 0));
+	  break;
+	default:
+	  // Unexpected REG_NOTE kind.
+	  gcc_unreachable ();
+	}
+    }
+
+  return result;
+}
+
+// Return the notes that should be attached to a combination of I1 and I2, where
+// *I1 < *I2.  LOAD_P is true for loads.
+static rtx
+combine_reg_notes (insn_info *i1, insn_info *i2, bool load_p)
+{
+  // Temporary storage for REG_FRAME_RELATED_EXPR notes.
+  rtx fr_expr[2] = {};
+
+  bool found_eh_region = false;
+  rtx result = NULL_RTX;
+  result = filter_notes (REG_NOTES (i2->rtl ()), result,
+			 &found_eh_region, fr_expr + 1);
+  result = filter_notes (REG_NOTES (i1->rtl ()), result,
+			 &found_eh_region, fr_expr);
+
+  if (!load_p)
+    {
+      // Simple frame-related sp-relative saves don't need CFI notes, but when
+      // we combine them into an stp we will need a CFI note as dwarf2cfi can't
+      // interpret the unspec pair representation directly.
+      if (RTX_FRAME_RELATED_P (i1->rtl ()) && !fr_expr[0])
+	fr_expr[0] = copy_rtx (PATTERN (i1->rtl ()));
+      if (RTX_FRAME_RELATED_P (i2->rtl ()) && !fr_expr[1])
+	fr_expr[1] = copy_rtx (PATTERN (i2->rtl ()));
+    }
+
+  rtx fr_pat = NULL_RTX;
+  if (fr_expr[0] && fr_expr[1])
+    {
+      // Combining two frame-related insns, need to construct
+      // a REG_FRAME_RELATED_EXPR note which represents the combined
+      // operation.
+      RTX_FRAME_RELATED_P (fr_expr[1]) = 1;
+      fr_pat = gen_rtx_PARALLEL (VOIDmode,
+				 gen_rtvec (2, fr_expr[0], fr_expr[1]));
+    }
+  else
+    fr_pat = fr_expr[0] ? fr_expr[0] : fr_expr[1];
+
+  if (fr_pat)
+    result = alloc_reg_note (REG_FRAME_RELATED_EXPR,
+			     fr_pat, result);
+
+  return result;
+}
+
+// Given two memory accesses in PATS, at least one of which is of a
+// writeback form, extract two non-writeback memory accesses addressed
+// relative to the initial value of the base register, and output these
+// in PATS.  Return an rtx that represents the overall change to the
+// base register.
+static rtx
+extract_writebacks (bool load_p, rtx pats[2], int changed)
+{
+  rtx base_reg = NULL_RTX;
+  poly_int64 current_offset = 0;
+
+  poly_int64 offsets[2];
+
+  for (int i = 0; i < 2; i++)
+    {
+      rtx mem = XEXP (pats[i], load_p);
+      rtx reg = XEXP (pats[i], !load_p);
+
+      rtx addr = XEXP (mem, 0);
+      const bool autoinc_p = GET_RTX_CLASS (GET_CODE (addr)) == RTX_AUTOINC;
+
+      poly_int64 offset;
+      rtx this_base = pair_mem_strip_offset (mem, &offset);
+      gcc_assert (REG_P (this_base));
+      if (base_reg)
+	gcc_assert (rtx_equal_p (base_reg, this_base));
+      else
+	base_reg = this_base;
+
+      // If we changed base for the current insn, then we already
+      // derived the correct mem for this insn from the effective
+      // address of the other access.
+      if (i == changed)
+	{
+	  gcc_checking_assert (!autoinc_p);
+	  offsets[i] = offset;
+	  continue;
+	}
+
+      if (autoinc_p && any_pre_modify_p (addr))
+	current_offset += offset;
+
+      poly_int64 this_off = current_offset;
+      if (!autoinc_p)
+	this_off += offset;
+
+      offsets[i] = this_off;
+      rtx new_mem = change_address (mem, GET_MODE (mem),
+				    plus_constant (GET_MODE (base_reg),
+						   base_reg, this_off));
+      pats[i] = load_p
+	? gen_rtx_SET (reg, new_mem)
+	: gen_rtx_SET (new_mem, reg);
+
+      if (autoinc_p && any_post_modify_p (addr))
+	current_offset += offset;
+    }
+
+  if (known_eq (current_offset, 0))
+    return NULL_RTX;
+
+  return gen_rtx_SET (base_reg, plus_constant (GET_MODE (base_reg),
+					       base_reg, current_offset));
+}
+
+// INSNS contains either {nullptr, pair insn} (when promoting an existing
+// non-writeback pair) or contains the candidate insns used to form the pair
+// (when fusing a new pair).
+//
+// PAIR_RANGE specifies where we want to form the final pair.
+// INITIAL_OFFSET gives the current base offset for the pair.
+// Bit I of INITIAL_WRITEBACK is set if INSNS[I] initially had writeback.
+// ACCESS_SIZE gives the access size for a single arm of the pair.
+// BASE_DEF gives the initial def of the base register consumed by the pair.
+//
+// Given the above, this function looks for a trailing destructive update of the
+// base register.  If there is one, we choose the first such update after
+// PAIR_DST that is still in the same BB as our pair.  We return the new def in
+// *ADD_DEF and the resulting writeback effect in *WRITEBACK_EFFECT.
+insn_info *
+pair_fusion::find_trailing_add (insn_info *insns[2],
+				const insn_range_info &pair_range,
+				int initial_writeback,
+				rtx *writeback_effect,
+				def_info **add_def,
+				def_info *base_def,
+				poly_int64 initial_offset,
+				unsigned access_size)
+{
+  // Punt on frame-related insns, it is better to be conservative and
+  // not try to form writeback pairs here, and means we don't have to
+  // worry about the writeback case in forming REG_FRAME_RELATED_EXPR
+  // notes (see combine_reg_notes).
+  if ((insns[0] && RTX_FRAME_RELATED_P (insns[0]->rtl ()))
+      || RTX_FRAME_RELATED_P (insns[1]->rtl ()))
+    return nullptr;
+
+  insn_info *pair_dst = pair_range.singleton ();
+  gcc_assert (pair_dst);
+
+  def_info *def = base_def->next_def ();
+
+  // In the case that either of the initial pair insns had writeback,
+  // then there will be intervening defs of the base register.
+  // Skip over these.
+  for (int i = 0; i < 2; i++)
+    if (initial_writeback & (1 << i))
+      {
+	gcc_assert (def->insn () == insns[i]);
+	def = def->next_def ();
+      }
+
+  if (!def || def->bb () != pair_dst->bb ())
+    return nullptr;
+
+  // DEF should now be the first def of the base register after PAIR_DST.
+  insn_info *cand = def->insn ();
+  gcc_assert (*cand > *pair_dst);
+
+  const auto base_regno = base_def->regno ();
+
+  // If CAND doesn't also use our base register,
+  // it can't destructively update it.
+  if (!find_access (cand->uses (), base_regno))
+    return nullptr;
+
+  auto rti = cand->rtl ();
+
+  if (!INSN_P (rti))
+    return nullptr;
+
+  auto pat = PATTERN (rti);
+  if (GET_CODE (pat) != SET)
+    return nullptr;
+
+  auto dest = XEXP (pat, 0);
+  if (!REG_P (dest) || REGNO (dest) != base_regno)
+    return nullptr;
+
+  poly_int64 offset;
+  rtx rhs_base = strip_offset (XEXP (pat, 1), &offset);
+  if (!REG_P (rhs_base)
+      || REGNO (rhs_base) != base_regno
+      || !offset.is_constant ())
+    return nullptr;
+
+  // If the initial base offset is zero, we can handle any add offset
+  // (post-inc).  Otherwise, we require the offsets to match (pre-inc).
+  if (!known_eq (initial_offset, 0) && !known_eq (offset, initial_offset))
+    return nullptr;
+
+  auto off_hwi = offset.to_constant ();
+
+  if (off_hwi % access_size != 0)
+    return nullptr;
+
+  off_hwi /= access_size;
+
+  if (!pair_mem_in_range_p (off_hwi))
+    return nullptr;
+
+  auto dump_prefix = [&]()
+    {
+      if (!insns[0])
+	fprintf (dump_file, "existing pair i%d: ", insns[1]->uid ());
+      else
+	fprintf (dump_file, "  (%d,%d)",
+		 insns[0]->uid (), insns[1]->uid ());
+    };
+
+  insn_info *hazard = latest_hazard_before (cand, nullptr, insns[1]);
+  if (!hazard || *hazard <= *pair_dst)
+    {
+      if (dump_file)
+	{
+	  dump_prefix ();
+	  fprintf (dump_file,
+		   "folding in trailing add (%d) to use writeback form\n",
+		   cand->uid ());
+	}
+
+      *add_def = def;
+      *writeback_effect = copy_rtx (pat);
+      return cand;
+    }
+
+  if (dump_file)
+    {
+      dump_prefix ();
+      fprintf (dump_file,
+	       "can't fold in trailing add (%d), hazard = %d\n",
+	       cand->uid (), hazard->uid ());
+    }
+
+  return nullptr;
+}
+
+// We just emitted a tombstone with uid UID, track it in a bitmap for
+// this BB so we can easily identify it later when cleaning up tombstones.
+void
+pair_fusion_bb_info::track_tombstone (int uid)
+{
+  if (!m_emitted_tombstone)
+    {
+      // Lazily initialize the bitmap for tracking tombstone insns.
+      bitmap_obstack_initialize (&m_bitmap_obstack);
+      bitmap_initialize (&m_tombstone_bitmap, &m_bitmap_obstack);
+      m_emitted_tombstone = true;
+    }
+
+  if (!bitmap_set_bit (&m_tombstone_bitmap, uid))
+    gcc_unreachable (); // Bit should have changed.
+}
+
+// Reset the debug insn containing USE (the debug insn has been
+// optimized away).
+static void
+reset_debug_use (use_info *use)
+{
+  auto use_insn = use->insn ();
+  auto use_rtl = use_insn->rtl ();
+  insn_change change (use_insn);
+  change.new_uses = {};
+  INSN_VAR_LOCATION_LOC (use_rtl) = gen_rtx_UNKNOWN_VAR_LOC ();
+  crtl->ssa->change_insn (change);
+}
+
+// USE is a debug use that needs updating because DEF (a def of the same
+// register) is being re-ordered over it.  If BASE is non-null, then DEF
+// is an update of the register BASE by a constant, given by WB_OFFSET,
+// and we can preserve debug info by accounting for the change in side
+// effects.
+static void
+fixup_debug_use (obstack_watermark &attempt,
+		 use_info *use,
+		 def_info *def,
+		 rtx base,
+		 poly_int64 wb_offset)
+{
+  auto use_insn = use->insn ();
+  if (base)
+    {
+      auto use_rtl = use_insn->rtl ();
+      insn_change change (use_insn);
+
+      gcc_checking_assert (REG_P (base) && use->regno () == REGNO (base));
+      change.new_uses = check_remove_regno_access (attempt,
+						   change.new_uses,
+						   use->regno ());
+
+      // The effect of the writeback is to add WB_OFFSET to BASE.  If
+      // we're re-ordering DEF below USE, then we update USE by adding
+      // WB_OFFSET to it.  Otherwise, if we're re-ordering DEF above
+      // USE, we update USE by undoing the effect of the writeback
+      // (subtracting WB_OFFSET).
+      use_info *new_use;
+      if (*def->insn () > *use_insn)
+	{
+	  // We now need USE_INSN to consume DEF.  Create a new use of DEF.
+	  //
+	  // N.B. this means until we call change_insns for the main change
+	  // group we will temporarily have a debug use consuming a def that
+	  // comes after it, but RTL-SSA doesn't currently support updating
+	  // debug insns as part of the main change group (together with
+	  // nondebug changes), so we will have to live with this update
+	  // leaving the IR being temporarily inconsistent.  It seems to
+	  // work out OK once the main change group is applied.
+	  wb_offset *= -1;
+	  new_use = crtl->ssa->create_use (attempt,
+					   use_insn,
+					   as_a<set_info *> (def));
+	}
+      else
+	new_use = find_access (def->insn ()->uses (), use->regno ());
+
+      change.new_uses = insert_access (attempt, new_use, change.new_uses);
+
+      if (dump_file)
+	{
+	  const char *dir = (*def->insn () < *use_insn) ? "down" : "up";
+	  pretty_printer pp;
+	  pp_string (&pp, "[");
+	  pp_access (&pp, use, 0);
+	  pp_string (&pp, "]");
+	  pp_string (&pp, " due to wb def ");
+	  pp_string (&pp, "[");
+	  pp_access (&pp, def, 0);
+	  pp_string (&pp, "]");
+	  fprintf (dump_file,
+		   "  i%d: fix up debug use %s re-ordered %s, "
+		   "sub r%u -> r%u + ",
+		   use_insn->uid (), pp_formatted_text (&pp),
+		   dir, REGNO (base), REGNO (base));
+	  print_dec (wb_offset, dump_file);
+	  fprintf (dump_file, "\n");
+	}
+
+      insn_propagation prop (use_rtl, base,
+			     plus_constant (GET_MODE (base), base, wb_offset));
+      if (prop.apply_to_pattern (&INSN_VAR_LOCATION_LOC (use_rtl)))
+	crtl->ssa->change_insn (change);
+      else
+	{
+	  if (dump_file)
+	    fprintf (dump_file, "  i%d: RTL substitution failed (%s)"
+		     ", resetting debug insn", use_insn->uid (),
+		     prop.failure_reason);
+	  reset_debug_use (use);
+	}
+    }
+  else
+    {
+      if (dump_file)
+	{
+	  pretty_printer pp;
+	  pp_string (&pp, "[");
+	  pp_access (&pp, use, 0);
+	  pp_string (&pp, "] due to re-ordered load def [");
+	  pp_access (&pp, def, 0);
+	  pp_string (&pp, "]");
+	  fprintf (dump_file, "  i%d: resetting debug use %s\n",
+		   use_insn->uid (), pp_formatted_text (&pp));
+	}
+      reset_debug_use (use);
+    }
+}
+
+// Update debug uses when folding in a trailing add insn to form a
+// writeback pair.
+//
+// ATTEMPT is used to allocate RTL-SSA temporaries for the changes,
+// the final pair is placed immediately after PAIR_DST, TRAILING_ADD
+// is a trailing add insn which is being folded into the pair to make it
+// use writeback addressing, and WRITEBACK_EFFECT is the pattern for
+// TRAILING_ADD.
+static void
+fixup_debug_uses_trailing_add (obstack_watermark &attempt,
+			       insn_info *pair_dst,
+			       insn_info *trailing_add,
+			       rtx writeback_effect)
+{
+  rtx base = SET_DEST (writeback_effect);
+
+  poly_int64 wb_offset;
+  rtx base2 = strip_offset (SET_SRC (writeback_effect), &wb_offset);
+  gcc_checking_assert (rtx_equal_p (base, base2));
+
+  auto defs = trailing_add->defs ();
+  gcc_checking_assert (defs.size () == 1);
+  def_info *def = defs[0];
+
+  if (auto set = safe_dyn_cast<set_info *> (def->prev_def ()))
+    for (auto use : iterate_safely (set->debug_insn_uses ()))
+      if (*use->insn () > *pair_dst)
+	// DEF is getting re-ordered above USE, fix up USE accordingly.
+	fixup_debug_use (attempt, use, def, base, wb_offset);
+}
+
+// Called from fuse_pair, fixes up any debug uses that will be affected
+// by the changes.
+//
+// ATTEMPT is the obstack watermark used to allocate RTL-SSA temporaries for
+// the changes, INSNS gives the candidate insns: at this point the use/def
+// information should still be as on entry to fuse_pair, but the patterns may
+// have changed, hence we pass ORIG_RTL which contains the original patterns
+// for the candidate insns.
+//
+// The final pair will be placed immediately after PAIR_DST, LOAD_P is true if
+// it is a load pair, bit I of WRITEBACK is set if INSNS[I] originally had
+// writeback, and WRITEBACK_EFFECT is an rtx describing the overall update to
+// the base register in the final pair (if any).  BASE_REGNO gives the register
+// number of the base register used in the final pair.
+static void
+fixup_debug_uses (obstack_watermark &attempt,
+		  insn_info *insns[2],
+		  rtx orig_rtl[2],
+		  insn_info *pair_dst,
+		  insn_info *trailing_add,
+		  bool load_p,
+		  int writeback,
+		  rtx writeback_effect,
+		  unsigned base_regno)
+{
+  // USE is a debug use that needs updating because DEF (a def of the
+  // resource) is being re-ordered over it.  If WRITEBACK_PAT is non-NULL,
+  // then it gives the original RTL pattern for DEF's insn, and DEF is a
+  // writeback update of the base register.
+  //
+  // This simply unpacks WRITEBACK_PAT if needed and calls fixup_debug_use.
+  auto update_debug_use = [&](use_info *use, def_info *def,
+			      rtx writeback_pat)
+    {
+      poly_int64 offset = 0;
+      rtx base = NULL_RTX;
+      if (writeback_pat)
+	{
+	  rtx mem = XEXP (writeback_pat, load_p);
+	  gcc_checking_assert (GET_RTX_CLASS (GET_CODE (XEXP (mem, 0)))
+			       == RTX_AUTOINC);
+
+	  base = pair_mem_strip_offset (mem, &offset);
+	  gcc_checking_assert (REG_P (base) && REGNO (base) == base_regno);
+	}
+      fixup_debug_use (attempt, use, def, base, offset);
+    };
+
+  // Reset any debug uses of mem over which we re-ordered a store.
+  //
+  // It would be nice to try and preserve debug info here, but it seems that
+  // would require doing alias analysis to see if the store aliases with the
+  // debug use, which seems a little extravagant just to preserve debug info.
+  if (!load_p)
+    {
+      auto def = memory_access (insns[0]->defs ());
+      auto last_def = memory_access (insns[1]->defs ());
+      for (; def != last_def; def = def->next_def ())
+	{
+	  auto set = as_a<set_info *> (def);
+	  for (auto use : iterate_safely (set->debug_insn_uses ()))
+	    {
+	      if (dump_file)
+		fprintf (dump_file, "  i%d: resetting debug use of mem\n",
+			 use->insn ()->uid ());
+	      reset_debug_use (use);
+	    }
+	}
+    }
+
+  // Now let's take care of register uses, starting with debug uses
+  // attached to defs from our first insn.
+  for (auto def : insns[0]->defs ())
+    {
+      auto set = dyn_cast<set_info *> (def);
+      if (!set || set->is_mem () || !set->first_debug_insn_use ())
+	continue;
+
+      def_info *defs[2] = {
+	def,
+	find_access (insns[1]->defs (), def->regno ())
+      };
+
+      rtx writeback_pats[2] = {};
+      if (def->regno () == base_regno)
+	for (int i = 0; i < 2; i++)
+	  if (writeback & (1 << i))
+	    {
+	      gcc_checking_assert (defs[i]);
+	      writeback_pats[i] = orig_rtl[i];
+	    }
+
+      // Now that we've characterized the defs involved, go through the
+      // debug uses and determine how to update them (if needed).
+      for (auto use : iterate_safely (set->debug_insn_uses ()))
+	{
+	  if (*pair_dst < *use->insn () && defs[1])
+	    // We're re-ordering defs[1] above a previous use of the
+	    // same resource.
+	    update_debug_use (use, defs[1], writeback_pats[1]);
+	  else if (*pair_dst >= *use->insn ())
+	    // We're re-ordering defs[0] below its use.
+	    update_debug_use (use, defs[0], writeback_pats[0]);
+	}
+    }
+
+  // Now let's look at registers which are def'd by the second insn
+  // but not by the first insn, there may still be debug uses of a
+  // previous def which can be affected by moving the second insn up.
+  for (auto def : insns[1]->defs ())
+    {
+      // This should be M log N where N is the number of defs in
+      // insns[0] and M is the number of defs in insns[1].
+      if (def->is_mem () || find_access (insns[0]->defs (), def->regno ()))
+	  continue;
+
+      auto prev_set = safe_dyn_cast<set_info *> (def->prev_def ());
+      if (!prev_set)
+	continue;
+
+      rtx writeback_pat = NULL_RTX;
+      if (def->regno () == base_regno && (writeback & 2))
+	writeback_pat = orig_rtl[1];
+
+      // We have a def in insns[1] which isn't def'd by the first insn.
+      // Look to the previous def and see if it has any debug uses.
+      for (auto use : iterate_safely (prev_set->debug_insn_uses ()))
+	if (*pair_dst < *use->insn ())
+	  // We're ordering DEF above a previous use of the same register.
+	  update_debug_use (use, def, writeback_pat);
+    }
+
+  if ((writeback & 2) && !writeback_effect)
+    {
+      // If the second insn initially had writeback but the final
+      // pair does not, then there may be trailing debug uses of the
+      // second writeback def which need re-parenting: do that.
+      auto def = find_access (insns[1]->defs (), base_regno);
+      gcc_assert (def);
+      auto set = as_a<set_info *> (def);
+      for (auto use : iterate_safely (set->debug_insn_uses ()))
+	{
+	  insn_change change (use->insn ());
+	  change.new_uses = check_remove_regno_access (attempt,
+						       change.new_uses,
+						       base_regno);
+	  auto new_use = find_access (insns[0]->uses (), base_regno);
+
+	  // N.B. insns must have already shared a common base due to writeback.
+	  gcc_assert (new_use);
+
+	  if (dump_file)
+	    fprintf (dump_file,
+		     "  i%d: cancelling wb, re-parenting trailing debug use\n",
+		     use->insn ()->uid ());
+
+	  change.new_uses = insert_access (attempt, new_use, change.new_uses);
+	  crtl->ssa->change_insn (change);
+	}
+    }
+  else if (trailing_add)
+    fixup_debug_uses_trailing_add (attempt, pair_dst, trailing_add,
+				   writeback_effect);
+}
+
+// Try and actually fuse the pair given by insns I1 and I2.
+//
+// Here we've done enough analysis to know this is safe, we only
+// reject the pair at this stage if either the tuning policy says to,
+// or recog fails on the final pair insn.
+//
+// LOAD_P is true for loads, ACCESS_SIZE gives the access size of each
+// candidate insn.  Bit i of WRITEBACK is set if the ith insn (in program
+// order) uses writeback.
+//
+// BASE gives the chosen base candidate for the pair and MOVE_RANGE is
+// a singleton range which says where to place the pair.
+bool
+pair_fusion_bb_info::fuse_pair (bool load_p,
+				unsigned access_size,
+				int writeback,
+				insn_info *i1, insn_info *i2,
+				base_cand &base,
+				const insn_range_info &move_range)
+{
+  auto attempt = crtl->ssa->new_change_attempt ();
+
+  auto make_change = [&attempt](insn_info *insn)
+    {
+      return crtl->ssa->change_alloc<insn_change> (attempt, insn);
+    };
+  auto make_delete = [&attempt](insn_info *insn)
+    {
+      return crtl->ssa->change_alloc<insn_change> (attempt,
+						   insn,
+						   insn_change::DELETE);
+    };
+
+  insn_info *first = (*i1 < *i2) ? i1 : i2;
+  insn_info *second = (first == i1) ? i2 : i1;
+
+  insn_info *pair_dst = move_range.singleton ();
+  gcc_assert (pair_dst);
+
+  insn_info *insns[2] = { first, second };
+
+  auto_vec<insn_change *> changes;
+  auto_vec<int, 2> tombstone_uids (2);
+
+  rtx pats[2] = {
+    PATTERN (first->rtl ()),
+    PATTERN (second->rtl ())
+  };
+
+  // Make copies of the patterns as we might need to refer to the original RTL
+  // later, for example when updating debug uses (which is after we've updated
+  // one or both of the patterns in the candidate insns).
+  rtx orig_rtl[2];
+  for (int i = 0; i < 2; i++)
+    orig_rtl[i] = copy_rtx (pats[i]);
+
+  use_array input_uses[2] = { first->uses (), second->uses () };
+  def_array input_defs[2] = { first->defs (), second->defs () };
+
+  int changed_insn = -1;
+  if (base.from_insn != -1)
+    {
+      // If we're not already using a shared base, we need
+      // to re-write one of the accesses to use the base from
+      // the other insn.
+      gcc_checking_assert (base.from_insn == 0 || base.from_insn == 1);
+      changed_insn = !base.from_insn;
+
+      rtx base_pat = pats[base.from_insn];
+      rtx change_pat = pats[changed_insn];
+      rtx base_mem = XEXP (base_pat, load_p);
+      rtx change_mem = XEXP (change_pat, load_p);
+
+      const bool lower_base_p = (insns[base.from_insn] == i1);
+      HOST_WIDE_INT adjust_amt = access_size;
+      if (!lower_base_p)
+	adjust_amt *= -1;
+
+      rtx change_reg = XEXP (change_pat, !load_p);
+      rtx effective_base = drop_writeback (base_mem);
+      rtx adjusted_addr = plus_constant (Pmode,
+					 XEXP (effective_base, 0),
+					 adjust_amt);
+      rtx new_mem = replace_equiv_address_nv (change_mem, adjusted_addr);
+      rtx new_set = load_p
+	? gen_rtx_SET (change_reg, new_mem)
+	: gen_rtx_SET (new_mem, change_reg);
+
+      pats[changed_insn] = new_set;
+
+      auto keep_use = [&](use_info *u)
+	{
+	  return refers_to_regno_p (u->regno (), u->regno () + 1,
+				    change_pat, &XEXP (change_pat, load_p));
+	};
+
+      // Drop any uses that only occur in the old address.
+      input_uses[changed_insn] = filter_accesses (attempt,
+						  input_uses[changed_insn],
+						  keep_use);
+    }
+
+  rtx writeback_effect = NULL_RTX;
+  if (writeback)
+    writeback_effect = extract_writebacks (load_p, pats, changed_insn);
+
+  const auto base_regno = base.def->regno ();
+
+  if (base.from_insn == -1 && (writeback & 1))
+    {
+      // If the first of the candidate insns had a writeback form, we'll need to
+      // drop the use of the updated base register from the second insn's uses.
+      //
+      // N.B. we needn't worry about the base register occurring as a store
+      // operand, as we checked that there was no non-address true dependence
+      // between the insns in try_fuse_pair.
+      gcc_checking_assert (find_access (input_uses[1], base_regno));
+      input_uses[1] = check_remove_regno_access (attempt,
+						 input_uses[1],
+						 base_regno);
+    }
+
+  // Go through and drop uses that only occur in register notes,
+  // as we won't be preserving those.
+  for (int i = 0; i < 2; i++)
+    {
+      auto rti = insns[i]->rtl ();
+      if (!REG_NOTES (rti))
+	continue;
+
+      input_uses[i] = remove_note_accesses (attempt, input_uses[i]);
+    }
+
+  // Edge case: if the first insn is a writeback load and the
+  // second insn is a non-writeback load which transfers into the base
+  // register, then we should drop the writeback altogether as the
+  // update of the base register from the second load should prevail.
+  //
+  // For example:
+  //   ldr x2, [x1], #8
+  //   ldr x1, [x1]
+  //   -->
+  //   ldp x2, x1, [x1]
+  if (writeback == 1
+      && load_p
+      && find_access (input_defs[1], base_regno))
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "  load pair: i%d has wb but subsequent i%d has non-wb "
+		 "update of base (r%d), dropping wb\n",
+		 insns[0]->uid (), insns[1]->uid (), base_regno);
+      gcc_assert (writeback_effect);
+      writeback_effect = NULL_RTX;
+    }
+
+  // So far the patterns have been in instruction order,
+  // now we want them in offset order.
+  if (i1 != first)
+    std::swap (pats[0], pats[1]);
+
+  poly_int64 offsets[2];
+  for (int i = 0; i < 2; i++)
+    {
+      rtx mem = XEXP (pats[i], load_p);
+      gcc_checking_assert (MEM_P (mem));
+      rtx base = strip_offset (XEXP (mem, 0), offsets + i);
+      gcc_checking_assert (REG_P (base));
+      gcc_checking_assert (base_regno == REGNO (base));
+    }
+
+  // If either of the original insns had writeback, but the resulting pair insn
+  // does not (can happen e.g. in the load pair edge case above, or if the
+  // writeback effects cancel out), then drop the def (s) of the base register
+  // as appropriate.
+  //
+  // Also drop the first def in the case that both of the original insns had
+  // writeback.  The second def could well have uses, but the first def should
+  // only be used by the second insn (and we dropped that use above).
+  for (int i = 0; i < 2; i++)
+    if ((!writeback_effect && (writeback & (1 << i)))
+	|| (i == 0 && writeback == 3))
+      input_defs[i] = check_remove_regno_access (attempt,
+						 input_defs[i],
+						 base_regno);
+
+  // If we don't currently have a writeback pair, and we don't have
+  // a load that clobbers the base register, look for a trailing destructive
+  // update of the base register and try and fold it in to make this into a
+  // writeback pair.
+  insn_info *trailing_add = nullptr;
+  if (m_pass->should_handle_writeback (writeback::ALL)
+      && !writeback_effect
+      && (!load_p || (!refers_to_regno_p (base_regno, base_regno + 1,
+					 XEXP (pats[0], 0), nullptr)
+		      && !refers_to_regno_p (base_regno, base_regno + 1,
+					     XEXP (pats[1], 0), nullptr))))
+    {
+      def_info *add_def;
+      trailing_add = m_pass->find_trailing_add (insns, move_range, writeback,
+						&writeback_effect,
+						&add_def, base.def, offsets[0],
+						access_size);
+      if (trailing_add)
+	{
+	  // The def of the base register from the trailing add should prevail.
+	  input_defs[0] = insert_access (attempt, add_def, input_defs[0]);
+	  gcc_assert (input_defs[0].is_valid ());
+	}
+    }
+
+  // Now that we know what base mem we're going to use, check if it's OK
+  // with the pair mem policy.
+  rtx first_mem = XEXP (pats[0], load_p);
+  if (!m_pass->pair_mem_ok_with_policy (first_mem, load_p))
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "punting on pair (%d,%d), pair mem policy says no\n",
+		 i1->uid (), i2->uid ());
+      return false;
+    }
+
+  rtx reg_notes = combine_reg_notes (first, second, load_p);
+
+  rtx pair_pat = m_pass->gen_pair (pats, writeback_effect, load_p);
+  insn_change *pair_change = nullptr;
+  auto set_pair_pat = [pair_pat,reg_notes](insn_change *change) {
+    rtx_insn *rti = change->insn ()->rtl ();
+    validate_unshare_change (rti, &PATTERN (rti), pair_pat, true);
+    validate_change (rti, &REG_NOTES (rti), reg_notes, true);
+  };
+
+  if (load_p)
+    {
+      changes.safe_push (make_delete (first));
+      pair_change = make_change (second);
+      changes.safe_push (pair_change);
+
+      pair_change->move_range = move_range;
+      pair_change->new_defs = merge_access_arrays (attempt,
+						   input_defs[0],
+						   input_defs[1]);
+      gcc_assert (pair_change->new_defs.is_valid ());
+
+      pair_change->new_uses
+	= merge_access_arrays (attempt,
+			       drop_memory_access (input_uses[0]),
+			       drop_memory_access (input_uses[1]));
+      gcc_assert (pair_change->new_uses.is_valid ());
+      set_pair_pat (pair_change);
+    }
+  else
+    {
+      using Action = store_change_builder::action;
+      insn_info *store_to_change = try_repurpose_store (first, second,
+							move_range);
+      store_change_builder builder (insns, store_to_change, pair_dst);
+      insn_change *change;
+      set_info *new_set = nullptr;
+      for (; !builder.done (); builder.advance ())
+	{
+	  auto action = builder.get_change ();
+	  change = (action.type == Action::INSERT)
+	    ? nullptr : make_change (action.insn);
+	  switch (action.type)
+	    {
+	    case Action::CHANGE:
+	    {
+	      set_pair_pat (change);
+	      change->new_uses = merge_access_arrays (attempt,
+						      input_uses[0],
+						      input_uses[1]);
+	      auto d1 = drop_memory_access (input_defs[0]);
+	      auto d2 = drop_memory_access (input_defs[1]);
+	      change->new_defs = merge_access_arrays (attempt, d1, d2);
+	      gcc_assert (change->new_defs.is_valid ());
+	      def_info *store_def = memory_access (change->insn ()->defs ());
+	      change->new_defs = insert_access (attempt,
+						store_def,
+						change->new_defs);
+	      gcc_assert (change->new_defs.is_valid ());
+	      change->move_range = move_range;
+	      pair_change = change;
+	      break;
+	    }
+	    case Action::TOMBSTONE:
+	    {
+	      tombstone_uids.quick_push (change->insn ()->uid ());
+	      rtx_insn *rti = change->insn ()->rtl ();
+	      validate_change (rti, &PATTERN (rti), gen_tombstone (), true);
+	      validate_change (rti, &REG_NOTES (rti), NULL_RTX, true);
+	      change->new_uses = use_array (nullptr, 0);
+	      break;
+	    }
+	    case Action::INSERT:
+	    {
+	      if (dump_file)
+		fprintf (dump_file,
+			 "  stp: cannot re-purpose candidate stores\n");
+
+	      auto new_insn = crtl->ssa->create_insn (attempt, INSN, pair_pat);
+	      change = make_change (new_insn);
+	      change->move_range = move_range;
+	      change->new_uses = merge_access_arrays (attempt,
+						      input_uses[0],
+						      input_uses[1]);
+	      gcc_assert (change->new_uses.is_valid ());
+
+	      auto d1 = drop_memory_access (input_defs[0]);
+	      auto d2 = drop_memory_access (input_defs[1]);
+	      change->new_defs = merge_access_arrays (attempt, d1, d2);
+	      gcc_assert (change->new_defs.is_valid ());
+
+	      new_set = crtl->ssa->create_set (attempt, new_insn, memory);
+	      change->new_defs = insert_access (attempt, new_set,
+						change->new_defs);
+	      gcc_assert (change->new_defs.is_valid ());
+	      pair_change = change;
+	      break;
+	    }
+	    case Action::FIXUP_USE:
+	    {
+	      // This use now needs to consume memory from our stp.
+	      if (dump_file)
+		fprintf (dump_file,
+			 "  stp: changing i%d to use mem from new stp "
+			 "(after i%d)\n",
+			 action.insn->uid (), pair_dst->uid ());
+	      change->new_uses = drop_memory_access (change->new_uses);
+	      gcc_assert (new_set);
+	      auto new_use = crtl->ssa->create_use (attempt, action.insn,
+						    new_set);
+	      change->new_uses = insert_access (attempt, new_use,
+						change->new_uses);
+	      break;
+	    }
+	    }
+	  changes.safe_push (change);
+	}
+    }
+
+  if (trailing_add)
+    changes.safe_push (make_delete (trailing_add));
+  else if ((writeback & 2) && !writeback_effect)
+    {
+      // The second insn initially had writeback but now the pair does not,
+      // need to update any nondebug uses of the base register def in the
+      // second insn.  We'll take care of debug uses later.
+      auto def = find_access (insns[1]->defs (), base_regno);
+      gcc_assert (def);
+      auto set = dyn_cast<set_info *> (def);
+      if (set && set->has_nondebug_uses ())
+	{
+	  auto orig_use = find_access (insns[0]->uses (), base_regno);
+	  for (auto use : set->nondebug_insn_uses ())
+	    {
+	      auto change = make_change (use->insn ());
+	      change->new_uses = check_remove_regno_access (attempt,
+							    change->new_uses,
+							    base_regno);
+	      change->new_uses = insert_access (attempt,
+						orig_use,
+						change->new_uses);
+	      changes.safe_push (change);
+	    }
+	}
+    }
+
+  auto is_changing = insn_is_changing (changes);
+  for (unsigned i = 0; i < changes.length (); i++)
+    gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
+
+  // Check the pair pattern is recog'd.
+  if (!rtl_ssa::recog_ignoring (attempt, *pair_change, is_changing))
+    {
+      if (dump_file)
+	fprintf (dump_file, "  failed to form pair, recog failed\n");
+
+      // Free any reg notes we allocated.
+      while (reg_notes)
+	{
+	  rtx next = XEXP (reg_notes, 1);
+	  free_EXPR_LIST_node (reg_notes);
+	  reg_notes = next;
+	}
+      cancel_changes (0);
+      return false;
+    }
+
+  gcc_assert (crtl->ssa->verify_insn_changes (changes));
+
+  // Fix up any debug uses that will be affected by the changes.
+  if (MAY_HAVE_DEBUG_INSNS)
+    fixup_debug_uses (attempt, insns, orig_rtl, pair_dst, trailing_add,
+		      load_p, writeback, writeback_effect, base_regno);
+
+  confirm_change_group ();
+  crtl->ssa->change_insns (changes);
+
+  gcc_checking_assert (tombstone_uids.length () <= 2);
+  for (auto uid : tombstone_uids)
+    track_tombstone (uid);
+
+  return true;
+}
+
+// Return true if STORE_INSN may modify mem rtx MEM.  Make sure we keep
+// within our BUDGET for alias analysis.
+static bool
+store_modifies_mem_p (rtx mem, insn_info *store_insn, int &budget)
+{
+  if (!budget)
+    {
+      if (dump_file)
+	{
+	  fprintf (dump_file,
+		   "exceeded budget, assuming store %d aliases with mem ",
+		   store_insn->uid ());
+	  print_simple_rtl (dump_file, mem);
+	  fprintf (dump_file, "\n");
+	}
+
+      return true;
+    }
+
+  budget--;
+  return memory_modified_in_insn_p (mem, store_insn->rtl ());
+}
+
+// Return true if LOAD may be modified by STORE.  Make sure we keep
+// within our BUDGET for alias analysis.
+static bool
+load_modified_by_store_p (insn_info *load,
+			  insn_info *store,
+			  int &budget)
+{
+  gcc_checking_assert (budget >= 0);
+
+  if (!budget)
+    {
+      if (dump_file)
+	{
+	  fprintf (dump_file,
+		   "exceeded budget, assuming load %d aliases with store %d\n",
+		   load->uid (), store->uid ());
+	}
+      return true;
+    }
+
+  // It isn't safe to re-order stores over calls.
+  if (CALL_P (load->rtl ()))
+    return true;
+
+  budget--;
+
+  // Iterate over all MEMs in the load, seeing if any alias with
+  // our store.
+  subrtx_var_iterator::array_type array;
+  rtx pat = PATTERN (load->rtl ());
+  FOR_EACH_SUBRTX_VAR (iter, array, pat, NONCONST)
+    if (MEM_P (*iter) && memory_modified_in_insn_p (*iter, store->rtl ()))
+      return true;
+
+  return false;
+}
+
+// Implement some common functionality used by both store_walker
+// and load_walker.
+template<bool reverse>
+class def_walker : public alias_walker
+{
+protected:
+  using def_iter_t = typename std::conditional<reverse,
+	reverse_def_iterator, def_iterator>::type;
+
+  static use_info *start_use_chain (def_iter_t &def_iter)
+  {
+    set_info *set = nullptr;
+    for (; *def_iter; def_iter++)
+      {
+	set = dyn_cast<set_info *> (*def_iter);
+	if (!set)
+	  continue;
+
+	use_info *use = reverse
+	  ? set->last_nondebug_insn_use ()
+	  : set->first_nondebug_insn_use ();
+
+	if (use)
+	  return use;
+      }
+
+    return nullptr;
+  }
+
+  def_iter_t def_iter;
+  insn_info *limit;
+  def_walker (def_info *def, insn_info *limit) :
+    def_iter (def), limit (limit) {}
+
+  virtual bool iter_valid () const { return *def_iter; }
+
+public:
+  insn_info *insn () const override { return (*def_iter)->insn (); }
+  void advance () override { def_iter++; }
+  bool valid () const override final
+  {
+    if (!iter_valid ())
+      return false;
+
+    if (reverse)
+      return *(insn ()) > *limit;
+    else
+      return *(insn ()) < *limit;
+  }
+};
+
+// alias_walker that iterates over stores.
+template<bool reverse, typename InsnPredicate>
+class store_walker : public def_walker<reverse>
+{
+  rtx cand_mem;
+  InsnPredicate tombstone_p;
+
+public:
+  store_walker (def_info *mem_def, rtx mem, insn_info *limit_insn,
+		InsnPredicate tombstone_fn) :
+    def_walker<reverse> (mem_def, limit_insn),
+    cand_mem (mem), tombstone_p (tombstone_fn) {}
+
+  bool conflict_p (int &budget) const override final
+  {
+    if (tombstone_p (this->insn ()))
+      return false;
+
+    return store_modifies_mem_p (cand_mem, this->insn (), budget);
+  }
+};
+
+// alias_walker that iterates over loads.
+template<bool reverse>
+class load_walker : public def_walker<reverse>
+{
+  using Base = def_walker<reverse>;
+  using use_iter_t = typename std::conditional<reverse,
+	reverse_use_iterator, nondebug_insn_use_iterator>::type;
+
+  use_iter_t use_iter;
+  insn_info *cand_store;
+
+  bool iter_valid () const override final { return *use_iter; }
+
+public:
+  void advance () override final
+  {
+    use_iter++;
+    if (*use_iter)
+      return;
+    this->def_iter++;
+    use_iter = Base::start_use_chain (this->def_iter);
+  }
+
+  insn_info *insn () const override final
+  {
+    return (*use_iter)->insn ();
+  }
+
+  bool conflict_p (int &budget) const override final
+  {
+    return load_modified_by_store_p (insn (), cand_store, budget);
+  }
+
+  load_walker (def_info *def, insn_info *store, insn_info *limit_insn)
+    : Base (def, limit_insn),
+      use_iter (Base::start_use_chain (this->def_iter)),
+      cand_store (store) {}
+};
+
+// Process our alias_walkers in a round-robin fashion, proceeding until
+// nothing more can be learned from alias analysis.
+//
+// We try to maintain the invariant that if a walker becomes invalid, we
+// set its pointer to null.
+void
+pair_fusion::do_alias_analysis (insn_info *alias_hazards[4],
+				alias_walker *walkers[4],
+				bool load_p)
+{
+  const int n_walkers = 2 + (2 * !load_p);
+  int budget = pair_mem_alias_check_limit ();
+
+  auto next_walker = [walkers,n_walkers](int current) -> int {
+    for (int j = 1; j <= n_walkers; j++)
+      {
+	int idx = (current + j) % n_walkers;
+	if (walkers[idx])
+	  return idx;
+      }
+    return -1;
+  };
+
+  int i = -1;
+  for (int j = 0; j < n_walkers; j++)
+    {
+      alias_hazards[j] = nullptr;
+      if (!walkers[j])
+	continue;
+
+      if (!walkers[j]->valid ())
+	walkers[j] = nullptr;
+      else if (i == -1)
+	i = j;
+    }
+
+  while (i >= 0)
+    {
+      int insn_i = i % 2;
+      int paired_i = (i & 2) + !insn_i;
+      int pair_fst = (i & 2);
+      int pair_snd = (i & 2) + 1;
+
+      if (walkers[i]->conflict_p (budget))
+	{
+	  alias_hazards[i] = walkers[i]->insn ();
+
+	  // We got an aliasing conflict for this {load,store} walker,
+	  // so we don't need to walk any further.
+	  walkers[i] = nullptr;
+
+	  // If we have a pair of alias conflicts that prevent
+	  // forming the pair, stop.  There's no need to do further
+	  // analysis.
+	  if (alias_hazards[paired_i]
+	      && (*alias_hazards[pair_fst] <= *alias_hazards[pair_snd]))
+	    return;
+
+	  if (!load_p)
+	    {
+	      int other_pair_fst = (pair_fst ? 0 : 2);
+	      int other_paired_i = other_pair_fst + !insn_i;
+
+	      int x_pair_fst = (i == pair_fst) ? i : other_paired_i;
+	      int x_pair_snd = (i == pair_fst) ? other_paired_i : i;
+
+	      // Similarly, handle the case where we have a {load,store}
+	      // or {store,load} alias hazard pair that prevents forming
+	      // the pair.
+	      if (alias_hazards[other_paired_i]
+		  && *alias_hazards[x_pair_fst] <= *alias_hazards[x_pair_snd])
+		return;
+	    }
+	}
+
+      if (walkers[i])
+	{
+	  walkers[i]->advance ();
+
+	  if (!walkers[i]->valid ())
+	    walkers[i] = nullptr;
+	}
+
+      i = next_walker (i);
+    }
+}
+
+// Given INSNS (in program order) which are known to be adjacent, look
+// to see if either insn has a suitable RTL (register) base that we can
+// use to form a pair.  Push these to BASE_CANDS if we find any.  CAND_MEMs
+// gives the relevant mems from the candidate insns, ACCESS_SIZE gives the
+// size of a single candidate access, and REVERSED says whether the accesses
+// are inverted in offset order.
+//
+// Returns an integer where bit (1 << i) is set if INSNS[i] uses writeback
+// addressing.
+int
+pair_fusion::get_viable_bases (insn_info *insns[2],
+			       vec<base_cand> &base_cands,
+			       rtx cand_mems[2],
+			       unsigned access_size,
+			       bool reversed)
+{
+  // We discovered this pair through a common base.  Need to ensure that
+  // we have a common base register that is live at both locations.
+  def_info *base_defs[2] = {};
+  int writeback = 0;
+  for (int i = 0; i < 2; i++)
+    {
+      const bool is_lower = (i == reversed);
+      poly_int64 poly_off;
+      rtx base = pair_mem_strip_offset (cand_mems[i], &poly_off);
+      if (GET_RTX_CLASS (GET_CODE (XEXP (cand_mems[i], 0))) == RTX_AUTOINC)
+	writeback |= (1 << i);
+
+      if (!REG_P (base) || !poly_off.is_constant ())
+	continue;
+
+      // Punt on accesses relative to eliminable regs.  See the comment in
+      // pair_fusion_bb_info::track_access for a detailed explanation of this.
+      if (!reload_completed
+	  && (REGNO (base) == FRAME_POINTER_REGNUM
+	      || REGNO (base) == ARG_POINTER_REGNUM))
+	continue;
+
+      HOST_WIDE_INT base_off = poly_off.to_constant ();
+
+      // It should be unlikely that we ever punt here, since MEM_EXPR offset
+      // alignment should be a good proxy for register offset alignment.
+      if (base_off % access_size != 0)
+	{
+	  if (dump_file)
+	    fprintf (dump_file,
+		     "base not viable, offset misaligned (insn %d)\n",
+		     insns[i]->uid ());
+	  continue;
+	}
+
+      base_off /= access_size;
+
+      if (!is_lower)
+	base_off--;
+
+      if (!pair_mem_in_range_p (base_off))
+	continue;
+
+      use_info *use = find_access (insns[i]->uses (), REGNO (base));
+      gcc_assert (use);
+      base_defs[i] = use->def ();
+    }
+
+  if (!base_defs[0] && !base_defs[1])
+    {
+      if (dump_file)
+	fprintf (dump_file, "no viable base register for pair (%d,%d)\n",
+		 insns[0]->uid (), insns[1]->uid ());
+      return writeback;
+    }
+
+  for (int i = 0; i < 2; i++)
+    if ((writeback & (1 << i)) && !base_defs[i])
+      {
+	if (dump_file)
+	  fprintf (dump_file, "insn %d has writeback but base isn't viable\n",
+		   insns[i]->uid ());
+	return writeback;
+      }
+
+  if (writeback == 3
+      && base_defs[0]->regno () != base_defs[1]->regno ())
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "pair (%d,%d): double writeback with distinct regs (%d,%d): "
+		 "punting\n",
+		 insns[0]->uid (), insns[1]->uid (),
+		 base_defs[0]->regno (), base_defs[1]->regno ());
+      return writeback;
+    }
+
+  if (base_defs[0] && base_defs[1]
+      && base_defs[0]->regno () == base_defs[1]->regno ())
+    {
+      // Easy case: insns already share the same base reg.
+      base_cands.quick_push (base_defs[0]);
+      return writeback;
+    }
+
+  // Otherwise, we know that one of the bases must change.
+  //
+  // Note that if there is writeback we must use the writeback base
+  // (we know now there is exactly one).
+  for (int i = 0; i < 2; i++)
+    if (base_defs[i] && (!writeback || (writeback & (1 << i))))
+      base_cands.quick_push (base_cand { base_defs[i], i });
+
+  return writeback;
+}
+
+// Given two adjacent memory accesses of the same size, I1 and I2, try
+// and see if we can merge them into a paired access.
+//
+// ACCESS_SIZE gives the (common) size of a single access, LOAD_P is true
+// if the accesses are both loads, otherwise they are both stores.
+bool
+pair_fusion_bb_info::try_fuse_pair (bool load_p, unsigned access_size,
+				    insn_info *i1, insn_info *i2)
+{
+  if (dump_file)
+    fprintf (dump_file, "analyzing pair (load=%d): (%d,%d)\n",
+	     load_p, i1->uid (), i2->uid ());
+
+  insn_info *insns[2];
+  bool reversed = false;
+  if (*i1 < *i2)
+    {
+      insns[0] = i1;
+      insns[1] = i2;
+    }
+  else
+    {
+      insns[0] = i2;
+      insns[1] = i1;
+      reversed = true;
+    }
+
+  rtx cand_mems[2];
+  rtx reg_ops[2];
+  rtx pats[2];
+  for (int i = 0; i < 2; i++)
+    {
+      pats[i] = PATTERN (insns[i]->rtl ());
+      cand_mems[i] = XEXP (pats[i], load_p);
+      reg_ops[i] = XEXP (pats[i], !load_p);
+    }
+
+  if (load_p && reg_overlap_mentioned_p (reg_ops[0], reg_ops[1]))
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "punting on load pair due to reg conflcits (%d,%d)\n",
+		 insns[0]->uid (), insns[1]->uid ());
+      return false;
+    }
+
+  if (cfun->can_throw_non_call_exceptions
+      && find_reg_note (insns[0]->rtl (), REG_EH_REGION, NULL_RTX)
+      && find_reg_note (insns[1]->rtl (), REG_EH_REGION, NULL_RTX))
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "can't combine insns with EH side effects (%d,%d)\n",
+		 insns[0]->uid (), insns[1]->uid ());
+      return false;
+    }
+
+  auto_vec<base_cand, 2> base_cands (2);
+
+  int writeback = m_pass->get_viable_bases (insns, base_cands, cand_mems,
+					    access_size, reversed);
+  if (base_cands.is_empty ())
+    {
+      if (dump_file)
+	fprintf (dump_file, "no viable base for pair (%d,%d)\n",
+		 insns[0]->uid (), insns[1]->uid ());
+      return false;
+    }
+
+  // Punt on frame-related insns with writeback.  We probably won't see
+  // these in practice, but this is conservative and ensures we don't
+  // have to worry about these later on.
+  if (writeback && (RTX_FRAME_RELATED_P (i1->rtl ())
+		    || RTX_FRAME_RELATED_P (i2->rtl ())))
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "rejecting pair (%d,%d): frame-related insn with writeback\n",
+		 i1->uid (), i2->uid ());
+      return false;
+    }
+
+  rtx *ignore = &XEXP (pats[1], load_p);
+  for (auto use : insns[1]->uses ())
+    if (!use->is_mem ()
+	&& refers_to_regno_p (use->regno (), use->regno () + 1, pats[1], ignore)
+	&& use->def () && use->def ()->insn () == insns[0])
+      {
+	// N.B. we allow a true dependence on the base address, as this
+	// happens in the case of auto-inc accesses.  Consider a post-increment
+	// load followed by a regular indexed load, for example.
+	if (dump_file)
+	  fprintf (dump_file,
+		   "%d has non-address true dependence on %d, rejecting pair\n",
+		   insns[1]->uid (), insns[0]->uid ());
+	return false;
+      }
+
+  unsigned i = 0;
+  while (i < base_cands.length ())
+    {
+      base_cand &cand = base_cands[i];
+
+      rtx *ignore[2] = {};
+      for (int j = 0; j < 2; j++)
+	if (cand.from_insn == !j)
+	  ignore[j] = &XEXP (cand_mems[j], 0);
+
+      insn_info *h = first_hazard_after (insns[0], ignore[0]);
+      if (h && *h < *insns[1])
+	cand.hazards[0] = h;
+
+      h = latest_hazard_before (insns[1], ignore[1]);
+      if (h && *h > *insns[0])
+	cand.hazards[1] = h;
+
+      if (!cand.viable ())
+	{
+	  if (dump_file)
+	    fprintf (dump_file,
+		     "pair (%d,%d): rejecting base %d due to dataflow "
+		     "hazards (%d,%d)\n",
+		     insns[0]->uid (),
+		     insns[1]->uid (),
+		     cand.def->regno (),
+		     cand.hazards[0]->uid (),
+		     cand.hazards[1]->uid ());
+
+	  base_cands.ordered_remove (i);
+	}
+      else
+	i++;
+    }
+
+  if (base_cands.is_empty ())
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "can't form pair (%d,%d) due to dataflow hazards\n",
+		 insns[0]->uid (), insns[1]->uid ());
+      return false;
+    }
+
+  insn_info *alias_hazards[4] = {};
+
+  // First def of memory after the first insn, and last def of memory
+  // before the second insn, respectively.
+  def_info *mem_defs[2] = {};
+  if (load_p)
+    {
+      if (!MEM_READONLY_P (cand_mems[0]))
+	{
+	  mem_defs[0] = memory_access (insns[0]->uses ())->def ();
+	  gcc_checking_assert (mem_defs[0]);
+	  mem_defs[0] = mem_defs[0]->next_def ();
+	}
+      if (!MEM_READONLY_P (cand_mems[1]))
+	{
+	  mem_defs[1] = memory_access (insns[1]->uses ())->def ();
+	  gcc_checking_assert (mem_defs[1]);
+	}
+    }
+  else
+    {
+      mem_defs[0] = memory_access (insns[0]->defs ())->next_def ();
+      mem_defs[1] = memory_access (insns[1]->defs ())->prev_def ();
+      gcc_checking_assert (mem_defs[0]);
+      gcc_checking_assert (mem_defs[1]);
+    }
+
+  auto tombstone_p = [&](insn_info *insn) -> bool {
+    return m_emitted_tombstone
+	   && bitmap_bit_p (&m_tombstone_bitmap, insn->uid ());
+  };
+
+  store_walker<false, decltype(tombstone_p)>
+    forward_store_walker (mem_defs[0], cand_mems[0], insns[1], tombstone_p);
+
+  store_walker<true, decltype(tombstone_p)>
+    backward_store_walker (mem_defs[1], cand_mems[1], insns[0], tombstone_p);
+
+  alias_walker *walkers[4] = {};
+  if (mem_defs[0])
+    walkers[0] = &forward_store_walker;
+  if (mem_defs[1])
+    walkers[1] = &backward_store_walker;
+
+  if (load_p && (mem_defs[0] || mem_defs[1]))
+    m_pass->do_alias_analysis (alias_hazards, walkers, load_p);
+  else
+    {
+      // We want to find any loads hanging off the first store.
+      mem_defs[0] = memory_access (insns[0]->defs ());
+      load_walker<false> forward_load_walker (mem_defs[0], insns[0], insns[1]);
+      load_walker<true> backward_load_walker (mem_defs[1], insns[1], insns[0]);
+      walkers[2] = &forward_load_walker;
+      walkers[3] = &backward_load_walker;
+      m_pass->do_alias_analysis (alias_hazards, walkers, load_p);
+      // Now consolidate hazards back down.
+      if (alias_hazards[2]
+	  && (!alias_hazards[0] || (*alias_hazards[2] < *alias_hazards[0])))
+	alias_hazards[0] = alias_hazards[2];
+
+      if (alias_hazards[3]
+	  && (!alias_hazards[1] || (*alias_hazards[3] > *alias_hazards[1])))
+	alias_hazards[1] = alias_hazards[3];
+    }
+
+  if (alias_hazards[0] && alias_hazards[1]
+      && *alias_hazards[0] <= *alias_hazards[1])
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "cannot form pair (%d,%d) due to alias conflicts (%d,%d)\n",
+		 i1->uid (), i2->uid (),
+		 alias_hazards[0]->uid (), alias_hazards[1]->uid ());
+      return false;
+    }
+
+  // Now narrow the hazards on each base candidate using
+  // the alias hazards.
+  i = 0;
+  while (i < base_cands.length ())
+    {
+      base_cand &cand = base_cands[i];
+      if (alias_hazards[0] && (!cand.hazards[0]
+			       || *alias_hazards[0] < *cand.hazards[0]))
+	cand.hazards[0] = alias_hazards[0];
+      if (alias_hazards[1] && (!cand.hazards[1]
+			       || *alias_hazards[1] > *cand.hazards[1]))
+	cand.hazards[1] = alias_hazards[1];
+
+      if (cand.viable ())
+	i++;
+      else
+	{
+	  if (dump_file)
+	    fprintf (dump_file, "pair (%d,%d): rejecting base %d due to "
+				"alias/dataflow hazards (%d,%d)",
+				insns[0]->uid (), insns[1]->uid (),
+				cand.def->regno (),
+				cand.hazards[0]->uid (),
+				cand.hazards[1]->uid ());
+
+	  base_cands.ordered_remove (i);
+	}
+    }
+
+  if (base_cands.is_empty ())
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "cannot form pair (%d,%d) due to alias/dataflow hazards",
+		 insns[0]->uid (), insns[1]->uid ());
+
+      return false;
+    }
+
+  base_cand *base = &base_cands[0];
+  if (base_cands.length () > 1)
+    {
+      // If there are still multiple viable bases, it makes sense
+      // to choose one that allows us to reduce register pressure,
+      // for loads this means moving further down, for stores this
+      // means moving further up.
+      gcc_checking_assert (base_cands.length () == 2);
+      const int hazard_i = !load_p;
+      if (base->hazards[hazard_i])
+	{
+	  if (!base_cands[1].hazards[hazard_i])
+	    base = &base_cands[1];
+	  else if (load_p
+		   && *base_cands[1].hazards[hazard_i]
+		      > *(base->hazards[hazard_i]))
+	    base = &base_cands[1];
+	  else if (!load_p
+		   && *base_cands[1].hazards[hazard_i]
+		      < *(base->hazards[hazard_i]))
+	    base = &base_cands[1];
+	}
+    }
+
+  // Otherwise, hazards[0] > hazards[1].
+  // Pair can be formed anywhere in (hazards[1], hazards[0]).
+  insn_range_info range (insns[0], insns[1]);
+  if (base->hazards[1])
+    range.first = base->hazards[1];
+  if (base->hazards[0])
+    range.last = base->hazards[0]->prev_nondebug_insn ();
+
+  // If the second insn can throw, narrow the move range to exactly that insn.
+  // This prevents us trying to move the second insn from the end of the BB.
+  if (cfun->can_throw_non_call_exceptions
+      && find_reg_note (insns[1]->rtl (), REG_EH_REGION, NULL_RTX))
+    {
+      gcc_assert (range.includes (insns[1]));
+      range = insn_range_info (insns[1]);
+    }
+
+  // Placement strategy: push loads down and pull stores up, this should
+  // help register pressure by reducing live ranges.
+  if (load_p)
+    range.first = range.last;
+  else
+    range.last = range.first;
+
+  if (dump_file)
+    {
+      auto print_hazard = [](insn_info *i)
+	{
+	  if (i)
+	    fprintf (dump_file, "%d", i->uid ());
+	  else
+	    fprintf (dump_file, "-");
+	};
+      auto print_pair = [print_hazard](insn_info **i)
+	{
+	  print_hazard (i[0]);
+	  fprintf (dump_file, ",");
+	  print_hazard (i[1]);
+	};
+
+      fprintf (dump_file, "fusing pair [L=%d] (%d,%d), base=%d, hazards: (",
+	      load_p, insns[0]->uid (), insns[1]->uid (),
+	      base->def->regno ());
+      print_pair (base->hazards);
+      fprintf (dump_file, "), move_range: (%d,%d)\n",
+	       range.first->uid (), range.last->uid ());
+    }
+
+  return fuse_pair (load_p, access_size, writeback,
+		    i1, i2, *base, range);
+}
+
+static void
+dump_insn_list (FILE *f, const insn_list_t &l)
+{
+  fprintf (f, "(");
+
+  auto i = l.begin ();
+  auto end = l.end ();
+
+  if (i != end)
+    fprintf (f, "%d", (*i)->uid ());
+  i++;
+
+  for (; i != end; i++)
+    fprintf (f, ", %d", (*i)->uid ());
+
+  fprintf (f, ")");
+}
+
+DEBUG_FUNCTION void
+debug (const insn_list_t &l)
+{
+  dump_insn_list (stderr, l);
+  fprintf (stderr, "\n");
+}
+
+// LEFT_LIST and RIGHT_LIST are lists of candidate instructions where all insns
+// in LEFT_LIST are known to be adjacent to those in RIGHT_LIST.
+//
+// This function traverses the resulting 2D matrix of possible pair candidates
+// and attempts to merge them into pairs.
+//
+// The algorithm is straightforward: if we consider a combined list of
+// candidates X obtained by merging LEFT_LIST and RIGHT_LIST in program order,
+// then we advance through X until we reach a crossing point (where X[i] and
+// X[i+1] come from different source lists).
+//
+// At this point we know X[i] and X[i+1] are adjacent accesses, and we try to
+// fuse them into a pair.  If this succeeds, we remove X[i] and X[i+1] from
+// their original lists and continue as above.
+//
+// In the failure case, we advance through the source list containing X[i] and
+// continue as above (proceeding to the next crossing point).
+//
+// The rationale for skipping over groups of consecutive candidates from the
+// same source list is as follows:
+//
+// In the store case, the insns in the group can't be re-ordered over each
+// other as they are guaranteed to store to the same location, so we're
+// guaranteed not to lose opportunities by doing this.
+//
+// In the load case, subsequent loads from the same location are either
+// redundant (in which case they should have been cleaned up by an earlier
+// optimization pass) or there is an intervening aliasing hazard, in which case
+// we can't re-order them anyway, so provided earlier passes have cleaned up
+// redundant loads, we shouldn't miss opportunities by doing this.
+void
+pair_fusion_bb_info::merge_pairs (insn_list_t &left_list,
+			  insn_list_t &right_list,
+			  bool load_p,
+			  unsigned access_size)
+{
+  if (dump_file)
+    {
+      fprintf (dump_file, "merge_pairs [L=%d], cand vecs ", load_p);
+      dump_insn_list (dump_file, left_list);
+      fprintf (dump_file, " x ");
+      dump_insn_list (dump_file, right_list);
+      fprintf (dump_file, "\n");
+    }
+
+  auto iter_l = left_list.begin ();
+  auto iter_r = right_list.begin ();
+
+  while (iter_l != left_list.end () && iter_r != right_list.end ())
+    {
+      auto next_l = std::next (iter_l);
+      auto next_r = std::next (iter_r);
+      if (**iter_l < **iter_r
+	  && next_l != left_list.end ()
+	  && **next_l < **iter_r)
+	iter_l = next_l;
+      else if (**iter_r < **iter_l
+	       && next_r != right_list.end ()
+	       && **next_r < **iter_l)
+	iter_r = next_r;
+      else if (try_fuse_pair (load_p, access_size, *iter_l, *iter_r))
+	{
+	  left_list.erase (iter_l);
+	  iter_l = next_l;
+	  right_list.erase (iter_r);
+	  iter_r = next_r;
+	}
+      else if (**iter_l < **iter_r)
+	iter_l = next_l;
+      else
+	iter_r = next_r;
+    }
+}
+
+// Iterate over the accesses in GROUP, looking for adjacent sets
+// of accesses.  If we find two sets of adjacent accesses, call
+// merge_pairs.
+void
+pair_fusion_bb_info::transform_for_base (int encoded_lfs,
+				 access_group &group)
+{
+  const auto lfs = decode_lfs (encoded_lfs);
+  const unsigned access_size = lfs.size;
+
+  bool skip_next = true;
+  access_record *prev_access = nullptr;
+
+  for (auto &access : group.list)
+    {
+      if (skip_next)
+	skip_next = false;
+      else if (known_eq (access.offset, prev_access->offset + access_size))
+	{
+	  merge_pairs (prev_access->cand_insns,
+		       access.cand_insns,
+		       lfs.load_p,
+		       access_size);
+	  skip_next = access.cand_insns.empty ();
+	}
+      prev_access = &access;
+    }
+}
+
+// If we emitted tombstone insns for this BB, iterate through the BB
+// and remove all the tombstone insns, being sure to reparent any uses
+// of mem to previous defs when we do this.
+void
+pair_fusion_bb_info::cleanup_tombstones ()
+{
+  // No need to do anything if we didn't emit a tombstone insn for this BB.
+  if (!m_emitted_tombstone)
+    return;
+
+  for (auto insn : iterate_safely (m_bb->nondebug_insns ()))
+    {
+      if (!insn->is_real ()
+	  || !bitmap_bit_p (&m_tombstone_bitmap, insn->uid ()))
+	continue;
+
+      auto set = as_a<set_info *> (memory_access (insn->defs ()));
+      if (set->has_any_uses ())
+	{
+	  auto prev_set = as_a<set_info *> (set->prev_def ());
+	  while (set->first_use ())
+	    crtl->ssa->reparent_use (set->first_use (), prev_set);
+	}
+
+      // Now set has no uses, we can delete it.
+      insn_change change (insn, insn_change::DELETE);
+      crtl->ssa->change_insn (change);
+    }
+}
+
+template<typename Map>
+void
+pair_fusion_bb_info::traverse_base_map (Map &map)
+{
+  for (auto kv : map)
+    {
+      const auto &key = kv.first;
+      auto &value = kv.second;
+      transform_for_base (key.second, value);
+    }
+}
+
+void
+pair_fusion_bb_info::transform ()
+{
+  traverse_base_map (expr_map);
+  traverse_base_map (def_map);
+}
+
+// the base register which we can fold in to make this pair use
+// a writeback addressing mode.
+void
+pair_fusion::try_promote_writeback (insn_info *insn, bool load_p)
+{
+  rtx regs[2];
+
+  rtx mem = destructure_pair (regs, PATTERN (insn->rtl ()), load_p);
+  gcc_checking_assert (MEM_P (mem));
+
+  poly_int64 offset;
+  rtx base = strip_offset (XEXP (mem, 0), &offset);
+  gcc_assert (REG_P (base));
+
+  const auto access_size = GET_MODE_SIZE (GET_MODE (mem)).to_constant () / 2;
+
+  if (find_access (insn->defs (), REGNO (base)))
+    {
+      gcc_assert (load_p);
+      if (dump_file)
+	fprintf (dump_file,
+		 "ldp %d clobbers base r%d, can't promote to writeback\n",
+		 insn->uid (), REGNO (base));
+      return;
+    }
+
+  auto base_use = find_access (insn->uses (), REGNO (base));
+  gcc_assert (base_use);
+
+  if (!base_use->def ())
+    {
+      if (dump_file)
+	fprintf (dump_file,
+		 "found pair (i%d, L=%d): but base r%d is upwards exposed\n",
+		 insn->uid (), load_p, REGNO (base));
+      return;
+    }
+
+  auto base_def = base_use->def ();
+
+  rtx wb_effect = NULL_RTX;
+  def_info *add_def;
+  const insn_range_info pair_range (insn);
+  insn_info *insns[2] = { nullptr, insn };
+  insn_info *trailing_add
+    = find_trailing_add (insns, pair_range, 0, &wb_effect,
+			 &add_def, base_def, offset,
+			 access_size);
+  if (!trailing_add)
+    return;
+
+  auto attempt = crtl->ssa->new_change_attempt ();
+
+  insn_change pair_change (insn);
+  insn_change del_change (trailing_add, insn_change::DELETE);
+  insn_change *changes[] = { &pair_change, &del_change };
+
+  rtx pair_pat = gen_promote_writeback_pair (wb_effect, mem, regs, load_p);
+  validate_unshare_change (insn->rtl (), &PATTERN (insn->rtl ()), pair_pat,
+			   true);
+
+  // The pair must gain the def of the base register from the add.
+  pair_change.new_defs = insert_access (attempt,
+					add_def,
+					pair_change.new_defs);
+  gcc_assert (pair_change.new_defs.is_valid ());
+
+  auto is_changing = insn_is_changing (changes);
+  for (unsigned i = 0; i < ARRAY_SIZE (changes); i++)
+    gcc_assert (rtl_ssa::restrict_movement_ignoring (*changes[i], is_changing));
+
+  if (!rtl_ssa::recog_ignoring (attempt, pair_change, is_changing))
+    {
+      if (dump_file)
+	fprintf (dump_file, "i%d: recog failed on wb pair, bailing out\n",
+		 insn->uid ());
+      cancel_changes (0);
+      return;
+    }
+
+  gcc_assert (crtl->ssa->verify_insn_changes (changes));
+
+  if (MAY_HAVE_DEBUG_INSNS)
+    fixup_debug_uses_trailing_add (attempt, insn, trailing_add, wb_effect);
+
+  confirm_change_group ();
+  crtl->ssa->change_insns (changes);
+}
+
+// Main function for the pass.  Iterate over the insns in BB looking
+// for load/store candidates.  If running after RA, also try and promote
+// non-writeback pairs to use writeback addressing.  Then try to fuse
+// candidates into pairs.
+void pair_fusion::process_block (bb_info *bb)
+{
+  const bool track_loads = track_loads_p ();
+  const bool track_stores = track_stores_p ();
+
+  pair_fusion_bb_info bb_state (bb, this);
+
+  for (auto insn : bb->nondebug_insns ())
+    {
+      rtx_insn *rti = insn->rtl ();
+
+      if (!rti || !INSN_P (rti))
+	continue;
+
+      rtx pat = PATTERN (rti);
+      bool load_p;
+      if (reload_completed
+	  && should_handle_writeback (writeback::ALL)
+	  && pair_mem_insn_p (rti, load_p))
+	try_promote_writeback (insn, load_p);
+
+      if (GET_CODE (pat) != SET)
+	continue;
+
+      if (track_stores && MEM_P (XEXP (pat, 0)))
+	bb_state.track_access (insn, false, XEXP (pat, 0));
+      else if (track_loads && MEM_P (XEXP (pat, 1)))
+	bb_state.track_access (insn, true, XEXP (pat, 1));
+    }
+
+  bb_state.transform ();
+  bb_state.cleanup_tombstones ();
+}
diff --git a/gcc/pair-fusion.h b/gcc/pair-fusion.h
new file mode 100644
index 00000000000..6c9b98bedd4
--- /dev/null
+++ b/gcc/pair-fusion.h
@@ -0,0 +1,201 @@ 
+// Pair Mem fusion generic header file.
+// Copyright (C) 2024 Free Software Foundation, Inc.
+//
+// This file is part of GCC.
+//
+// GCC is free software; you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// GCC is distributed in the hope that it will be useful, but
+// WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#define INCLUDE_ALGORITHM
+#define INCLUDE_FUNCTIONAL
+#define INCLUDE_LIST
+#define INCLUDE_TYPE_TRAITS
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "rtl.h"
+#include "df.h"
+#include "rtl-iter.h"
+#include "rtl-ssa.h"
+
+using namespace rtl_ssa;
+
+// Information about a potential base candidate, used in try_fuse_pair.
+// There may be zero, one, or two viable RTL bases for a given pair.
+struct base_cand
+{
+  // DEF is the def of the base register to be used by the pair.
+  def_info *def;
+
+  // FROM_INSN is -1 if the base candidate is already shared by both
+  // candidate insns.  Otherwise it holds the index of the insn from
+  // which the base originated.
+  //
+  // In the case that the base is shared, either DEF is already used
+  // by both candidate accesses, or both accesses see different versions
+  // of the same regno, in which case DEF is the def consumed by the
+  // first candidate access.
+  int from_insn;
+
+  // To form a pair, we do so by moving the first access down and the second
+  // access up.  To determine where to form the pair, and whether or not
+  // it is safe to form the pair, we track instructions which cannot be
+  // re-ordered past due to either dataflow or alias hazards.
+  //
+  // Since we allow changing the base used by an access, the choice of
+  // base can change which instructions act as re-ordering hazards for
+  // this pair (due to different dataflow).  We store the initial
+  // dataflow hazards for this choice of base candidate in HAZARDS.
+  //
+  // These hazards act as re-ordering barriers to each candidate insn
+  // respectively, in program order.
+  //
+  // Later on, when we take alias analysis into account, we narrow
+  // HAZARDS accordingly.
+  insn_info *hazards[2];
+
+  base_cand (def_info *def, int insn)
+    : def (def), from_insn (insn), hazards {nullptr, nullptr} {}
+
+  base_cand (def_info *def) : base_cand (def, -1) {}
+
+  // Test if this base candidate is viable according to HAZARDS.
+  bool viable () const
+  {
+    return !hazards[0] || !hazards[1] || (*hazards[0] > *hazards[1]);
+  }
+};
+
+struct alias_walker;
+
+// When querying should_handle_writeback, this enum is used to
+// qualify which opportunities we are asking about.
+enum class writeback {
+  // Only those writeback opportunities that arise from existing
+  // auto-increment accesses.
+  EXISTING,
+
+  // All writeback opportunities, including those that involve folding
+  // base register updates into a non-writeback pair.
+  ALL
+};
+
+// This class can be overriden by targets to give a pass that fuses
+// adjacent loads and stores into load/store pair instructions.
+//
+// The target can override the various virtual functions to customize
+// the behaviour of the pass as appropriate for the target.
+struct pair_fusion {
+  pair_fusion ();
+
+  // Given:
+  // - an rtx REG_OP, the non-memory operand in a load/store insn,
+  // - a machine_mode MEM_MODE, the mode of the MEM in that insn, and
+  // - a boolean LOAD_P (true iff the insn is a load), then:
+  // return true if the access should be considered an FP/SIMD access.
+  // Such accesses are segregated from GPR accesses, since we only want
+  // to form pairs for accesses that use the same register file.
+  virtual bool fpsimd_op_p (rtx, machine_mode, bool)
+  {
+    return false;
+  }
+
+  // Return true if we should consider forming pairs from memory
+  // accesses with operand mode MODE at this stage in compilation.
+  virtual bool pair_operand_mode_ok_p (machine_mode mode) = 0;
+
+  // Return true iff REG_OP is a suitable register operand for a paired
+  // memory access, where LOAD_P is true if we're asking about loads and
+  // false for stores.  MODE gives the mode of the operand.
+  virtual bool pair_reg_operand_ok_p (bool load_p, rtx reg_op,
+				      machine_mode mode) = 0;
+
+  // Return alias check limit.
+  // This is needed to avoid unbounded quadratic behaviour when
+  // performing alias analysis.
+  virtual int pair_mem_alias_check_limit () = 0;
+
+  // Return true if we should try to handle writeback opportunities.
+  // WHICH determines the kinds of writeback opportunities the caller
+  // is asking about.
+  virtual bool should_handle_writeback (enum writeback which) = 0;
+
+  // Given BASE_MEM, the mem from the lower candidate access for a pair,
+  // and LOAD_P (true if the access is a load), check if we should proceed
+  // to form the pair given the target's code generation policy on
+  // paired accesses.
+  virtual bool pair_mem_ok_with_policy (rtx base_mem, bool load_p) = 0;
+
+  // Generate the pattern for a paired access.  PATS gives the patterns
+  // for the individual memory accesses (which by this point must share a
+  // common base register).  If WRITEBACK is non-NULL, then this rtx
+  // describes the update to the base register that should be performed by
+  // the resulting insn.  LOAD_P is true iff the accesses are loads.
+  virtual rtx gen_pair (rtx *pats, rtx writeback, bool load_p) = 0;
+
+  // Return true if INSN is a paired memory access.  If so, set LOAD_P to
+  // true iff INSN is a load pair.
+  virtual bool pair_mem_insn_p (rtx_insn *insn, bool &load_p) = 0;
+
+  // Return true if we should track loads.
+  virtual bool track_loads_p ()
+  {
+    return true;
+  }
+
+  // Return true if we should track stores.
+  virtual bool track_stores_p ()
+  {
+    return true;
+  }
+
+  // Return true if OFFSET is in range for a paired memory access.
+  virtual bool pair_mem_in_range_p (HOST_WIDE_INT offset) = 0;
+
+  // Given a load/store pair insn in PATTERN, unpack the insn, storing
+  // the register operands in REGS, and returning the mem.  LOAD_P is
+  // true for loads and false for stores.
+  virtual rtx destructure_pair (rtx regs[2], rtx pattern, bool load_p) = 0;
+
+  // Given a pair mem in MEM, register operands in REGS, and an rtx
+  // representing the effect of writeback on the base register in WB_EFFECT,
+  // return an insn representing a writeback variant of this pair.
+  // LOAD_P is true iff the pair is a load.
+  // This is used when promoting existing non-writeback pairs to writeback
+  // variants.
+  virtual rtx gen_promote_writeback_pair (rtx wb_effect, rtx mem,
+					  rtx regs[2], bool load_p) = 0;
+
+  inline void process_block (bb_info *bb);
+  inline insn_info *find_trailing_add (insn_info *insns[2],
+				       const insn_range_info &pair_range,
+				       int initial_writeback,
+				       rtx *writeback_effect,
+				       def_info **add_def,
+				       def_info *base_def,
+				       poly_int64 initial_offset,
+				       unsigned access_size);
+  inline int get_viable_bases (insn_info *insns[2],
+			       vec<base_cand> &base_cands,
+			       rtx cand_mems[2],
+			       unsigned access_size,
+			       bool reversed);
+  inline void do_alias_analysis (insn_info *alias_hazards[4],
+				 alias_walker *walkers[4],
+				 bool load_p);
+  inline void try_promote_writeback (insn_info *insn, bool load_p);
+  void run ();
+  ~pair_fusion ();
+};