[PATCHv2] gdb: Add switch to disable DWARF stack unwinders

diff --git a/gdb/NEWS b/gdb/NEWS
index 76b963e2bc1..669ed2d0ebd 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -27,6 +27,10 @@  tfaas COMMAND
   output).
   Shortcut for 'thread apply all -s frame apply all -s COMMAND'.
 
+maint set dwarf unwinders (on|off)
+maint show dwarf unwinders
+  Control whether DWARF unwinders can be used.
+
 * Changed commands
 
 thread apply [all | COUNT | -COUNT] [FLAG]... COMMAND
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index b36a39b5b4c..b95c2b4f49a 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -35986,6 +35986,30 @@ 
 memory will be used.  Setting it to zero disables caching, which will
 slow down @value{GDBN} startup, but reduce memory consumption.
 
+@kindex maint set dwarf unwinders
+@kindex maint show dwarf unwinders
+@item maint set dwarf unwinders
+@itemx maint show dwarf unwinders
+Control use of the DWARF frame unwinders.
+
+@cindex DWARF frame unwinders
+Many targets that support DWARF debugging use @value{GDBN}'s DWARF
+frame unwinders to build the backtrace.  Many of these targets will
+also have a second mechanism for building the backtrace for use in
+cases where DWARF information is not available, this second mechanism
+is often an analysis of a function's prologue.
+
+In order to extend testing coverage of the second level stack
+unwinding mechanisms it is helpful to be able to disable the DWARF
+stack unwinders, this can be done with this switch.
+
+In normal use of @value{GDBN} disabling the DWARF unwinders is not
+advisable, there are cases that are better handled through DWARF than
+prologue analysis, and the debug experience is likely to be better
+with the DWARF frame unwinders enabled.
+
+If DWARF frame unwinders are not supported for a particular target
+architecture, then enabling this flag does not cause them to be used.
 @kindex maint set profile
 @kindex maint show profile
 @cindex profiling GDB
diff --git a/gdb/dwarf2-frame-tailcall.c b/gdb/dwarf2-frame-tailcall.c
index 1d3e1f445bb..f565a2eecc8 100644
--- a/gdb/dwarf2-frame-tailcall.c
+++ b/gdb/dwarf2-frame-tailcall.c
@@ -318,6 +318,9 @@  tailcall_frame_sniffer (const struct frame_unwind *self,
   int next_levels;
   struct tailcall_cache *cache;
 
+  if (!dwarf2_frame_unwinders_enabled_p)
+    return 0;
+
   /* Inner tail call element does not make sense for a sentinel frame.  */
   next_frame = get_next_frame (this_frame);
   if (next_frame == NULL)
diff --git a/gdb/dwarf2-frame.c b/gdb/dwarf2-frame.c
index 91e16cf024e..2e1110bbe8b 100644
--- a/gdb/dwarf2-frame.c
+++ b/gdb/dwarf2-frame.c
@@ -35,6 +35,7 @@ 
 
 #include "complaints.h"
 #include "dwarf2-frame.h"
+#include "dwarf2read.h"
 #include "ax.h"
 #include "dwarf2loc.h"
 #include "dwarf2-frame-tailcall.h"
@@ -169,6 +170,9 @@  static CORE_ADDR read_encoded_value (struct comp_unit *unit, gdb_byte encoding,
 				     CORE_ADDR func_base);
 
 
+/* See dwarf2-frame.h.  */
+int dwarf2_frame_unwinders_enabled_p = 1;
+
 /* Store the length the expression for the CFA in the `cfa_reg' field,
    which is unused in that case.  */
 #define cfa_exp_len cfa_reg
@@ -1326,6 +1330,9 @@  static int
 dwarf2_frame_sniffer (const struct frame_unwind *self,
 		      struct frame_info *this_frame, void **this_cache)
 {
+  if (!dwarf2_frame_unwinders_enabled_p)
+    return 0;
+
   /* Grab an address that is guarenteed to reside somewhere within the
      function.  get_frame_pc(), with a no-return next function, can
      end up returning something past the end of this function's body.
@@ -2394,12 +2401,36 @@  dwarf2_build_frame_info (struct objfile *objfile)
   set_objfile_data (objfile, dwarf2_frame_objfile_data, fde_table2);
 }
 
+/* Handle 'maintenance show dwarf unwinders'.  */
+
+static void
+show_dwarf_unwinders_enabled_p (struct ui_file *file, int from_tty,
+				struct cmd_list_element *c,
+				const char *value)
+{
+  fprintf_filtered (file,
+		    _("The DWARF stack unwinders are currently %s.\n"),
+		    value);
+}
+
 void
 _initialize_dwarf2_frame (void)
 {
   dwarf2_frame_data = gdbarch_data_register_pre_init (dwarf2_frame_init);
   dwarf2_frame_objfile_data = register_objfile_data ();
 
+  add_setshow_boolean_cmd ("unwinders", class_obscure,
+			   &dwarf2_frame_unwinders_enabled_p , _("\
+Set whether the DWARF stack frame unwinders are used."), _("\
+Show whether the DWARF stack frame unwinders are used."), _("\
+When enabled the DWARF stack frame unwinders can be used for architectures\n\
+that support the DWARF unwinders.  Enabling the dwarf unwinders for an\n\
+architecture that doesn't support them will have no effect."),
+			   NULL,
+			   show_dwarf_unwinders_enabled_p,
+			   &set_dwarf_cmdlist,
+			   &show_dwarf_cmdlist);
+
 #if GDB_SELF_TEST
   selftests::register_test_foreach_arch ("execute_cfa_program",
 					 selftests::execute_cfa_program_test);
diff --git a/gdb/dwarf2-frame.h b/gdb/dwarf2-frame.h
index 471281a2c3f..52316e5e168 100644
--- a/gdb/dwarf2-frame.h
+++ b/gdb/dwarf2-frame.h
@@ -210,6 +210,12 @@  struct dwarf2_frame_state
   bool armcc_cfa_offsets_reversed = false;
 };
 
+/* When this is true the DWARF frame unwinders can be used if they are
+   registered with the gdbarch.  Not all architectures can or do use the
+   DWARF unwinders.  Setting this to true on a target that does not
+   otherwise support the DWARF unwinders has no effect.  */
+extern int dwarf2_frame_unwinders_enabled_p;
+
 /* Set the architecture-specific register state initialization
    function for GDBARCH to INIT_REG.  */
 
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 45280fec410..8455d948412 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -25330,8 +25330,8 @@  partial_die_eq (const void *item_lhs, const void *item_rhs)
   return part_die_lhs->sect_off == part_die_rhs->sect_off;
 }
 
-static struct cmd_list_element *set_dwarf_cmdlist;
-static struct cmd_list_element *show_dwarf_cmdlist;
+struct cmd_list_element *set_dwarf_cmdlist;
+struct cmd_list_element *show_dwarf_cmdlist;
 
 static void
 set_dwarf_cmd (const char *args, int from_tty)
diff --git a/gdb/dwarf2read.h b/gdb/dwarf2read.h
index 74335d77db0..6e37c5d06f5 100644
--- a/gdb/dwarf2read.h
+++ b/gdb/dwarf2read.h
@@ -23,6 +23,10 @@ 
 #include "filename-seen-cache.h"
 #include "gdb_obstack.h"
 
+/* Hold 'maintenance (set|show) dwarf' commands.  */
+extern struct cmd_list_element *set_dwarf_cmdlist;
+extern struct cmd_list_element *show_dwarf_cmdlist;
+
 typedef struct dwarf2_per_cu_data *dwarf2_per_cu_ptr;
 DEF_VEC_P (dwarf2_per_cu_ptr);
 
diff --git a/gdb/testsuite/gdb.base/maint.exp b/gdb/testsuite/gdb.base/maint.exp
index 92086eec862..6125b6ec6f2 100644
--- a/gdb/testsuite/gdb.base/maint.exp
+++ b/gdb/testsuite/gdb.base/maint.exp
@@ -526,6 +526,10 @@  gdb_test_no_output "maint info line-table xxx.c" \
 
 set timeout $oldtimeout
 
+# Just check that the DWARF unwinders control flag is visible.
+gdb_test "maint show dwarf unwinders" \
+    "The DWARF stack unwinders are currently (on\|off)\."
+
 #============test help on maint commands
 
 test_prefix_command_help {"maint info" "maintenance info"} {