libgcc: Special-case BFD ld unwind table encodings in find_fde_tail
Commit Message
BFD ld (and the other linkers) only produce one encoding of these
values. It is not necessary to use the general
read_encoded_value_with_base decoding routine. This avoids the
data-dependent branches in its implementation.
libgcc/
* unwind-dw2-fde-dip.c (find_fde_tail): Special-case encoding
values actually used by BFD ld.
---
libgcc/unwind-dw2-fde-dip.c | 58 +++++++++++++++++++++++++++++++++++++--------
1 file changed, 48 insertions(+), 10 deletions(-)
base-commit: de7d6310862c6045cf2dfb0ef209ff0e0923e648
Comments
On 10/17/22 03:06, Florian Weimer via Gcc-patches wrote:
> BFD ld (and the other linkers) only produce one encoding of these
> values. It is not necessary to use the general
> read_encoded_value_with_base decoding routine. This avoids the
> data-dependent branches in its implementation.
>
> libgcc/
>
> * unwind-dw2-fde-dip.c (find_fde_tail): Special-case encoding
> values actually used by BFD ld.
OK.
jeff
@@ -396,10 +396,21 @@ find_fde_tail (_Unwind_Ptr pc,
if (hdr->version != 1)
return NULL;
- p = read_encoded_value_with_base (hdr->eh_frame_ptr_enc,
- base_from_cb_data (hdr->eh_frame_ptr_enc,
- dbase),
- p, &eh_frame);
+ if (__builtin_expect (hdr->eh_frame_ptr_enc == (DW_EH_PE_sdata4
+ | DW_EH_PE_pcrel), 1))
+ {
+ /* Specialized version of read_encoded_value_with_base, based on what
+ BFD ld generates. */
+ signed value __attribute__ ((mode (SI)));
+ memcpy (&value, p, sizeof (value));
+ p += sizeof (value);
+ dbase = value; /* No adjustment because pcrel has base 0. */
+ }
+ else
+ p = read_encoded_value_with_base (hdr->eh_frame_ptr_enc,
+ base_from_cb_data (hdr->eh_frame_ptr_enc,
+ dbase),
+ p, &eh_frame);
/* We require here specific table encoding to speed things up.
Also, DW_EH_PE_datarel here means using PT_GNU_EH_FRAME start
@@ -409,10 +420,20 @@ find_fde_tail (_Unwind_Ptr pc,
{
_Unwind_Ptr fde_count;
- p = read_encoded_value_with_base (hdr->fde_count_enc,
- base_from_cb_data (hdr->fde_count_enc,
- dbase),
- p, &fde_count);
+ if (__builtin_expect (hdr->fde_count_enc == DW_EH_PE_udata4, 1))
+ {
+ /* Specialized version of read_encoded_value_with_base, based on
+ what BFD ld generates. */
+ unsigned value __attribute__ ((mode (SI)));
+ memcpy (&value, p, sizeof (value));
+ p += sizeof (value);
+ fde_count = value;
+ }
+ else
+ p = read_encoded_value_with_base (hdr->fde_count_enc,
+ base_from_cb_data (hdr->fde_count_enc,
+ dbase),
+ p, &fde_count);
/* Shouldn't happen. */
if (fde_count == 0)
return NULL;
@@ -454,8 +475,25 @@ find_fde_tail (_Unwind_Ptr pc,
f = (fde *) (table[mid].fde + data_base);
f_enc = get_fde_encoding (f);
f_enc_size = size_of_encoded_value (f_enc);
- read_encoded_value_with_base (f_enc & 0x0f, 0,
- &f->pc_begin[f_enc_size], &range);
+
+ /* BFD ld uses DW_EH_PE_sdata4 | DW_EH_PE_pcrel on non-FDPIC targets,
+ so optimize for that.
+
+ This optimization is not valid for FDPIC targets. f_enc & 0x0f as
+ passed to read_encoded_value_with_base masks away the base flags,
+ but they are implicit for FDPIC. */
+#ifndef __FDPIC__
+ if (__builtin_expect (f_enc == (DW_EH_PE_sdata4 | DW_EH_PE_pcrel),
+ 1))
+ {
+ signed value __attribute__ ((mode (SI)));
+ memcpy (&value, &f->pc_begin[f_enc_size], sizeof (value));
+ range = value;
+ }
+ else
+#endif
+ read_encoded_value_with_base (f_enc & 0x0f, 0,
+ &f->pc_begin[f_enc_size], &range);
_Unwind_Ptr func = table[mid].initial_loc + data_base;
if (pc < table[mid].initial_loc + data_base + range)
{