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libgcc: Special-case BFD ld unwind table encodings in find_fde_tail

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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.
This commit is contained in:
Florian Weimer
2022-11-04 10:18:03 +01:00
parent a05d8e1d15
commit e724b0480b
1 changed files with 48 additions and 10 deletions
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58
libgcc/unwind-dw2-fde-dip.c
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@@ -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)
{