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bfd: optimize bfd_elf_hash

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The bfd_elf_hash loop is taken straight from the sysV document, but it
is poorly optimized. This refactoring removes about 5 x86 insns from
the 15 insn loop.

1) The if (..) is meaningless -- we're xoring with that value, and of
course xor 0 is a nop. On x86 (at least) we actually compute the xor'd
value and then cmov.  Removing the if test removes the cmov.

2) The 'h ^ g' to clear the top 4 bits is not needed, as those 4 bits
will be shifted out in the next iteration.  All we need to do is sink
a mask of those 4 bits out of the loop.

3) anding with 0xf0 after shifting by 24 bits can allow betterin
encoding on RISC ISAs than masking with '0xf0 << 24' before shifting.
RISC ISAs often require materializing larger constants.

	bfd/
	* elf.c (bfd_elf_hash): Refactor to optimize loop.
	(bfd_elf_gnu_hash): Refactor to use 32-bit type.
This commit is contained in:
Nathan Sidwell
2023-04-11 17:47:31 -04:00
parent d8ca1d2fc5
commit 0481d5dce3
1 changed files with 11 additions and 20 deletions
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31
bfd/elf.c
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@@ -196,23 +196,15 @@ _bfd_elf_swap_versym_out (bfd *abfd,
unsigned long
bfd_elf_hash (const char *namearg)
{
const unsigned char *name = (const unsigned char *) namearg;
unsigned long h = 0;
unsigned long g;
int ch;
uint32_t h = 0;
while ((ch = *name++) != '\0')
for (const unsigned char *name = (const unsigned char *) namearg;
*name; name++)
{
h = (h << 4) + ch;
if ((g = (h & 0xf0000000)) != 0)
{
h ^= g >> 24;
/* The ELF ABI says `h &= ~g', but this is equivalent in
this case and on some machines one insn instead of two. */
h ^= g;
}
h = (h << 4) + *name;
h ^= (h >> 24) & 0xf0;
}
return h & 0xffffffff;
return h & 0x0fffffff;
}
/* DT_GNU_HASH hash function. Do not change this function; you will
@@ -221,13 +213,12 @@ bfd_elf_hash (const char *namearg)
unsigned long
bfd_elf_gnu_hash (const char *namearg)
{
const unsigned char *name = (const unsigned char *) namearg;
unsigned long h = 5381;
unsigned char ch;
uint32_t h = 5381;
while ((ch = *name++) != '\0')
h = (h << 5) + h + ch;
return h & 0xffffffff;
for (const unsigned char *name = (const unsigned char *) namearg;
*name; name++)
h = (h << 5) + h + *name;
return h;
}
/* Create a tdata field OBJECT_SIZE bytes in length, zeroed out and with