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glibc/string/memcmp.c

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/* Copyright (C) 1991-2024 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Torbjorn Granlund (tege@sics.se).
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#if defined HAVE_STRING_H || defined _LIBC
# include <string.h>
#endif
#undef memcmp
#ifndef MEMCMP
# define MEMCMP memcmp
#endif
#ifdef _LIBC
# include <memcopy.h>
# include <endian.h>
# if __BYTE_ORDER == __BIG_ENDIAN
# define WORDS_BIGENDIAN
# endif
#else /* Not in the GNU C library. */
# include <sys/types.h>
/* Type to use for aligned memory operations.
This should normally be the biggest type supported by a single load
and store. Must be an unsigned type. */
# define OPSIZ (sizeof (op_t))
/* Type to use for unaligned operations. */
typedef unsigned char byte;
# ifndef WORDS_BIGENDIAN
# define MERGE(w0, sh_1, w1, sh_2) (((w0) >> (sh_1)) | ((w1) << (sh_2)))
# else
# define MERGE(w0, sh_1, w1, sh_2) (((w0) << (sh_1)) | ((w1) >> (sh_2)))
# endif
#endif /* In the GNU C library. */
#ifdef WORDS_BIGENDIAN
# define CMP_LT_OR_GT(a, b) ((a) > (b) ? 1 : -1)
#else
# define CMP_LT_OR_GT(a, b) memcmp_bytes ((a), (b))
#endif
/* BE VERY CAREFUL IF YOU CHANGE THIS CODE! */
/* The strategy of this memcmp is:
1. Compare bytes until one of the block pointers is aligned.
2. Compare using memcmp_common_alignment or
memcmp_not_common_alignment, regarding the alignment of the other
block after the initial byte operations. The maximum number of
full words (of type op_t) are compared in this way.
3. Compare the few remaining bytes. */
#ifndef WORDS_BIGENDIAN
/* memcmp_bytes -- Compare A and B bytewise in the byte order of the machine.
A and B are known to be different.
This is needed only on little-endian machines. */
static int memcmp_bytes (op_t, op_t) __THROW;
static int
memcmp_bytes (op_t a, op_t b)
{
long int srcp1 = (long int) &a;
long int srcp2 = (long int) &b;
op_t a0, b0;
do
{
a0 = ((byte *) srcp1)[0];
b0 = ((byte *) srcp2)[0];
srcp1 += 1;
srcp2 += 1;
}
while (a0 == b0);
return a0 - b0;
}
#endif
static int memcmp_common_alignment (long, long, size_t) __THROW;
/* memcmp_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN `op_t'
objects (not LEN bytes!). Both SRCP1 and SRCP2 should be aligned for
memory operations on `op_t's. */
static int
memcmp_common_alignment (long int srcp1, long int srcp2, size_t len)
{
op_t a0, a1;
op_t b0, b1;
switch (len % 4)
{
default: /* Avoid warning about uninitialized local variables. */
case 2:
a0 = ((op_t *) srcp1)[0];
b0 = ((op_t *) srcp2)[0];
srcp1 -= 2 * OPSIZ;
srcp2 -= 2 * OPSIZ;
len += 2;
goto do1;
case 3:
a1 = ((op_t *) srcp1)[0];
b1 = ((op_t *) srcp2)[0];
srcp1 -= OPSIZ;
srcp2 -= OPSIZ;
len += 1;
goto do2;
case 0:
if (OP_T_THRES <= 3 * OPSIZ && len == 0)
return 0;
a0 = ((op_t *) srcp1)[0];
b0 = ((op_t *) srcp2)[0];
goto do3;
case 1:
a1 = ((op_t *) srcp1)[0];
b1 = ((op_t *) srcp2)[0];
srcp1 += OPSIZ;
srcp2 += OPSIZ;
len -= 1;
if (OP_T_THRES <= 3 * OPSIZ && len == 0)
goto do0;
/* Fall through. */
}
do
{
a0 = ((op_t *) srcp1)[0];
b0 = ((op_t *) srcp2)[0];
if (a1 != b1)
return CMP_LT_OR_GT (a1, b1);
do3:
a1 = ((op_t *) srcp1)[1];
b1 = ((op_t *) srcp2)[1];
if (a0 != b0)
return CMP_LT_OR_GT (a0, b0);
do2:
a0 = ((op_t *) srcp1)[2];
b0 = ((op_t *) srcp2)[2];
if (a1 != b1)
return CMP_LT_OR_GT (a1, b1);
do1:
a1 = ((op_t *) srcp1)[3];
b1 = ((op_t *) srcp2)[3];
if (a0 != b0)
return CMP_LT_OR_GT (a0, b0);
srcp1 += 4 * OPSIZ;
srcp2 += 4 * OPSIZ;
len -= 4;
}
while (len != 0);
/* This is the right position for do0. Please don't move
it into the loop. */
do0:
if (a1 != b1)
return CMP_LT_OR_GT (a1, b1);
return 0;
}
static int memcmp_not_common_alignment (long, long, size_t) __THROW;
/* memcmp_not_common_alignment -- Compare blocks at SRCP1 and SRCP2 with LEN
`op_t' objects (not LEN bytes!). SRCP2 should be aligned for memory
operations on `op_t', but SRCP1 *should be unaligned*. */
static int
memcmp_not_common_alignment (long int srcp1, long int srcp2, size_t len)
{
op_t a0, a1, a2, a3;
op_t b0, b1, b2, b3;
op_t x;
int shl, shr;
/* Calculate how to shift a word read at the memory operation
aligned srcp1 to make it aligned for comparison. */
shl = 8 * (srcp1 % OPSIZ);
shr = 8 * OPSIZ - shl;
/* Make SRCP1 aligned by rounding it down to the beginning of the `op_t'
it points in the middle of. */
srcp1 &= -OPSIZ;
switch (len % 4)
{
default: /* Avoid warning about uninitialized local variables. */
case 2:
a1 = ((op_t *) srcp1)[0];
a2 = ((op_t *) srcp1)[1];
b2 = ((op_t *) srcp2)[0];
srcp1 -= 1 * OPSIZ;
srcp2 -= 2 * OPSIZ;
len += 2;
goto do1;
case 3:
a0 = ((op_t *) srcp1)[0];
a1 = ((op_t *) srcp1)[1];
b1 = ((op_t *) srcp2)[0];
srcp2 -= 1 * OPSIZ;
len += 1;
goto do2;
case 0:
if (OP_T_THRES <= 3 * OPSIZ && len == 0)
return 0;
a3 = ((op_t *) srcp1)[0];
a0 = ((op_t *) srcp1)[1];
b0 = ((op_t *) srcp2)[0];
srcp1 += 1 * OPSIZ;
goto do3;
case 1:
a2 = ((op_t *) srcp1)[0];
a3 = ((op_t *) srcp1)[1];
b3 = ((op_t *) srcp2)[0];
srcp1 += 2 * OPSIZ;
srcp2 += 1 * OPSIZ;
len -= 1;
if (OP_T_THRES <= 3 * OPSIZ && len == 0)
goto do0;
/* Fall through. */
}
do
{
a0 = ((op_t *) srcp1)[0];
b0 = ((op_t *) srcp2)[0];
x = MERGE(a2, shl, a3, shr);
if (x != b3)
return CMP_LT_OR_GT (x, b3);
do3:
a1 = ((op_t *) srcp1)[1];
b1 = ((op_t *) srcp2)[1];
x = MERGE(a3, shl, a0, shr);
if (x != b0)
return CMP_LT_OR_GT (x, b0);
do2:
a2 = ((op_t *) srcp1)[2];
b2 = ((op_t *) srcp2)[2];
x = MERGE(a0, shl, a1, shr);
if (x != b1)
return CMP_LT_OR_GT (x, b1);
do1:
a3 = ((op_t *) srcp1)[3];
b3 = ((op_t *) srcp2)[3];
x = MERGE(a1, shl, a2, shr);
if (x != b2)
return CMP_LT_OR_GT (x, b2);
srcp1 += 4 * OPSIZ;
srcp2 += 4 * OPSIZ;
len -= 4;
}
while (len != 0);
/* This is the right position for do0. Please don't move
it into the loop. */
do0:
x = MERGE(a2, shl, a3, shr);
if (x != b3)
return CMP_LT_OR_GT (x, b3);
return 0;
}
int
MEMCMP (const void *s1, const void *s2, size_t len)
{
op_t a0;
op_t b0;
long int srcp1 = (long int) s1;
long int srcp2 = (long int) s2;
op_t res;
if (len >= OP_T_THRES)
{
/* There are at least some bytes to compare. No need to test
for LEN == 0 in this alignment loop. */
while (srcp2 % OPSIZ != 0)
{
a0 = ((byte *) srcp1)[0];
b0 = ((byte *) srcp2)[0];
srcp1 += 1;
srcp2 += 1;
res = a0 - b0;
if (res != 0)
return res;
len -= 1;
}
/* SRCP2 is now aligned for memory operations on `op_t'.
SRCP1 alignment determines if we can do a simple,
aligned compare or need to shuffle bits. */
if (srcp1 % OPSIZ == 0)
res = memcmp_common_alignment (srcp1, srcp2, len / OPSIZ);
else
res = memcmp_not_common_alignment (srcp1, srcp2, len / OPSIZ);
if (res != 0)
return res;
/* Number of bytes remaining in the interval [0..OPSIZ-1]. */
srcp1 += len & -OPSIZ;
srcp2 += len & -OPSIZ;
len %= OPSIZ;
}
/* There are just a few bytes to compare. Use byte memory operations. */
while (len != 0)
{
a0 = ((byte *) srcp1)[0];
b0 = ((byte *) srcp2)[0];
srcp1 += 1;
srcp2 += 1;
res = a0 - b0;
if (res != 0)
return res;
len -= 1;
}
return 0;
}
libc_hidden_builtin_def(memcmp)
#ifdef weak_alias
# undef bcmp
weak_alias (memcmp, bcmp)
#endif
#undef __memcmpeq
strong_alias (memcmp, __memcmpeq)
libc_hidden_def(__memcmpeq)
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