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string: Improve generic memrchr

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New algorithm read the lastaligned address and mask off the unwanted
bytes.  The loop now read word-aligned address and check using the
has_eq macro.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
and powerpc64-linux-gnu by removing the arch-specific assembly
implementation and disabling multi-arch (it covers both LE and BE
for 64 and 32 bits).

Co-authored-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
This commit is contained in:
Adhemerval Zanella 2023-01-10 18:01:00 -03:00
parent 0f4254311e
commit 9d4fa7a1ca
1 changed files with 39 additions and 157 deletions
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196
string/memrchr.c
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@ -1,11 +1,6 @@
/* memrchr -- find the last occurrence of a byte in a memory block
Copyright (C) 1991-2023 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
with help from Dan Sahlin (dan@sics.se) and
commentary by Jim Blandy (jimb@ai.mit.edu);
adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
and implemented by Roland McGrath (roland@ai.mit.edu).
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
@ -21,177 +16,64 @@
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <stdlib.h>
#ifdef HAVE_CONFIG_H
# include <config.h>
#endif
#if defined _LIBC
# include <string.h>
# include <memcopy.h>
#endif
#if defined HAVE_LIMITS_H || defined _LIBC
# include <limits.h>
#endif
#define LONG_MAX_32_BITS 2147483647
#ifndef LONG_MAX
# define LONG_MAX LONG_MAX_32_BITS
#endif
#include <sys/types.h>
#include <string-fzb.h>
#include <string-fzc.h>
#include <string-fzi.h>
#include <string-shift.h>
#include <string.h>
#include <libc-pointer-arith.h>
#undef __memrchr
#undef memrchr
#ifndef weak_alias
# define __memrchr memrchr
#ifdef MEMRCHR
# define __memrchr MEMRCHR
#endif
/* Search no more than N bytes of S for C. */
void *
#ifndef MEMRCHR
__memrchr
#else
MEMRCHR
#endif
(const void *s, int c_in, size_t n)
__memrchr (const void *s, int c_in, size_t n)
{
const unsigned char *char_ptr;
const unsigned long int *longword_ptr;
unsigned long int longword, magic_bits, charmask;
unsigned char c;
if (__glibc_unlikely (n == 0))
return NULL;
c = (unsigned char) c_in;
const op_t *word_ptr = (const op_t *) PTR_ALIGN_UP (s + n, sizeof (op_t));
uintptr_t s_int = (uintptr_t) s + n;
/* Handle the last few characters by reading one character at a time.
Do this until CHAR_PTR is aligned on a longword boundary. */
for (char_ptr = (const unsigned char *) s + n;
n > 0 && ((unsigned long int) char_ptr
& (sizeof (longword) - 1)) != 0;
--n)
if (*--char_ptr == c)
return (void *) char_ptr;
op_t word = *--word_ptr;
op_t repeated_c = repeat_bytes (c_in);
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to 8-byte longwords. */
/* Compute the address of the word containing the initial byte. */
const op_t *sword = (const op_t *) PTR_ALIGN_DOWN (s, sizeof (op_t));
longword_ptr = (const unsigned long int *) char_ptr;
/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
the "holes." Note that there is a hole just to the left of
each byte, with an extra at the end:
bits: 01111110 11111110 11111110 11111111
bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
The 1-bits make sure that carries propagate to the next 0-bit.
The 0-bits provide holes for carries to fall into. */
magic_bits = -1;
magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
/* Set up a longword, each of whose bytes is C. */
charmask = c | (c << 8);
charmask |= charmask << 16;
#if LONG_MAX > LONG_MAX_32_BITS
charmask |= charmask << 32;
#endif
/* Instead of the traditional loop which tests each character,
we will test a longword at a time. The tricky part is testing
if *any of the four* bytes in the longword in question are zero. */
while (n >= sizeof (longword))
/* If the end of buffer is not op_t aligned, mask off the undesirable bits
before find the last byte position. */
find_t mask = shift_find_last (find_eq_all (word, repeated_c), s_int);
if (mask != 0)
{
/* We tentatively exit the loop if adding MAGIC_BITS to
LONGWORD fails to change any of the hole bits of LONGWORD.
char *ret = (char *) word_ptr + index_last (mask);
return ret >= (char *) s ? ret : NULL;
}
if (word_ptr == sword)
return NULL;
word = *--word_ptr;
1) Is this safe? Will it catch all the zero bytes?
Suppose there is a byte with all zeros. Any carry bits
propagating from its left will fall into the hole at its
least significant bit and stop. Since there will be no
carry from its most significant bit, the LSB of the
byte to the left will be unchanged, and the zero will be
detected.
2) Is this worthwhile? Will it ignore everything except
zero bytes? Suppose every byte of LONGWORD has a bit set
somewhere. There will be a carry into bit 8. If bit 8
is set, this will carry into bit 16. If bit 8 is clear,
one of bits 9-15 must be set, so there will be a carry
into bit 16. Similarly, there will be a carry into bit
24. If one of bits 24-30 is set, there will be a carry
into bit 31, so all of the hole bits will be changed.
The one misfire occurs when bits 24-30 are clear and bit
31 is set; in this case, the hole at bit 31 is not
changed. If we had access to the processor carry flag,
we could close this loophole by putting the fourth hole
at bit 32!
So it ignores everything except 128's, when they're aligned
properly.
3) But wait! Aren't we looking for C, not zero?
Good point. So what we do is XOR LONGWORD with a longword,
each of whose bytes is C. This turns each byte that is C
into a zero. */
longword = *--longword_ptr ^ charmask;
/* Add MAGIC_BITS to LONGWORD. */
if ((((longword + magic_bits)
/* Set those bits that were unchanged by the addition. */
^ ~longword)
/* Look at only the hole bits. If any of the hole bits
are unchanged, most likely one of the bytes was a
zero. */
& ~magic_bits) != 0)
{
/* Which of the bytes was C? If none of them were, it was
a misfire; continue the search. */
const unsigned char *cp = (const unsigned char *) longword_ptr;
#if LONG_MAX > 2147483647
if (cp[7] == c)
return (void *) &cp[7];
if (cp[6] == c)
return (void *) &cp[6];
if (cp[5] == c)
return (void *) &cp[5];
if (cp[4] == c)
return (void *) &cp[4];
#endif
if (cp[3] == c)
return (void *) &cp[3];
if (cp[2] == c)
return (void *) &cp[2];
if (cp[1] == c)
return (void *) &cp[1];
if (cp[0] == c)
return (void *) cp;
}
n -= sizeof (longword);
while (word_ptr != sword)
{
if (has_eq (word, repeated_c))
return (char *) word_ptr + index_last_eq (word, repeated_c);
word = *--word_ptr;
}
char_ptr = (const unsigned char *) longword_ptr;
while (n-- > 0)
if (has_eq (word, repeated_c))
{
if (*--char_ptr == c)
return (void *) char_ptr;
/* We found a match, but it might be in a byte past the end of the
array. */
char *ret = (char *) word_ptr + index_last_eq (word, repeated_c);
if (ret >= (char *) s)
return ret;
}
return 0;
return NULL;
}
#ifndef MEMRCHR
# ifdef weak_alias
weak_alias (__memrchr, memrchr)
# endif
#endif