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string: Improve generic strchrnul

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New algorithm read the first aligned address and mask off the unwanted
bytes (this strategy is similar to arch-specific  implementations used
on powerpc, sparc, and sh).

The loop now read word-aligned address and check using the has_zero_eq
function.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu,
and powerpc-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 18:00:57 -03:00
parent 350d8d1366
commit 685e844a97
3 changed files with 22 additions and 139 deletions
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155
string/strchrnul.c
View File

@ -1,10 +1,5 @@
/* 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
bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);
adaptation to strchr 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
@ -20,147 +15,41 @@
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <libc-pointer-arith.h>
#include <string-fzb.h>
#include <string-fzc.h>
#include <string-fzi.h>
#include <string-shift.h>
#include <string.h>
#include <memcopy.h>
#include <stdlib.h>
#undef __strchrnul
#undef strchrnul
#ifndef STRCHRNUL
# define STRCHRNUL __strchrnul
#ifdef STRCHRNUL
# define __strchrnul STRCHRNUL
#endif
/* Find the first occurrence of C in S or the final NUL byte. */
char *
STRCHRNUL (const char *s, int c_in)
__strchrnul (const char *str, int c_in)
{
const unsigned char *char_ptr;
const unsigned long int *longword_ptr;
unsigned long int longword, magic_bits, charmask;
unsigned char c;
/* Align pointer to sizeof op_t. */
uintptr_t s_int = (uintptr_t) str;
const op_t *word_ptr = (const op_t *) PTR_ALIGN_DOWN (str, sizeof (op_t));
c = (unsigned char) c_in;
op_t repeated_c = repeat_bytes (c_in);
/* Handle the first 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;
((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;
++char_ptr)
if (*char_ptr == c || *char_ptr == '\0')
return (void *) char_ptr;
op_t word = *word_ptr;
find_t mask = shift_find (find_zero_eq_all (word, repeated_c), s_int);
if (mask != 0)
return (char *) str + index_first (mask);
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to 8-byte longwords. */
do
word = *++word_ptr;
while (! has_zero_eq (word, repeated_c));
longword_ptr = (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 (sizeof (longword) > 4)
/* Do the shift in two steps to avoid a warning if long has 32 bits. */
charmask |= (charmask << 16) << 16;
if (sizeof (longword) > 8)
abort ();
/* 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. */
for (;;)
{
/* We tentatively exit the loop if adding MAGIC_BITS to
LONGWORD fails to change any of the hole bits of LONGWORD.
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 as well as 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++;
/* 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
/* That caught zeroes. Now test for C. */
|| ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))
& ~magic_bits) != 0)
{
/* Which of the bytes was C or zero?
If none of them were, it was a misfire; continue the search. */
const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);
if (*cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (sizeof (longword) > 4)
{
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
if (*++cp == c || *cp == '\0')
return (char *) cp;
}
}
}
/* This should never happen. */
return NULL;
return (char *) word_ptr + index_first_zero_eq (word, repeated_c);
}
#ifndef STRCHRNUL
weak_alias (__strchrnul, strchrnul)
#endif

4
sysdeps/powerpc/powerpc32/power4/multiarch/strchrnul-ppc32.c
View File

@ -19,10 +19,6 @@
#include <string.h>
#define STRCHRNUL __strchrnul_ppc
#undef weak_alias
#define weak_alias(a,b )
extern __typeof (strchrnul) __strchrnul_ppc attribute_hidden;
#include <string/strchrnul.c>

2
sysdeps/s390/strchrnul-c.c
View File

@ -22,8 +22,6 @@
# if HAVE_STRCHRNUL_IFUNC
# define STRCHRNUL STRCHRNUL_C
# define __strchrnul STRCHRNUL
# undef weak_alias
# define weak_alias(name, alias)
# endif
# include <string/strchrnul.c>