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string: Improve generic strlen

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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
macro.

Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu,
and powercp64-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 18:00:54 -03:00
parent 00cb84dde7
commit 350d8d1366
2 changed files with 29 additions and 75 deletions
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92
string/strlen.c
View File

@ -15,86 +15,38 @@
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 <stdlib.h>
#undef strlen
#ifndef STRLEN
# define STRLEN strlen
#ifdef STRLEN
# define __strlen STRLEN
#endif
/* Return the length of the null-terminated string STR. Scan for
the null terminator quickly by testing four bytes at a time. */
size_t
STRLEN (const char *str)
__strlen (const char *str)
{
const char *char_ptr;
const unsigned long int *longword_ptr;
unsigned long int longword, himagic, lomagic;
/* Align pointer to sizeof op_t. */
const uintptr_t s_int = (uintptr_t) str;
const op_t *word_ptr = (const op_t*) PTR_ALIGN_DOWN (str, sizeof (op_t));
/* 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 = str; ((unsigned long int) char_ptr
& (sizeof (longword) - 1)) != 0;
++char_ptr)
if (*char_ptr == '\0')
return char_ptr - str;
op_t word = *word_ptr;
find_t mask = shift_find (find_zero_all (word), s_int);
if (mask != 0)
return 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 (word));
longword_ptr = (unsigned long int *) char_ptr;
/* Computing (longword - lomagic) sets the high bit of any corresponding
byte that is either zero or greater than 0x80. The latter case can be
filtered out by computing (~longword & himagic). The final result
will always be non-zero if one of the bytes of longword is zero. */
himagic = 0x80808080L;
lomagic = 0x01010101L;
if (sizeof (longword) > 4)
{
/* 64-bit version of the magic. */
/* Do the shift in two steps to avoid a warning if long has 32 bits. */
himagic = ((himagic << 16) << 16) | himagic;
lomagic = ((lomagic << 16) << 16) | lomagic;
}
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 (;;)
{
longword = *longword_ptr++;
if (((longword - lomagic) & ~longword & himagic) != 0)
{
/* Which of the bytes was the zero? */
const char *cp = (const char *) (longword_ptr - 1);
if (cp[0] == 0)
return cp - str;
if (cp[1] == 0)
return cp - str + 1;
if (cp[2] == 0)
return cp - str + 2;
if (cp[3] == 0)
return cp - str + 3;
if (sizeof (longword) > 4)
{
if (cp[4] == 0)
return cp - str + 4;
if (cp[5] == 0)
return cp - str + 5;
if (cp[6] == 0)
return cp - str + 6;
if (cp[7] == 0)
return cp - str + 7;
}
}
}
return ((const char *) word_ptr) + index_first_zero (word) - str;
}
#ifndef STRLEN
weak_alias (__strlen, strlen)
libc_hidden_builtin_def (strlen)
#endif

12
sysdeps/s390/strlen-c.c
View File

@ -21,12 +21,14 @@
#if HAVE_STRLEN_C
# if HAVE_STRLEN_IFUNC
# define STRLEN STRLEN_C
# if defined SHARED && IS_IN (libc)
# undef libc_hidden_builtin_def
# define libc_hidden_builtin_def(name) \
__hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c);
# endif
# endif
# include <string/strlen.c>
# if HAVE_STRLEN_IFUNC
# if defined SHARED && IS_IN (libc)
__hidden_ver1 (__strlen_c, __GI_strlen, __strlen_c);
# endif
# endif
#endif