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string: Improve generic strnlen with memchr

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It also cleanups the multiple inclusion by leaving the ifunc
implementation to undef the weak_alias and libc_hidden_def.

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 18:00:55 -03:00
parent 2a8867a17f
commit 0f4254311e
4 changed files with 30 additions and 155 deletions
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137
string/strnlen.c
View file

@ -1,10 +1,6 @@
/* Find the length of STRING, but scan at most MAXLEN characters. /* Find the length of STRING, but scan at most MAXLEN characters.
Copyright (C) 1991-2023 Free Software Foundation, Inc. Copyright (C) 1991-2023 Free Software Foundation, Inc.
Based on strlen written by Torbjorn Granlund (tege@sics.se),
with help from Dan Sahlin (dan@sics.se);
commentary by Jim Blandy (jimb@ai.mit.edu).
The GNU C Library is free software; you can redistribute it and/or 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 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 published by the Free Software Foundation; either version 2.1 of the
@ -20,7 +16,6 @@
not, see <https://www.gnu.org/licenses/>. */ not, see <https://www.gnu.org/licenses/>. */
#include <string.h> #include <string.h>
#include <stdlib.h>
/* Find the length of S, but scan at most MAXLEN characters. If no /* Find the length of S, but scan at most MAXLEN characters. If no
'\0' terminator is found in that many characters, return MAXLEN. */ '\0' terminator is found in that many characters, return MAXLEN. */
@ -32,134 +27,12 @@
size_t size_t
__strnlen (const char *str, size_t maxlen) __strnlen (const char *str, size_t maxlen)
{ {
const char *char_ptr, *end_ptr = str + maxlen; const char *found = memchr (str, '\0', maxlen);
const unsigned long int *longword_ptr; return found ? found - str : maxlen;
unsigned long int longword, himagic, lomagic;
if (maxlen == 0)
return 0;
if (__glibc_unlikely (end_ptr < str))
end_ptr = (const char *) ~0UL;
/* 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')
{
if (char_ptr > end_ptr)
char_ptr = end_ptr;
return char_ptr - str;
}
/* All these elucidatory comments refer to 4-byte longwords,
but the theory applies equally well to 8-byte longwords. */
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. */
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. */
while (longword_ptr < (unsigned long int *) end_ptr)
{
/* 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. */
longword = *longword_ptr++;
if ((longword - lomagic) & himagic)
{
/* Which of the bytes was the zero? If none of them were, it was
a misfire; continue the search. */
const char *cp = (const char *) (longword_ptr - 1);
char_ptr = cp;
if (cp[0] == 0)
break;
char_ptr = cp + 1;
if (cp[1] == 0)
break;
char_ptr = cp + 2;
if (cp[2] == 0)
break;
char_ptr = cp + 3;
if (cp[3] == 0)
break;
if (sizeof (longword) > 4)
{
char_ptr = cp + 4;
if (cp[4] == 0)
break;
char_ptr = cp + 5;
if (cp[5] == 0)
break;
char_ptr = cp + 6;
if (cp[6] == 0)
break;
char_ptr = cp + 7;
if (cp[7] == 0)
break;
}
}
char_ptr = end_ptr;
}
if (char_ptr > end_ptr)
char_ptr = end_ptr;
return char_ptr - str;
} }
#ifndef STRNLEN #ifndef STRNLEN
libc_hidden_def (__strnlen)
weak_alias (__strnlen, strnlen) weak_alias (__strnlen, strnlen)
#endif libc_hidden_def (__strnlen)
libc_hidden_def (strnlen) libc_hidden_def (strnlen)
#endif

16
sysdeps/i386/i686/multiarch/strnlen-c.c
View file

@ -1,10 +1,10 @@
#define STRNLEN __strnlen_ia32 #define STRNLEN __strnlen_ia32
#ifdef SHARED #include <string/strnlen.c>
# undef libc_hidden_def
# define libc_hidden_def(name) \
__hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32); \
strong_alias (__strnlen_ia32, __strnlen_ia32_1); \
__hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
#endif
#include "string/strnlen.c" #ifdef SHARED
/* Alias for internal symbol to avoid PLT generation, it redirects the
libc_hidden_def (__strnlen/strlen) to default implementation. */
__hidden_ver1 (__strnlen_ia32, __GI_strnlen, __strnlen_ia32);
strong_alias (__strnlen_ia32, __strnlen_ia32_1);
__hidden_ver1 (__strnlen_ia32_1, __GI___strnlen, __strnlen_ia32_1);
#endif

16
sysdeps/powerpc/powerpc32/power4/multiarch/strnlen-ppc32.c
View file

@ -17,12 +17,12 @@
<https://www.gnu.org/licenses/>. */ <https://www.gnu.org/licenses/>. */
#define STRNLEN __strnlen_ppc #define STRNLEN __strnlen_ppc
#ifdef SHARED
# undef libc_hidden_def
# define libc_hidden_def(name) \
__hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
__hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
#endif
#include <string/strnlen.c> #include <string/strnlen.c>
#ifdef SHARED
/* Alias for internal symbol to avoid PLT generation, it redirects the
libc_hidden_def (__strnlen/strlen) to default implementation. */
__hidden_ver1 (__strnlen_ppc, __GI_strnlen, __strnlen_ppc); \
strong_alias (__strnlen_ppc, __strnlen_ppc_1); \
__hidden_ver1 (__strnlen_ppc_1, __GI___strnlen, __strnlen_ppc_1);
#endif

16
sysdeps/s390/strnlen-c.c
View file

@ -21,14 +21,16 @@
#if HAVE_STRNLEN_C #if HAVE_STRNLEN_C
# if HAVE_STRNLEN_IFUNC # if HAVE_STRNLEN_IFUNC
# define STRNLEN STRNLEN_C # define STRNLEN STRNLEN_C
# if defined SHARED && IS_IN (libc)
# undef libc_hidden_def
# define libc_hidden_def(name) \
__hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c); \
strong_alias (__strnlen_c, __strnlen_c_1); \
__hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
# endif
# endif # endif
# include <string/strnlen.c> # include <string/strnlen.c>
# if HAVE_STRNLEN_IFUNC
# if defined SHARED && IS_IN (libc)
__hidden_ver1 (__strnlen_c, __GI_strnlen, __strnlen_c);
strong_alias (__strnlen_c, __strnlen_c_1);
__hidden_ver1 (__strnlen_c_1, __GI___strnlen, __strnlen_c_1);
# endif
# endif
#endif #endif