[PATCH v11 05/29] string: Improve generic strnlen with memchr
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Wed Feb 1 20:02:57 GMT 2023
On 01/02/23 16:39, Noah Goldstein wrote:
> On Wed, Feb 1, 2023 at 11:04 AM Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>>
>> 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>
>> ---
>> string/strnlen.c | 137 +-----------------
>> sysdeps/i386/i686/multiarch/strnlen-c.c | 14 +-
>> .../power4/multiarch/strnlen-ppc32.c | 14 +-
>> sysdeps/s390/strnlen-c.c | 14 +-
>> 4 files changed, 27 insertions(+), 152 deletions(-)
>>
>> diff --git a/string/strnlen.c b/string/strnlen.c
>> index 6ff294eab1..dc23354ec8 100644
>> --- a/string/strnlen.c
>> +++ b/string/strnlen.c
>> @@ -1,10 +1,6 @@
>> /* Find the length of STRING, but scan at most MAXLEN characters.
>> 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
>> 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
>> @@ -20,7 +16,6 @@
>> not, see <https://www.gnu.org/licenses/>. */
>>
>> #include <string.h>
>> -#include <stdlib.h>
>>
>> /* Find the length of S, but scan at most MAXLEN characters. If no
>> '\0' terminator is found in that many characters, return MAXLEN. */
>> @@ -32,134 +27,12 @@
>> size_t
>> __strnlen (const char *str, size_t maxlen)
>> {
>> - const char *char_ptr, *end_ptr = str + maxlen;
>> - const unsigned long int *longword_ptr;
>> - 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;
>> + const char *found = memchr (str, '\0', maxlen);
> Can be __memchr to skip PLT no?
The internal memchr alias is defined with libc_hidden_builtin_def (memchr), so
calling 'memchr' will issue the internal alias. For instance, on loongarch
(which uses generic strnlen):
$ objdump -t string/strnlen.os | grep memchr
0000000000000000 *UND* 0000000000000000 __GI_memchr
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