diff options
Diffstat (limited to 'string/memchr.c')
| -rw-r--r-- | string/memchr.c | 176 |
1 files changed, 51 insertions, 125 deletions
diff --git a/string/memchr.c b/string/memchr.c index f800d47dce..4f497d6166 100644 --- a/string/memchr.c +++ b/string/memchr.c @@ -1,10 +1,6 @@ -/* Copyright (C) 1991-2023 Free Software Foundation, Inc. +/* Scan memory for a character. Generic version + 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 @@ -20,143 +16,73 @@ License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. */ -#ifndef _LIBC -# include <config.h> -#endif - +#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 <stddef.h> +#undef memchr -#include <limits.h> - -#undef __memchr -#ifdef _LIBC -# undef memchr +#ifdef MEMCHR +# define __memchr MEMCHR #endif -#ifndef weak_alias -# define __memchr memchr -#endif - -#ifndef MEMCHR -# define MEMCHR __memchr -#endif +static __always_inline const char * +sadd (uintptr_t x, uintptr_t y) +{ + return (const char *)(y > UINTPTR_MAX - x ? UINTPTR_MAX : x + y); +} /* Search no more than N bytes of S for C. */ void * -MEMCHR (void const *s, int c_in, size_t n) +__memchr (void const *s, int c_in, size_t n) { - /* On 32-bit hardware, choosing longword to be a 32-bit unsigned - long instead of a 64-bit uintmax_t tends to give better - performance. On 64-bit hardware, unsigned long is generally 64 - bits already. Change this typedef to experiment with - performance. */ - typedef unsigned long int longword; - - const unsigned char *char_ptr; - const longword *longword_ptr; - longword repeated_one; - longword repeated_c; - unsigned char c; - - c = (unsigned char) c_in; - - /* Handle the first few bytes by reading one byte at a time. - Do this until CHAR_PTR is aligned on a longword boundary. */ - for (char_ptr = (const unsigned char *) s; - n > 0 && (size_t) char_ptr % sizeof (longword) != 0; - --n, ++char_ptr) - if (*char_ptr == c) - return (void *) char_ptr; - - longword_ptr = (const longword *) char_ptr; - - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to any size longwords. */ - - /* Compute auxiliary longword values: - repeated_one is a value which has a 1 in every byte. - repeated_c has c in every byte. */ - repeated_one = 0x01010101; - repeated_c = c | (c << 8); - repeated_c |= repeated_c << 16; - if (0xffffffffU < (longword) -1) + if (__glibc_unlikely (n == 0)) + return NULL; + + /* Read the first word, but munge it so that bytes before the array + will not match goal. */ + const op_t *word_ptr = PTR_ALIGN_DOWN (s, sizeof (op_t)); + uintptr_t s_int = (uintptr_t) s; + + op_t word = *word_ptr; + op_t repeated_c = repeat_bytes (c_in); + /* Compute the address of the last byte taking in consideration possible + overflow. */ + const char *lbyte = sadd (s_int, n - 1); + /* And also the address of the word containing the last byte. */ + const op_t *lword = (const op_t *) PTR_ALIGN_DOWN (lbyte, sizeof (op_t)); + + find_t mask = shift_find (find_eq_all (word, repeated_c), s_int); + if (mask != 0) { - repeated_one |= repeated_one << 31 << 1; - repeated_c |= repeated_c << 31 << 1; - if (8 < sizeof (longword)) - { - size_t i; - - for (i = 64; i < sizeof (longword) * 8; i *= 2) - { - repeated_one |= repeated_one << i; - repeated_c |= repeated_c << i; - } - } + char *ret = (char *) s + index_first (mask); + return (ret <= lbyte) ? ret : NULL; } + if (word_ptr == lword) + return NULL; - /* Instead of the traditional loop which tests each byte, 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 equal to c. We first use an xor - with repeated_c. This reduces the task to testing whether *any of the - four* bytes in longword1 is zero. - - We compute tmp = - ((longword1 - repeated_one) & ~longword1) & (repeated_one << 7). - That is, we perform the following operations: - 1. Subtract repeated_one. - 2. & ~longword1. - 3. & a mask consisting of 0x80 in every byte. - Consider what happens in each byte: - - If a byte of longword1 is zero, step 1 and 2 transform it into 0xff, - and step 3 transforms it into 0x80. A carry can also be propagated - to more significant bytes. - - If a byte of longword1 is nonzero, let its lowest 1 bit be at - position k (0 <= k <= 7); so the lowest k bits are 0. After step 1, - the byte ends in a single bit of value 0 and k bits of value 1. - After step 2, the result is just k bits of value 1: 2^k - 1. After - step 3, the result is 0. And no carry is produced. - So, if longword1 has only non-zero bytes, tmp is zero. - Whereas if longword1 has a zero byte, call j the position of the least - significant zero byte. Then the result has a zero at positions 0, ..., - j-1 and a 0x80 at position j. We cannot predict the result at the more - significant bytes (positions j+1..3), but it does not matter since we - already have a non-zero bit at position 8*j+7. - - So, the test whether any byte in longword1 is zero is equivalent to - testing whether tmp is nonzero. */ - - while (n >= sizeof (longword)) + word = *++word_ptr; + while (word_ptr != lword) { - longword longword1 = *longword_ptr ^ repeated_c; - - if ((((longword1 - repeated_one) & ~longword1) - & (repeated_one << 7)) != 0) - break; - longword_ptr++; - n -= sizeof (longword); + if (has_eq (word, repeated_c)) + return (char *) word_ptr + index_first_eq (word, repeated_c); + word = *++word_ptr; } - char_ptr = (const unsigned char *) longword_ptr; - - /* At this point, we know that either n < sizeof (longword), or one of the - sizeof (longword) bytes starting at char_ptr is == c. On little-endian - machines, we could determine the first such byte without any further - memory accesses, just by looking at the tmp result from the last loop - iteration. But this does not work on big-endian machines. Choose code - that works in both cases. */ - - for (; n > 0; --n, ++char_ptr) + 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_first_eq (word, repeated_c); + if (ret <= lbyte) + return ret; } - return NULL; } -#ifdef weak_alias +#ifndef MEMCHR weak_alias (__memchr, memchr) -#endif libc_hidden_builtin_def (memchr) +#endif |