diff options
Diffstat (limited to 'string/memrchr.c')
| -rw-r--r-- | string/memrchr.c | 196 |
1 files changed, 39 insertions, 157 deletions
diff --git a/string/memrchr.c b/string/memrchr.c index 18b20ff76a..b37f2a68c8 100644 --- a/string/memrchr.c +++ b/string/memrchr.c @@ -1,11 +1,6 @@ /* memrchr -- find the last occurrence of a byte in a memory block 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 @@ -21,177 +16,64 @@ License along with the GNU C Library; if not, see <https://www.gnu.org/licenses/>. */ -#include <stdlib.h> - -#ifdef HAVE_CONFIG_H -# include <config.h> -#endif - -#if defined _LIBC -# include <string.h> -# include <memcopy.h> -#endif - -#if defined HAVE_LIMITS_H || defined _LIBC -# include <limits.h> -#endif - -#define LONG_MAX_32_BITS 2147483647 - -#ifndef LONG_MAX -# define LONG_MAX LONG_MAX_32_BITS -#endif - -#include <sys/types.h> +#include <string-fzb.h> +#include <string-fzc.h> +#include <string-fzi.h> +#include <string-shift.h> +#include <string.h> +#include <libc-pointer-arith.h> #undef __memrchr #undef memrchr -#ifndef weak_alias -# define __memrchr memrchr +#ifdef MEMRCHR +# define __memrchr MEMRCHR #endif -/* Search no more than N bytes of S for C. */ void * -#ifndef MEMRCHR -__memrchr -#else -MEMRCHR -#endif - (const void *s, int c_in, size_t n) +__memrchr (const void *s, int c_in, size_t n) { - const unsigned char *char_ptr; - const unsigned long int *longword_ptr; - unsigned long int longword, magic_bits, charmask; - unsigned char c; - - c = (unsigned char) c_in; - - /* Handle the last 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 + n; - n > 0 && ((unsigned long int) char_ptr - & (sizeof (longword) - 1)) != 0; - --n) - if (*--char_ptr == c) - return (void *) char_ptr; - - /* All these elucidatory comments refer to 4-byte longwords, - but the theory applies equally well to 8-byte longwords. */ - - longword_ptr = (const 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: + if (__glibc_unlikely (n == 0)) + return NULL; - bits: 01111110 11111110 11111110 11111111 - bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD + const op_t *word_ptr = (const op_t *) PTR_ALIGN_UP (s + n, sizeof (op_t)); + uintptr_t s_int = (uintptr_t) s + n; - 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; + op_t word = *--word_ptr; + op_t repeated_c = repeat_bytes (c_in); - /* Set up a longword, each of whose bytes is C. */ - charmask = c | (c << 8); - charmask |= charmask << 16; -#if LONG_MAX > LONG_MAX_32_BITS - charmask |= charmask << 32; -#endif + /* Compute the address of the word containing the initial byte. */ + const op_t *sword = (const op_t *) PTR_ALIGN_DOWN (s, sizeof (op_t)); - /* 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 (n >= sizeof (longword)) + /* If the end of buffer is not op_t aligned, mask off the undesirable bits + before find the last byte position. */ + find_t mask = shift_find_last (find_eq_all (word, repeated_c), s_int); + if (mask != 0) { - /* 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, not 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 ^ charmask; - - /* 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) - { - /* Which of the bytes was C? If none of them were, it was - a misfire; continue the search. */ - - const unsigned char *cp = (const unsigned char *) longword_ptr; - -#if LONG_MAX > 2147483647 - if (cp[7] == c) - return (void *) &cp[7]; - if (cp[6] == c) - return (void *) &cp[6]; - if (cp[5] == c) - return (void *) &cp[5]; - if (cp[4] == c) - return (void *) &cp[4]; -#endif - if (cp[3] == c) - return (void *) &cp[3]; - if (cp[2] == c) - return (void *) &cp[2]; - if (cp[1] == c) - return (void *) &cp[1]; - if (cp[0] == c) - return (void *) cp; - } - - n -= sizeof (longword); + char *ret = (char *) word_ptr + index_last (mask); + return ret >= (char *) s ? ret : NULL; } + if (word_ptr == sword) + return NULL; + word = *--word_ptr; - char_ptr = (const unsigned char *) longword_ptr; - - while (n-- > 0) + while (word_ptr != sword) { - if (*--char_ptr == c) - return (void *) char_ptr; + if (has_eq (word, repeated_c)) + return (char *) word_ptr + index_last_eq (word, repeated_c); + word = *--word_ptr; } - return 0; + if (has_eq (word, repeated_c)) + { + /* We found a match, but it might be in a byte past the end of the + array. */ + char *ret = (char *) word_ptr + index_last_eq (word, repeated_c); + if (ret >= (char *) s) + return ret; + } + return NULL; } #ifndef MEMRCHR -# ifdef weak_alias weak_alias (__memrchr, memrchr) -# endif #endif |