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string: Improve generic memrchr
New algorithm read the lastaligned address and mask off the unwanted bytes. The loop now read word-aligned address and check using the has_eq macro. Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc-linux-gnu, and powerpc64-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
parent
0f4254311e
commit
9d4fa7a1ca
196
string/memrchr.c
196
string/memrchr.c
@ -1,11 +1,6 @@
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/* memrchr -- find the last occurrence of a byte in a memory block
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Copyright (C) 1991-2023 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Based on strlen implementation by Torbjorn Granlund (tege@sics.se),
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with help from Dan Sahlin (dan@sics.se) and
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commentary by Jim Blandy (jimb@ai.mit.edu);
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adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu),
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and implemented by Roland McGrath (roland@ai.mit.edu).
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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@ -21,177 +16,64 @@
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <stdlib.h>
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#ifdef HAVE_CONFIG_H
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# include <config.h>
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#endif
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#if defined _LIBC
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# include <string.h>
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# include <memcopy.h>
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#endif
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#if defined HAVE_LIMITS_H || defined _LIBC
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# include <limits.h>
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#endif
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#define LONG_MAX_32_BITS 2147483647
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#ifndef LONG_MAX
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# define LONG_MAX LONG_MAX_32_BITS
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#endif
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#include <sys/types.h>
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#include <string-fzb.h>
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#include <string-fzc.h>
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#include <string-fzi.h>
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#include <string-shift.h>
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#include <string.h>
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#include <libc-pointer-arith.h>
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#undef __memrchr
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#undef memrchr
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#ifndef weak_alias
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# define __memrchr memrchr
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#ifdef MEMRCHR
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# define __memrchr MEMRCHR
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#endif
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/* Search no more than N bytes of S for C. */
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void *
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#ifndef MEMRCHR
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__memrchr
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#else
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MEMRCHR
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#endif
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(const void *s, int c_in, size_t n)
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__memrchr (const void *s, int c_in, size_t n)
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{
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const unsigned char *char_ptr;
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const unsigned long int *longword_ptr;
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unsigned long int longword, magic_bits, charmask;
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unsigned char c;
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if (__glibc_unlikely (n == 0))
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return NULL;
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c = (unsigned char) c_in;
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const op_t *word_ptr = (const op_t *) PTR_ALIGN_UP (s + n, sizeof (op_t));
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uintptr_t s_int = (uintptr_t) s + n;
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/* Handle the last few characters by reading one character at a time.
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Do this until CHAR_PTR is aligned on a longword boundary. */
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for (char_ptr = (const unsigned char *) s + n;
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n > 0 && ((unsigned long int) char_ptr
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& (sizeof (longword) - 1)) != 0;
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--n)
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if (*--char_ptr == c)
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return (void *) char_ptr;
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op_t word = *--word_ptr;
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op_t repeated_c = repeat_bytes (c_in);
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/* All these elucidatory comments refer to 4-byte longwords,
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but the theory applies equally well to 8-byte longwords. */
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/* Compute the address of the word containing the initial byte. */
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const op_t *sword = (const op_t *) PTR_ALIGN_DOWN (s, sizeof (op_t));
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longword_ptr = (const unsigned long int *) char_ptr;
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/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
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the "holes." Note that there is a hole just to the left of
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each byte, with an extra at the end:
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bits: 01111110 11111110 11111110 11111111
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bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
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The 1-bits make sure that carries propagate to the next 0-bit.
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The 0-bits provide holes for carries to fall into. */
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magic_bits = -1;
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magic_bits = magic_bits / 0xff * 0xfe << 1 >> 1 | 1;
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/* Set up a longword, each of whose bytes is C. */
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charmask = c | (c << 8);
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charmask |= charmask << 16;
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#if LONG_MAX > LONG_MAX_32_BITS
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charmask |= charmask << 32;
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#endif
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/* Instead of the traditional loop which tests each character,
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we will test a longword at a time. The tricky part is testing
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if *any of the four* bytes in the longword in question are zero. */
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while (n >= sizeof (longword))
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/* If the end of buffer is not op_t aligned, mask off the undesirable bits
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before find the last byte position. */
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find_t mask = shift_find_last (find_eq_all (word, repeated_c), s_int);
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if (mask != 0)
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{
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/* We tentatively exit the loop if adding MAGIC_BITS to
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LONGWORD fails to change any of the hole bits of LONGWORD.
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char *ret = (char *) word_ptr + index_last (mask);
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return ret >= (char *) s ? ret : NULL;
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}
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if (word_ptr == sword)
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return NULL;
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word = *--word_ptr;
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1) Is this safe? Will it catch all the zero bytes?
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Suppose there is a byte with all zeros. Any carry bits
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propagating from its left will fall into the hole at its
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least significant bit and stop. Since there will be no
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carry from its most significant bit, the LSB of the
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byte to the left will be unchanged, and the zero will be
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detected.
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2) Is this worthwhile? Will it ignore everything except
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zero bytes? Suppose every byte of LONGWORD has a bit set
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somewhere. There will be a carry into bit 8. If bit 8
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is set, this will carry into bit 16. If bit 8 is clear,
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one of bits 9-15 must be set, so there will be a carry
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into bit 16. Similarly, there will be a carry into bit
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24. If one of bits 24-30 is set, there will be a carry
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into bit 31, so all of the hole bits will be changed.
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The one misfire occurs when bits 24-30 are clear and bit
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31 is set; in this case, the hole at bit 31 is not
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changed. If we had access to the processor carry flag,
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we could close this loophole by putting the fourth hole
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at bit 32!
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So it ignores everything except 128's, when they're aligned
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properly.
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3) But wait! Aren't we looking for C, not zero?
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Good point. So what we do is XOR LONGWORD with a longword,
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each of whose bytes is C. This turns each byte that is C
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into a zero. */
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longword = *--longword_ptr ^ charmask;
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/* Add MAGIC_BITS to LONGWORD. */
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if ((((longword + magic_bits)
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/* Set those bits that were unchanged by the addition. */
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^ ~longword)
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/* Look at only the hole bits. If any of the hole bits
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are unchanged, most likely one of the bytes was a
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zero. */
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& ~magic_bits) != 0)
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{
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/* Which of the bytes was C? If none of them were, it was
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a misfire; continue the search. */
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const unsigned char *cp = (const unsigned char *) longword_ptr;
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#if LONG_MAX > 2147483647
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if (cp[7] == c)
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return (void *) &cp[7];
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if (cp[6] == c)
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return (void *) &cp[6];
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if (cp[5] == c)
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return (void *) &cp[5];
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if (cp[4] == c)
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return (void *) &cp[4];
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#endif
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if (cp[3] == c)
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return (void *) &cp[3];
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if (cp[2] == c)
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return (void *) &cp[2];
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if (cp[1] == c)
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return (void *) &cp[1];
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if (cp[0] == c)
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return (void *) cp;
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}
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n -= sizeof (longword);
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while (word_ptr != sword)
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{
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if (has_eq (word, repeated_c))
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return (char *) word_ptr + index_last_eq (word, repeated_c);
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word = *--word_ptr;
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}
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char_ptr = (const unsigned char *) longword_ptr;
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while (n-- > 0)
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if (has_eq (word, repeated_c))
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{
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if (*--char_ptr == c)
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return (void *) char_ptr;
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/* We found a match, but it might be in a byte past the end of the
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array. */
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char *ret = (char *) word_ptr + index_last_eq (word, repeated_c);
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if (ret >= (char *) s)
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return ret;
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}
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return 0;
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return NULL;
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}
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#ifndef MEMRCHR
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# ifdef weak_alias
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weak_alias (__memrchr, memrchr)
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# endif
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#endif
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