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c8dd67e7c9
As done in commit 284f42bc77, memcmp
can be used after memchr to avoid the initialization overhead of the
two-way algorithm for the first match. This has shown improvement
>40% for first match.
87 lines
3.1 KiB
C
87 lines
3.1 KiB
C
/* Copyright (C) 1991-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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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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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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/* This particular implementation was written by Eric Blake, 2008. */
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#ifndef _LIBC
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# include <config.h>
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#endif
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/* Specification of memmem. */
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#include <string.h>
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#ifndef _LIBC
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# define __builtin_expect(expr, val) (expr)
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# define __memmem memmem
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#endif
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#define RETURN_TYPE void *
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#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
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#define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
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#include "str-two-way.h"
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#undef memmem
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/* Return the first occurrence of NEEDLE in HAYSTACK. Return HAYSTACK
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if NEEDLE_LEN is 0, otherwise NULL if NEEDLE is not found in
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HAYSTACK. */
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void *
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__memmem (const void *haystack_start, size_t haystack_len,
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const void *needle_start, size_t needle_len)
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{
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/* Abstract memory is considered to be an array of 'unsigned char' values,
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not an array of 'char' values. See ISO C 99 section 6.2.6.1. */
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const unsigned char *haystack = (const unsigned char *) haystack_start;
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const unsigned char *needle = (const unsigned char *) needle_start;
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if (needle_len == 0)
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/* The first occurrence of the empty string is deemed to occur at
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the beginning of the string. */
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return (void *) haystack;
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/* Sanity check, otherwise the loop might search through the whole
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memory. */
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if (__glibc_unlikely (haystack_len < needle_len))
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return NULL;
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/* Use optimizations in memchr when possible, to reduce the search
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size of haystack using a linear algorithm with a smaller
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coefficient. However, avoid memchr for long needles, since we
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can often achieve sublinear performance. */
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if (needle_len < LONG_NEEDLE_THRESHOLD)
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{
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haystack = memchr (haystack, *needle, haystack_len);
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if (!haystack || __builtin_expect (needle_len == 1, 0))
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return (void *) haystack;
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haystack_len -= haystack - (const unsigned char *) haystack_start;
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if (haystack_len < needle_len)
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return NULL;
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/* Check whether we have a match. This improves performance since we
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avoid the initialization overhead of the two-way algorithm. */
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if (memcmp (haystack, needle, needle_len) == 0)
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return (void *) haystack;
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return two_way_short_needle (haystack, haystack_len, needle, needle_len);
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}
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else
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return two_way_long_needle (haystack, haystack_len, needle, needle_len);
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}
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libc_hidden_def (__memmem)
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weak_alias (__memmem, memmem)
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libc_hidden_weak (memmem)
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#undef LONG_NEEDLE_THRESHOLD
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