261 lines
9.7 KiB
Diff
261 lines
9.7 KiB
Diff
commit 5e0a7ecb6629461b28adc1a5aabcc0ede122f201
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Author: Wilco Dijkstra <wdijkstr@arm.com>
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Date: Wed Jun 12 11:38:52 2019 +0100
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Improve performance of strstr
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This patch significantly improves performance of strstr using a novel
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modified Horspool algorithm. Needles up to size 256 use a bad-character
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table indexed by hashed pairs of characters to quickly skip past mismatches.
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Long needles use a self-adapting filtering step to avoid comparing the whole
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needle repeatedly.
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By limiting the needle length to 256, the shift table only requires 8 bits
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per entry, lowering preprocessing overhead and minimizing cache effects.
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This limit also implies worst-case performance is linear.
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Small needles up to size 3 use a dedicated linear search. Very long needles
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use the Two-Way algorithm.
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The performance gain using the improved bench-strstr on Cortex-A72 is 5.8
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times basic_strstr and 3.7 times twoway_strstr.
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Tested against GLIBC testsuite, randomized tests and the GNULIB strstr test
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(https://git.savannah.gnu.org/cgit/gnulib.git/tree/tests/test-strstr.c).
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Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
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* string/str-two-way.h (two_way_short_needle): Add inline to avoid
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warning.
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(two_way_long_needle): Block inlining.
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* string/strstr.c (strstr2): Add new function.
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(strstr3): Likewise.
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(STRSTR): Completely rewrite strstr to improve performance.
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diff --git a/string/str-two-way.h b/string/str-two-way.h
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index 523d946c59412e1f..358959bef0fd6f74 100644
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--- a/string/str-two-way.h
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+++ b/string/str-two-way.h
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@@ -221,7 +221,7 @@ critical_factorization (const unsigned char *needle, size_t needle_len,
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most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
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If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
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HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */
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-static RETURN_TYPE
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+static inline RETURN_TYPE
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two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
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const unsigned char *needle, size_t needle_len)
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{
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@@ -382,8 +382,11 @@ two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
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and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible.
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If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
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HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and
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- sublinear performance is not possible. */
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-static RETURN_TYPE
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+ sublinear performance is not possible.
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+
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+ Since this function is large and complex, block inlining to avoid
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+ slowing down the common case of small needles. */
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+__attribute__((noinline)) static RETURN_TYPE
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two_way_long_needle (const unsigned char *haystack, size_t haystack_len,
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const unsigned char *needle, size_t needle_len)
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{
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diff --git a/string/strstr.c b/string/strstr.c
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index f74d7189ed1319f6..7ffb18ab4285b060 100644
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--- a/string/strstr.c
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+++ b/string/strstr.c
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@@ -16,29 +16,17 @@
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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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-
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#ifndef _LIBC
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# include <config.h>
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#endif
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-/* Specification of strstr. */
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#include <string.h>
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-#include <stdbool.h>
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-
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-#ifndef _LIBC
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-# define __builtin_expect(expr, val) (expr)
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-#endif
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-
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#define RETURN_TYPE char *
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#define AVAILABLE(h, h_l, j, n_l) \
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(((j) + (n_l) <= (h_l)) \
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|| ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
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(j) + (n_l) <= (h_l)))
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-#define CHECK_EOL (1)
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-#define RET0_IF_0(a) if (!a) goto ret0
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-#define FASTSEARCH(S,C,N) (void*) strchr ((void*)(S), (C))
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#include "str-two-way.h"
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#undef strstr
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@@ -47,47 +35,128 @@
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#define STRSTR strstr
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#endif
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-/* Return the first occurrence of NEEDLE in HAYSTACK. Return HAYSTACK
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- if NEEDLE is empty, otherwise NULL if NEEDLE is not found in
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- HAYSTACK. */
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-char *
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-STRSTR (const char *haystack, const char *needle)
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+static inline char *
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+strstr2 (const unsigned char *hs, const unsigned char *ne)
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{
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- size_t needle_len; /* Length of NEEDLE. */
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- size_t haystack_len; /* Known minimum length of HAYSTACK. */
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-
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- /* Handle empty NEEDLE special case. */
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- if (needle[0] == '\0')
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- return (char *) haystack;
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+ uint32_t h1 = (ne[0] << 16) | ne[1];
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+ uint32_t h2 = 0;
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+ for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
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+ h2 = (h2 << 16) | c;
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+ return h1 == h2 ? (char *)hs - 2 : NULL;
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+}
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- /* Skip until we find the first matching char from NEEDLE. */
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- haystack = strchr (haystack, needle[0]);
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- if (haystack == NULL || needle[1] == '\0')
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- return (char *) haystack;
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+static inline char *
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+strstr3 (const unsigned char *hs, const unsigned char *ne)
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+{
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+ uint32_t h1 = ((uint32_t)ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
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+ uint32_t h2 = 0;
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+ for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
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+ h2 = (h2 | c) << 8;
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+ return h1 == h2 ? (char *)hs - 3 : NULL;
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+}
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- /* Ensure HAYSTACK length is at least as long as NEEDLE length.
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- Since a match may occur early on in a huge HAYSTACK, use strnlen
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+/* Hash character pairs so a small shift table can be used. All bits of
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+ p[0] are included, but not all bits from p[-1]. So if two equal hashes
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+ match on p[-1], p[0] matches too. Hash collisions are harmless and result
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+ in smaller shifts. */
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+#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
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+
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+/* Fast strstr algorithm with guaranteed linear-time performance.
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+ Small needles up to size 3 use a dedicated linear search. Longer needles
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+ up to size 256 use a novel modified Horspool algorithm. It hashes pairs
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+ of characters to quickly skip past mismatches. The main search loop only
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+ exits if the last 2 characters match, avoiding unnecessary calls to memcmp
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+ and allowing for a larger skip if there is no match. A self-adapting
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+ filtering check is used to quickly detect mismatches in long needles.
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+ By limiting the needle length to 256, the shift table can be reduced to 8
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+ bits per entry, lowering preprocessing overhead and minimizing cache effects.
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+ The limit also implies worst-case performance is linear.
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+ Needles larger than 256 characters use the linear-time Two-Way algorithm. */
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+char *
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+STRSTR (const char *haystack, const char *needle)
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+{
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+ const unsigned char *hs = (const unsigned char *) haystack;
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+ const unsigned char *ne = (const unsigned char *) needle;
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+
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+ /* Handle short needle special cases first. */
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+ if (ne[0] == '\0')
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+ return (char *)hs;
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+ hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
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+ if (hs == NULL || ne[1] == '\0')
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+ return (char*)hs;
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+ if (ne[2] == '\0')
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+ return strstr2 (hs, ne);
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+ if (ne[3] == '\0')
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+ return strstr3 (hs, ne);
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+
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+ /* Ensure haystack length is at least as long as needle length.
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+ Since a match may occur early on in a huge haystack, use strnlen
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and read ahead a few cachelines for improved performance. */
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- needle_len = strlen (needle);
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- haystack_len = __strnlen (haystack, needle_len + 256);
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- if (haystack_len < needle_len)
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+ size_t ne_len = strlen ((const char*)ne);
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+ size_t hs_len = __strnlen ((const char*)hs, ne_len | 512);
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+ if (hs_len < ne_len)
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return NULL;
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- /* Check whether we have a match. This improves performance since we avoid
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- the initialization overhead of the two-way algorithm. */
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- if (memcmp (haystack, needle, needle_len) == 0)
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- return (char *) haystack;
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-
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- /* Perform the search. Abstract memory is considered to be an array
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- of 'unsigned char' values, not an array of 'char' values. See
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- ISO C 99 section 6.2.6.1. */
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- if (needle_len < LONG_NEEDLE_THRESHOLD)
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- return two_way_short_needle ((const unsigned char *) haystack,
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- haystack_len,
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- (const unsigned char *) needle, needle_len);
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- return two_way_long_needle ((const unsigned char *) haystack, haystack_len,
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- (const unsigned char *) needle, needle_len);
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+ /* Check whether we have a match. This improves performance since we
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+ avoid initialization overheads. */
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+ if (memcmp (hs, ne, ne_len) == 0)
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+ return (char *) hs;
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+
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+ /* Use Two-Way algorithm for very long needles. */
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+ if (__glibc_unlikely (ne_len > 256))
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+ return two_way_long_needle (hs, hs_len, ne, ne_len);
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+
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+ const unsigned char *end = hs + hs_len - ne_len;
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+ uint8_t shift[256];
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+ size_t tmp, shift1;
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+ size_t m1 = ne_len - 1;
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+ size_t offset = 0;
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+
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+ /* Initialize bad character shift hash table. */
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+ memset (shift, 0, sizeof (shift));
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+ for (int i = 1; i < m1; i++)
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+ shift[hash2 (ne + i)] = i;
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+ /* Shift1 is the amount we can skip after matching the hash of the
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+ needle end but not the full needle. */
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+ shift1 = m1 - shift[hash2 (ne + m1)];
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+ shift[hash2 (ne + m1)] = m1;
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+
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+ while (1)
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+ {
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+ if (__glibc_unlikely (hs > end))
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+ {
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+ end += __strnlen ((const char*)end + m1 + 1, 2048);
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+ if (hs > end)
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+ return NULL;
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+ }
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+
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+ /* Skip past character pairs not in the needle. */
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+ do
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+ {
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+ hs += m1;
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+ tmp = shift[hash2 (hs)];
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+ }
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+ while (tmp == 0 && hs <= end);
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+
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+ /* If the match is not at the end of the needle, shift to the end
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+ and continue until we match the hash of the needle end. */
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+ hs -= tmp;
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+ if (tmp < m1)
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+ continue;
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+
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+ /* Hash of the last 2 characters matches. If the needle is long,
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+ try to quickly filter out mismatches. */
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+ if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
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+ {
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+ if (memcmp (hs, ne, m1) == 0)
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+ return (void *) hs;
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+
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+ /* Adjust filter offset when it doesn't find the mismatch. */
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+ offset = (offset >= 8 ? offset : m1) - 8;
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+ }
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+
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+ /* Skip based on matching the hash of the needle end. */
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+ hs += shift1;
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+ }
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}
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libc_hidden_builtin_def (strstr)
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-
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-#undef LONG_NEEDLE_THRESHOLD
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