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Improve performance of strstr

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This patch significantly improves performance of strstr using a novel
modified Horspool algorithm.  Needles up to size 256 use a bad-character
table indexed by hashed pairs of characters to quickly skip past mismatches.
Long needles use a self-adapting filtering step to avoid comparing the whole
needle repeatedly.

By limiting the needle length to 256, the shift table only requires 8 bits
per entry, lowering preprocessing overhead and minimizing cache effects.
This limit also implies worst-case performance is linear.

Small needles up to size 3 use a dedicated linear search.  Very long needles
use the Two-Way algorithm.

The performance gain using the improved bench-strstr on Cortex-A72 is 5.8
times basic_strstr and 3.7 times twoway_strstr.

Tested against GLIBC testsuite, randomized tests and the GNULIB strstr test
(https://git.savannah.gnu.org/cgit/gnulib.git/tree/tests/test-strstr.c).

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

	* string/str-two-way.h (two_way_short_needle): Add inline to avoid
	warning.
	(two_way_long_needle): Block inlining.
	* string/strstr.c (strstr2): Add new function.
	(strstr3): Likewise.
	(STRSTR): Completely rewrite strstr to improve performance.
This commit is contained in:
Wilco Dijkstra 2019-06-12 11:38:52 +01:00
parent 80b2bfb535
commit 5e0a7ecb66
3 changed files with 129 additions and 48 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
ChangeLog
View File

@ -1,3 +1,12 @@
2019-06-12 Wilco Dijkstra <wdijkstr@arm.com>
* string/str-two-way.h (two_way_short_needle): Add inline to avoid
warning.
(two_way_long_needle): Block inlining.
* string/strstr.c (strstr2): Add new function.
(strstr3): Likewise.
(STRSTR): Completely rewrite strstr to improve performance.
2019-06-11 Wilco Dijkstra <wdijkstr@arm.com>
* benchtests/bench-strstr.c (test_hard_needle): New function.

9
string/str-two-way.h
View File

@ -221,7 +221,7 @@ critical_factorization (const unsigned char *needle, size_t needle_len,
most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching. */
static RETURN_TYPE
static inline RETURN_TYPE
two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
const unsigned char *needle, size_t needle_len)
{
@ -382,8 +382,11 @@ two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
and sublinear performance O(HAYSTACK_LEN / NEEDLE_LEN) is possible.
If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching, and
sublinear performance is not possible. */
static RETURN_TYPE
sublinear performance is not possible.
Since this function is large and complex, block inlining to avoid
slowing down the common case of small needles. */
__attribute__((noinline)) static RETURN_TYPE
two_way_long_needle (const unsigned char *haystack, size_t haystack_len,
const unsigned char *needle, size_t needle_len)
{

159
string/strstr.c
View File

@ -16,29 +16,17 @@
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
/* This particular implementation was written by Eric Blake, 2008. */
#ifndef _LIBC
# include <config.h>
#endif
/* Specification of strstr. */
#include <string.h>
#include <stdbool.h>
#ifndef _LIBC
# define __builtin_expect(expr, val) (expr)
#endif
#define RETURN_TYPE char *
#define AVAILABLE(h, h_l, j, n_l) \
(((j) + (n_l) <= (h_l)) \
|| ((h_l) += __strnlen ((void*)((h) + (h_l)), (n_l) + 512), \
(j) + (n_l) <= (h_l)))
#define CHECK_EOL (1)
#define RET0_IF_0(a) if (!a) goto ret0
#define FASTSEARCH(S,C,N) (void*) strchr ((void*)(S), (C))
#include "str-two-way.h"
#undef strstr
@ -47,47 +35,128 @@
#define STRSTR strstr
#endif
/* Return the first occurrence of NEEDLE in HAYSTACK. Return HAYSTACK
if NEEDLE is empty, otherwise NULL if NEEDLE is not found in
HAYSTACK. */
static inline char *
strstr2 (const unsigned char *hs, const unsigned char *ne)
{
uint32_t h1 = (ne[0] << 16) | ne[1];
uint32_t h2 = 0;
for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
h2 = (h2 << 16) | c;
return h1 == h2 ? (char *)hs - 2 : NULL;
}
static inline char *
strstr3 (const unsigned char *hs, const unsigned char *ne)
{
uint32_t h1 = ((uint32_t)ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
uint32_t h2 = 0;
for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
h2 = (h2 | c) << 8;
return h1 == h2 ? (char *)hs - 3 : NULL;
}
/* Hash character pairs so a small shift table can be used. All bits of
p[0] are included, but not all bits from p[-1]. So if two equal hashes
match on p[-1], p[0] matches too. Hash collisions are harmless and result
in smaller shifts. */
#define hash2(p) (((size_t)(p)[0] - ((size_t)(p)[-1] << 3)) % sizeof (shift))
/* Fast strstr algorithm with guaranteed linear-time performance.
Small needles up to size 3 use a dedicated linear search. Longer needles
up to size 256 use a novel modified Horspool algorithm. It hashes pairs
of characters to quickly skip past mismatches. The main search loop only
exits if the last 2 characters match, avoiding unnecessary calls to memcmp
and allowing for a larger skip if there is no match. A self-adapting
filtering check is used to quickly detect mismatches in long needles.
By limiting the needle length to 256, the shift table can be reduced to 8
bits per entry, lowering preprocessing overhead and minimizing cache effects.
The limit also implies worst-case performance is linear.
Needles larger than 256 characters use the linear-time Two-Way algorithm. */
char *
STRSTR (const char *haystack, const char *needle)
{
size_t needle_len; /* Length of NEEDLE. */
size_t haystack_len; /* Known minimum length of HAYSTACK. */
const unsigned char *hs = (const unsigned char *) haystack;
const unsigned char *ne = (const unsigned char *) needle;
/* Handle empty NEEDLE special case. */
if (needle[0] == '\0')
return (char *) haystack;
/* Handle short needle special cases first. */
if (ne[0] == '\0')
return (char *)hs;
hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
if (hs == NULL || ne[1] == '\0')
return (char*)hs;
if (ne[2] == '\0')
return strstr2 (hs, ne);
if (ne[3] == '\0')
return strstr3 (hs, ne);
/* Skip until we find the first matching char from NEEDLE. */
haystack = strchr (haystack, needle[0]);
if (haystack == NULL || needle[1] == '\0')
return (char *) haystack;
/* Ensure HAYSTACK length is at least as long as NEEDLE length.
Since a match may occur early on in a huge HAYSTACK, use strnlen
/* Ensure haystack length is at least as long as needle length.
Since a match may occur early on in a huge haystack, use strnlen
and read ahead a few cachelines for improved performance. */
needle_len = strlen (needle);
haystack_len = __strnlen (haystack, needle_len + 256);
if (haystack_len < needle_len)
size_t ne_len = strlen ((const char*)ne);
size_t hs_len = __strnlen ((const char*)hs, ne_len | 512);
if (hs_len < ne_len)
return NULL;
/* Check whether we have a match. This improves performance since we avoid
the initialization overhead of the two-way algorithm. */
if (memcmp (haystack, needle, needle_len) == 0)
return (char *) haystack;
/* Check whether we have a match. This improves performance since we
avoid initialization overheads. */
if (memcmp (hs, ne, ne_len) == 0)
return (char *) hs;
/* Perform the search. Abstract memory is considered to be an array
of 'unsigned char' values, not an array of 'char' values. See
ISO C 99 section 6.2.6.1. */
if (needle_len < LONG_NEEDLE_THRESHOLD)
return two_way_short_needle ((const unsigned char *) haystack,
haystack_len,
(const unsigned char *) needle, needle_len);
return two_way_long_needle ((const unsigned char *) haystack, haystack_len,
(const unsigned char *) needle, needle_len);
/* Use Two-Way algorithm for very long needles. */
if (__glibc_unlikely (ne_len > 256))
return two_way_long_needle (hs, hs_len, ne, ne_len);
const unsigned char *end = hs + hs_len - ne_len;
uint8_t shift[256];
size_t tmp, shift1;
size_t m1 = ne_len - 1;
size_t offset = 0;
/* Initialize bad character shift hash table. */
memset (shift, 0, sizeof (shift));
for (int i = 1; i < m1; i++)
shift[hash2 (ne + i)] = i;
/* Shift1 is the amount we can skip after matching the hash of the
needle end but not the full needle. */
shift1 = m1 - shift[hash2 (ne + m1)];
shift[hash2 (ne + m1)] = m1;
while (1)
{
if (__glibc_unlikely (hs > end))
{
end += __strnlen ((const char*)end + m1 + 1, 2048);
if (hs > end)
return NULL;
}
/* Skip past character pairs not in the needle. */
do
{
hs += m1;
tmp = shift[hash2 (hs)];
}
while (tmp == 0 && hs <= end);
/* If the match is not at the end of the needle, shift to the end
and continue until we match the hash of the needle end. */
hs -= tmp;
if (tmp < m1)
continue;
/* Hash of the last 2 characters matches. If the needle is long,
try to quickly filter out mismatches. */
if (m1 < 15 || memcmp (hs + offset, ne + offset, 8) == 0)
{
if (memcmp (hs, ne, m1) == 0)
return (void *) hs;
/* Adjust filter offset when it doesn't find the mismatch. */
offset = (offset >= 8 ? offset : m1) - 8;
}
/* Skip based on matching the hash of the needle end. */
hs += shift1;
}
}
libc_hidden_builtin_def (strstr)
#undef LONG_NEEDLE_THRESHOLD