libs/newlib-cygwin
libs
/
newlib-cygwin
mirror of git://sourceware.org/git/newlib-cygwin.git
4
0
Fork 0
Code Issues Projects Releases Wiki Activity

Improve performance of strstr 

v3: Add support for read ahead using strnlen, giving an additional 25% speedup
on large inputs (both short and long needles).

This patch significantly improves performance of strstr by using Sunday's
Quick-Search algorithm.  Due to its simplicity it has the best average
performance of string matching algorithms on almost all inputs.  It uses a
bad-character shift table to skip past mismatches.

The needle length is limited to 254 - this reduces the shift table memory
4 to 8 times, lowering preprocessing overhead and minimizing cache effects.
The limit also implies its worst-case performance is linear.

Larger needles are processed by the Two-Way algorithm.  The macro AVAILABLE
has been improved to use strnlen to read the input in chunks.  This results
in a 2.5 times speedup for large needles, reducing the performance drop when
the Quick-Search algorithm can't be used.

The code for 1-4 byte needles has been simplified and now uses unsigned
char.  Since the optimized code relies on 8-bit chars, we defer to the
size-optimized implementation if CHAR_BIT > 8.

The performance gain of finding a set of randomly chosen words of size 8 in
256 bytes of English text is 14 times on AArch64. For longer haystacks the
gain is well over 20 times.

The size-optimized strstr has also been rewritten from scratch to improve
performance.  On the same test the performance gain is 69%.

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

--
This commit is contained in:
Wilco Dijkstra 2018-10-18 17:29:36 +00:00 committed by Corinna Vinschen
parent 4f7a6c326a
commit 473f1a3a5d
1 changed files with 125 additions and 94 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

219
newlib/libc/string/strstr.c
View File

@ -1,3 +1,31 @@
/* Optimized strstr function.
Copyright (c) 2018 Arm Ltd. All rights reserved.
SPDX-License-Identifier: BSD-3-Clause
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. The name of the company may not be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY ARM LTD ``AS IS'' AND ANY EXPRESS OR IMPLIED
WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL ARM LTD BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */
/* /*
FUNCTION FUNCTION
<<strstr>>---find string segment <<strstr>>---find string segment
@ -29,141 +57,144 @@ QUICKREF
*/ */
#include <string.h> #include <string.h>
#include <limits.h>
#if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__) #if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__) \
|| CHAR_BIT > 8
/* Small and efficient strstr implementation. */
char * char *
strstr (const char *searchee, strstr (const char *hs, const char *ne)
const char *lookfor)
{ {
/* Less code size, but quadratic performance in the worst case. */ size_t i;
if (*searchee == 0) int c = ne[0];
if (c == 0)
return (char*)hs;
for ( ; hs[0] != '\0'; hs++)
{ {
if (*lookfor) if (hs[0] != c)
return (char *) NULL; continue;
return (char *) searchee; for (i = 1; ne[i] != 0; i++)
if (hs[i] != ne[i])
break;
if (ne[i] == '\0')
return (char*)hs;
} }
while (*searchee) return NULL;
{ }
size_t i;
i = 0;
while (1)
{
if (lookfor[i] == 0)
{
return (char *) searchee;
}
if (lookfor[i] != searchee[i])
{
break;
}
i++;
}
searchee++;
}
return (char *) NULL;
#else /* compilation for speed */ #else /* compilation for speed */
# define RETURN_TYPE char * # define RETURN_TYPE char *
# define AVAILABLE(h, h_l, j, n_l) \ # define AVAILABLE(h, h_l, j, n_l) (((j) <= (h_l) - (n_l)) \
(!memchr ((h) + (h_l), '\0', (j) + (n_l) - (h_l)) \ || ((h_l) += strnlen ((h) + (h_l), (n_l) | 2048), ((j) <= (h_l) - (n_l))))
&& ((h_l) = (j) + (n_l)))
# include "str-two-way.h" # include "str-two-way.h"
/* Number of bits used to index shift table. */
#define SHIFT_TABLE_BITS 6
static inline char * static inline char *
strstr2 (const char *hs, const char *ne) strstr2 (const unsigned char *hs, const unsigned char *ne)
{ {
uint32_t h1 = (ne[0] << 16) | ne[1]; uint32_t h1 = (ne[0] << 16) | ne[1];
uint32_t h2 = 0; uint32_t h2 = 0;
int c = hs[0]; for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
while (h1 != h2 && c != 0)
{
h2 = (h2 << 16) | c; h2 = (h2 << 16) | c;
c = *++hs;
}
return h1 == h2 ? (char *)hs - 2 : NULL; return h1 == h2 ? (char *)hs - 2 : NULL;
} }
static inline char * static inline char *
strstr3 (const char *hs, const char *ne) strstr3 (const unsigned char *hs, const unsigned char *ne)
{ {
uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8); uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
uint32_t h2 = 0; uint32_t h2 = 0;
int c = hs[0]; for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
while (h1 != h2 && c != 0)
{
h2 = (h2 | c) << 8; h2 = (h2 | c) << 8;
c = *++hs;
}
return h1 == h2 ? (char *)hs - 3 : NULL; return h1 == h2 ? (char *)hs - 3 : NULL;
} }
static inline char * static inline char *
strstr4 (const char *hs, const char *ne) strstr4 (const unsigned char *hs, const unsigned char *ne)
{ {
uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8) | ne[3]; uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8) | ne[3];
uint32_t h2 = 0; uint32_t h2 = 0;
int c = hs[0]; for (int c = hs[0]; c != 0 && h1 != h2; c = *++hs)
while (h1 != h2 && c != 0) h2 = (h2 << 8) | c;
{
h2 = (h2 << 8) | c;
c = *++hs;
}
return h1 == h2 ? (char *)hs - 4 : NULL; return h1 == h2 ? (char *)hs - 4 : NULL;
} }
/* Extremely fast strstr algorithm with guaranteed linear-time performance.
Small needles up to size 4 use a dedicated linear search. Longer needles
up to size 254 use Sunday's Quick-Search algorithm. Due to its simplicity
it has the best average performance of string matching algorithms on almost
all inputs. It uses a bad-character shift table to skip past mismatches.
By limiting the needle length to 254, the shift table can be reduced to 8
bits per entry, lowering preprocessing overhead and minimizing cache effects.
The limit also implies the worst-case performance is linear.
Even larger needles are processed by the linear-time Two-Way algorithm.
*/
char * char *
strstr (const char *searchee, strstr (const char *haystack, const char *needle)
const char *lookfor)
{ {
/* Larger code size, but guaranteed linear performance. */ const unsigned char *hs = (const unsigned char *) haystack;
const char *haystack = searchee; const unsigned char *ne = (const unsigned char *) needle;
const char *needle = lookfor;
size_t needle_len; /* Length of NEEDLE. */
size_t haystack_len; /* Known minimum length of HAYSTACK. */
int ok = 1; /* True if NEEDLE is prefix of HAYSTACK. */
/* Handle short needle special cases first. */ /* Handle short needle special cases first. */
if (needle[0] == '\0') if (ne[0] == '\0')
return (char *) haystack; return (char *) hs;
if (needle[1] == '\0') if (ne[1] == '\0')
return strchr (haystack, needle[0]); return (char*)strchr (hs, ne[0]);
if (needle[2] == '\0') if (ne[2] == '\0')
return strstr2 (haystack, needle); return strstr2 (hs, ne);
if (needle[3] == '\0') if (ne[3] == '\0')
return strstr3 (haystack, needle); return strstr3 (hs, ne);
if (needle[4] == '\0') if (ne[4] == '\0')
return strstr4 (haystack, needle); return strstr4 (hs, ne);
/* Determine length of NEEDLE, and in the process, make sure size_t ne_len = strlen (ne);
HAYSTACK is at least as long (no point processing all of a long size_t hs_len = strnlen (hs, ne_len | 512);
NEEDLE if HAYSTACK is too short). */
while (*haystack && *needle) /* Ensure haystack length is >= needle length. */
ok &= *haystack++ == *needle++; if (hs_len < ne_len)
if (*needle)
return NULL; return NULL;
if (ok)
return (char *) searchee;
/* Reduce the size of haystack using strchr, since it has a smaller /* Use the Quick-Search algorithm for needle lengths less than 255. */
linear coefficient than the Two-Way algorithm. */ if (__builtin_expect (ne_len < 255, 1))
needle_len = needle - lookfor; {
haystack = strchr (searchee + 1, *lookfor); uint8_t shift[1 << SHIFT_TABLE_BITS];
if (!haystack || needle_len == 1) const unsigned char *end = hs + hs_len - ne_len;
return (char *) haystack;
haystack_len = (haystack > searchee + needle_len ? 1
: needle_len + searchee - haystack);
/* Perform the search. */ /* Initialize bad character shift hash table. */
if (needle_len < LONG_NEEDLE_THRESHOLD) memset (shift, ne_len + 1, sizeof (shift));
return two_way_short_needle ((const unsigned char *) haystack, for (int i = 0; i < ne_len; i++)
haystack_len, shift[ne[i] % sizeof (shift)] = ne_len - i;
(const unsigned char *) lookfor, needle_len);
return two_way_long_needle ((const unsigned char *) haystack, haystack_len, do
(const unsigned char *) lookfor, needle_len); {
#endif /* compilation for speed */ hs--;
/* Search by skipping past bad characters. */
size_t tmp = shift[hs[ne_len] % sizeof (shift)];
for (hs += tmp; hs <= end; hs += tmp)
{
tmp = shift[hs[ne_len] % sizeof (shift)];
if (memcmp (hs, ne, ne_len) == 0)
return (char*) hs;
}
if (end[ne_len] == 0)
return NULL;
end += strnlen (end + ne_len, 2048);
}
while (hs <= end);
return NULL;
}
/* Use Two-Way algorithm for very long needles. */
return two_way_long_needle (hs, hs_len, ne, ne_len);
} }
#endif /* compilation for speed */