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Simplify and speedup strstr/strcasestr first match

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Looking at the benchtests, both strstr and strcasestr spend a lot of time
in a slow initialization loop handling one character per iteration.
This can be simplified and use the much faster strlen/strnlen/strchr/memcmp.
Read ahead a few cachelines to reduce the number of strnlen calls, which
improves performance by ~3-4%.  This patch improves the time taken for the
full strstr benchtest by >40%.

	* string/strcasestr.c (STRCASESTR): Simplify and speedup first match.
	* string/strstr.c (AVAILABLE): Likewise.
This commit is contained in:
Wilco Dijkstra
2018-08-03 17:24:12 +01:00
parent 430388d5dc
commit 284f42bc77
3 changed files with 39 additions and 44 deletions
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43
string/strstr.c
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@ -50,33 +50,32 @@
if NEEDLE is empty, otherwise NULL if NEEDLE is not found in
HAYSTACK. */
char *
STRSTR (const char *haystack_start, const char *needle_start)
STRSTR (const char *haystack, const char *needle)
{
const char *haystack = haystack_start;
const char *needle = needle_start;
size_t needle_len; /* Length of NEEDLE. */
size_t haystack_len; /* Known minimum length of HAYSTACK. */
bool ok = true; /* True if NEEDLE is prefix of HAYSTACK. */
/* Determine length of NEEDLE, and in the process, make sure
HAYSTACK is at least as long (no point processing all of a long
NEEDLE if HAYSTACK is too short). */
while (*haystack && *needle)
ok &= *haystack++ == *needle++;
if (*needle)
return NULL;
if (ok)
return (char *) haystack_start;
/* Reduce the size of haystack using strchr, since it has a smaller
linear coefficient than the Two-Way algorithm. */
needle_len = needle - needle_start;
haystack = strchr (haystack_start + 1, *needle_start);
if (!haystack || __builtin_expect (needle_len == 1, 0))
/* Handle empty NEEDLE special case. */
if (needle[0] == '\0')
return (char *) haystack;
/* 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
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)
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;
needle -= needle_len;
haystack_len = (haystack > haystack_start + needle_len ? 1
: needle_len + haystack_start - haystack);
/* Perform the search. Abstract memory is considered to be an array
of 'unsigned char' values, not an array of 'char' values. See