Improve strstr performance
Commit Message
Improve strstr performance. Strstr tends to be slow because it uses
many calls to memchr and a slow byte loop to scan for the next match.
Performance is significantly improved by using strnlen on larger blocks
and using strchr to search for the next matching character. strcasestr
can also use strnlen to scan ahead, and memmem can use memchr to check
for the next match.
On the GLIBC bench tests the overall performance gains on Cortex-A72 are:
strstr: +25%
strcasestr: +4.3%
memmem: +18%
On a 256KB dataset strstr performance improves by 67%, strcasestr by 47%.
Tested on AArch64, passes GLIBC tests.
ChangeLog:
2018-06-08 Wilco Dijkstra <wdijkstr@arm.com>
* string/memmem.c (FASTSEARCH): Define.
* string/str-two-way.h (two_way_short_needle): Minor cleanups.
Add support for FASTSEARCH.
* string/strcasestr.c (AVAILABLE): Use read-ahead strnlen.
* string/strstr.c (AVAILABLE): Use read-ahead strnlen.
(FASTSEARCH): Define.
--
Comments
On 11/06/2018 07:31, Wilco Dijkstra wrote:
> Improve strstr performance. Strstr tends to be slow because it uses
> many calls to memchr and a slow byte loop to scan for the next match.
> Performance is significantly improved by using strnlen on larger blocks
> and using strchr to search for the next matching character. strcasestr
> can also use strnlen to scan ahead, and memmem can use memchr to check
> for the next match.
>
> On the GLIBC bench tests the overall performance gains on Cortex-A72 are:
> strstr: +25%
> strcasestr: +4.3%
> memmem: +18%
>
> On a 256KB dataset strstr performance improves by 67%, strcasestr by 47%.
>
> Tested on AArch64, passes GLIBC tests.
I am checking if it is worth to keep the implementation in sync with gnulib,
since it contains some changes over the years which did not make into glibc.
I create a branch with these changes [1] along with your changes, but
results are mixed:
- For strstr/strcasestr performance is the same for neddles larger than
4, but faster for shorter needles.
- For memmem performance is faster for needle of size 2, but slower for
large haystack.
My plan is to check if we can simplify the code a bit, as gnulib one has
done; but in overall the change should be ok (modulo one fix below).
[1] https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/azanella/str-two-way-optimization
> diff --git a/string/strcasestr.c b/string/strcasestr.c
> index 90ba189790481a58eb7627eb8c396530f988f985..d54c4b56853735fb8f5ebd59419b222ca8827651 100644
> --- a/string/strcasestr.c
> +++ b/string/strcasestr.c
> @@ -37,8 +37,8 @@
> /* Two-Way algorithm. */
> #define RETURN_TYPE char *
> #define AVAILABLE(h, h_l, j, n_l) \
> - (!memchr ((h) + (h_l), '\0', (j) + (n_l) - (h_l)) \
> - && ((h_l) = (j) + (n_l)))
> + (((j) + (n_l) <= (h_l)) || ((h_l) += strnlen ((void*)((h) + (h_l)), 512), \
> + (j) + (n_l) <= (h_l)))
Why did it not trigger linknamespace issues with strnlen usage? For instance:
$ cat conform/ISO/assert.h/linknamespace.out
[initial] __assert_fail -> [libc.a(assert.o)] __dcgettext -> [libc.a(dcgettext.o)] __dcigettext -> [libc.a(dcigettext.o)] strstr -> [libc.a(strstr.o)] strnlen
On the branch I created it contains a fix for this [2].
[2] https://sourceware.org/git/?p=glibc.git;a=commit;h=6cc58a9708ff256323c34d200fd9362f25bf66f7
> #define CHECK_EOL (1)
> #define RET0_IF_0(a) if (!a) goto ret0
> #define CANON_ELEMENT(c) TOLOWER (c)
> diff --git a/string/strstr.c b/string/strstr.c
> index b3b5deb673758510616d32849bcf2fc15ce941cd..346f303c7423c95de0fbcdeb1a9f31293b303372 100644
> --- a/string/strstr.c
> +++ b/string/strstr.c
> @@ -33,10 +33,11 @@
>
> #define RETURN_TYPE char *
> #define AVAILABLE(h, h_l, j, n_l) \
> - (!memchr ((h) + (h_l), '\0', (j) + (n_l) - (h_l)) \
> - && ((h_l) = (j) + (n_l)))
> + (((j) + (n_l) <= (h_l)) || ((h_l) += strnlen ((void*)((h) + (h_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
>
@@ -31,6 +31,7 @@
#define RETURN_TYPE void *
#define AVAILABLE(h, h_l, j, n_l) ((j) <= (h_l) - (n_l))
+#define FASTSEARCH(S,C,N) (void*) memchr ((void *)(S), (C), (N))
#include "str-two-way.h"
#undef memmem
@@ -281,50 +281,50 @@ two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
}
else
{
- const unsigned char *phaystack = &haystack[suffix];
+ const unsigned char *phaystack;
/* The comparison always starts from needle[suffix], so cache it
and use an optimized first-character loop. */
unsigned char needle_suffix = CANON_ELEMENT (needle[suffix]);
-#if CHECK_EOL
- /* We start matching from the SUFFIX'th element, so make sure we
- don't hit '\0' before that. */
- if (haystack_len < suffix + 1
- && !AVAILABLE (haystack, haystack_len, 0, suffix + 1))
- return NULL;
-#endif
-
/* The two halves of needle are distinct; no extra memory is
required, and any mismatch results in a maximal shift. */
period = MAX (suffix, needle_len - suffix) + 1;
j = 0;
- while (1
-#if !CHECK_EOL
- && AVAILABLE (haystack, haystack_len, j, needle_len)
-#endif
- )
+ while (AVAILABLE (haystack, haystack_len, j, needle_len))
{
unsigned char haystack_char;
const unsigned char *pneedle;
- /* TODO: The first-character loop can be sped up by adapting
- longword-at-a-time implementation of memchr/strchr. */
- if (needle_suffix
+ phaystack = &haystack[suffix + j];
+
+#ifdef FASTSEARCH
+ if (*phaystack++ != needle_suffix)
+ {
+ phaystack = FASTSEARCH (phaystack, needle_suffix,
+ haystack_len - needle_len - j);
+ if (phaystack == NULL)
+ goto ret0;
+ j = phaystack - &haystack[suffix];
+ phaystack++;
+ }
+#else
+ while (needle_suffix
!= (haystack_char = CANON_ELEMENT (*phaystack++)))
{
RET0_IF_0 (haystack_char);
-#if !CHECK_EOL
+# if !CHECK_EOL
++j;
-#endif
- continue;
+ if (!AVAILABLE (haystack, haystack_len, j, needle_len))
+ goto ret0;
+# endif
}
-#if CHECK_EOL
+# if CHECK_EOL
/* Calculate J if it wasn't kept up-to-date in the first-character
loop. */
j = phaystack - &haystack[suffix] - 1;
+# endif
#endif
-
/* Scan for matches in right half. */
i = suffix + 1;
pneedle = &needle[i];
@@ -338,6 +338,11 @@ two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
}
++i;
}
+#if CHECK_EOL
+ /* Update minimal length of haystack. */
+ if (phaystack > haystack + haystack_len)
+ haystack_len = phaystack - haystack;
+#endif
if (needle_len <= i)
{
/* Scan for matches in left half. */
@@ -360,13 +365,6 @@ two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
}
else
j += i - suffix + 1;
-
-#if CHECK_EOL
- if (!AVAILABLE (haystack, haystack_len, j, needle_len))
- break;
-#endif
-
- phaystack = &haystack[suffix + j];
}
}
ret0: __attribute__ ((unused))
@@ -37,8 +37,8 @@
/* Two-Way algorithm. */
#define RETURN_TYPE char *
#define AVAILABLE(h, h_l, j, n_l) \
- (!memchr ((h) + (h_l), '\0', (j) + (n_l) - (h_l)) \
- && ((h_l) = (j) + (n_l)))
+ (((j) + (n_l) <= (h_l)) || ((h_l) += strnlen ((void*)((h) + (h_l)), 512), \
+ (j) + (n_l) <= (h_l)))
#define CHECK_EOL (1)
#define RET0_IF_0(a) if (!a) goto ret0
#define CANON_ELEMENT(c) TOLOWER (c)
@@ -33,10 +33,11 @@
#define RETURN_TYPE char *
#define AVAILABLE(h, h_l, j, n_l) \
- (!memchr ((h) + (h_l), '\0', (j) + (n_l) - (h_l)) \
- && ((h_l) = (j) + (n_l)))
+ (((j) + (n_l) <= (h_l)) || ((h_l) += strnlen ((void*)((h) + (h_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