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Benchmark strstr hard needles

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Benchmark needles which exhibit worst-case performance.  This shows that
basic_strstr is quadratic and thus unsuitable for large needles.
On the other hand the Two-way and new strstr implementations are linear with
increasing needle sizes.  The slowest cases of the two implementations are
within a factor of 2 on several different microarchitectures.  Two-way is
slowest on inputs which cause a branch mispredict on almost every character.
The new strstr is slowest on inputs which almost match and result in many
calls to memcmp.  Thanks to Szabolcs for providing various hard needles.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

	* benchtests/bench-strstr.c (test_hard_needle): New function.
This commit is contained in:
Wilco Dijkstra 2019-06-11 15:52:21 +01:00
parent e6e2424390
commit 80b2bfb535
2 changed files with 83 additions and 0 deletions
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4
ChangeLog
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@ -1,3 +1,7 @@
2019-06-11 Wilco Dijkstra <wdijkstr@arm.com>
* benchtests/bench-strstr.c (test_hard_needle): New function.
2019-06-10 Joseph Myers <joseph@codesourcery.com>
* malloc/tst-calloc.c: Include <libc-diag.h>.

79
benchtests/bench-strstr.c
View File

@ -203,6 +203,81 @@ do_test (size_t align1, size_t align2, size_t len1, size_t len2,
putchar ('\n');
}
/* Test needles which exhibit worst-case performance. This shows that
basic_strstr is quadratic and thus unsuitable for large needles.
On the other hand Two-way and skip table implementations are linear with
increasing needle sizes. The slowest cases of the two implementations are
within a factor of 2 on several different microarchitectures. */
static void
test_hard_needle (size_t ne_len, size_t hs_len)
{
char *ne = (char *) buf1;
char *hs = (char *) buf2;
/* Hard needle for strstr algorithm using skip table. This results in many
memcmp calls comparing most of the needle. */
{
memset (ne, 'a', ne_len);
ne[ne_len] = '\0';
ne[ne_len - 14] = 'b';
memset (hs, 'a', hs_len);
for (size_t i = ne_len; i <= hs_len; i += ne_len)
{
hs[i-5] = 'b';
hs[i-62] = 'b';
}
printf ("Length %4zd/%3zd, complex needle 1:", hs_len, ne_len);
FOR_EACH_IMPL (impl, 0)
do_one_test (impl, hs, ne, NULL);
putchar ('\n');
}
/* 2nd hard needle for strstr algorithm using skip table. This results in
many memcmp calls comparing most of the needle. */
{
memset (ne, 'a', ne_len);
ne[ne_len] = '\0';
ne[ne_len - 6] = 'b';
memset (hs, 'a', hs_len);
for (size_t i = ne_len; i <= hs_len; i += ne_len)
{
hs[i-5] = 'b';
hs[i-6] = 'b';
}
printf ("Length %4zd/%3zd, complex needle 2:", hs_len, ne_len);
FOR_EACH_IMPL (impl, 0)
do_one_test (impl, hs, ne, NULL);
putchar ('\n');
}
/* Hard needle for Two-way algorithm - the random input causes a large number
of branch mispredictions which significantly reduces performance on modern
micro architectures. */
{
for (int i = 0; i < hs_len; i++)
hs[i] = (rand () & 255) > 155 ? 'a' : 'b';
hs[hs_len] = 0;
memset (ne, 'a', ne_len);
ne[ne_len-2] = 'b';
ne[0] = 'b';
ne[ne_len] = 0;
printf ("Length %4zd/%3zd, complex needle 3:", hs_len, ne_len);
FOR_EACH_IMPL (impl, 0)
do_one_test (impl, hs, ne, NULL);
putchar ('\n');
}
}
static int
test_main (void)
{
@ -227,6 +302,10 @@ test_main (void)
do_test (14, 5, hlen, klen, 1);
}
test_hard_needle (64, 65536);
test_hard_needle (256, 65536);
test_hard_needle (1024, 65536);
return ret;
}