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| -rw-r--r-- | ChangeLog | 4 | ||||
| -rw-r--r-- | benchtests/bench-strstr.c | 79 |
2 files changed, 83 insertions, 0 deletions
diff --git a/ChangeLog b/ChangeLog index e3f60702ea..2c9836d4e6 100644 --- a/ChangeLog +++ b/ChangeLog @@ -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>. diff --git a/benchtests/bench-strstr.c b/benchtests/bench-strstr.c index b4cd141083..2cbe13e9ac 100644 --- a/benchtests/bench-strstr.c +++ b/benchtests/bench-strstr.c @@ -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; } |