Benchmark strstr hard needles

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.

diff --git a/ChangeLog b/ChangeLog
index e3f60702ea278c3cee1aa774b1bb6cc6ca410424..2c9836d4e689c1cfe0bfe9938a8f6904007a311c 100644 (file)
--- 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 b4cd141083d65388dda042d195d7137e8f19480c..2cbe13e9ac06eed303cdb9d61da31f111d55a736 100644 (file)
--- 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;
 }