[glibc/zack/no-nested-includes] Benchmark strstr hard needles

[Zack Weinberg <zack at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=80b2bfb5350442ef1a781b0ee9dd44d61bd88f8a

commit 80b2bfb5350442ef1a781b0ee9dd44d61bd88f8a
Author: Wilco Dijkstra <wdijkstr@arm.com>
Date:   Tue Jun 11 15:52:21 2019 +0100

    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:
---
 ChangeLog                 |  4 +++
 benchtests/bench-strstr.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index e3f6070..2c9836d 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 b4cd141..2cbe13e 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;
 }