[PATCH] Use Quicksearch in strstr

[Wilco Dijkstra <Wilco dot Dijkstra at arm dot com>]

This patch significantly improves performance of strstr by using Sunday's
Quick-Search algorithm.  Due to its simplicity it has the best average
performance of string matching algorithms on almost all inputs.  It uses a
bad-character shift table to skip past mismatches.

The needle length is limited to 254 - combined with a hashed shift table
this reduces the shift table memory 16 to 32 times, lowering preprocessing
overhead and minimizing cache effects.  The needle limit also implies
worst-case performance remains linear.

Small 1-4 byte needles use special case code which is typically faster.
Very long needles continue to use the linear-time Two-Way algorithm.

The performance gain using the improved bench-strstr on Cortex-A72 is 2.1x.
Due to the low overhead the gain is larger on small haystacks: haystacks of 64
chars are ~3 times faster for needles of 5-16 chars.

Tested against GLIBC testsuite, randomized tests and the GNULIB strstr test
(https://git.savannah.gnu.org/cgit/gnulib.git/tree/tests/test-strstr.c).

ChangeLog:
2018-10-29  Wilco Dijkstra  <wdijkstr@arm.com>

	* string/str-two-way.h (two_way_short_needle): Add inline to avoid warning.
	* string/strstr.c (strstr2): Add new function.
	(strstr3): Likewise.
	(strstr3): Likewise.
	(STRSTR): Completely rewrite strstr to improve performance

---

diff --git a/string/str-two-way.h b/string/str-two-way.h
index 523d946c59412e1f1f65b8ec3778553b83191952..31c3f18fb057cdd999c3ac9e9d894a8b62a98a70 100644
--- a/string/str-two-way.h
+++ b/string/str-two-way.h
@@ -221,7 +221,7 @@ critical_factorization (const unsigned char *needle, size_t needle_len,
    most 2 * HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.
    If AVAILABLE modifies HAYSTACK_LEN (as in strstr), then at most 3 *
    HAYSTACK_LEN - NEEDLE_LEN comparisons occur in searching.  */
-static RETURN_TYPE
+static inline RETURN_TYPE
 two_way_short_needle (const unsigned char *haystack, size_t haystack_len,
 		      const unsigned char *needle, size_t needle_len)
 {
diff --git a/string/strstr.c b/string/strstr.c
index f74d7189ed1319f6225525cc2d32380745de1523..206f20ddfcf10f61ba68725b516ef00356e9028c 100644
--- a/string/strstr.c
+++ b/string/strstr.c
@@ -22,11 +22,8 @@
 # include <config.h>
 #endif
 
-/* Specification of strstr.  */
 #include <string.h>
 
-#include <stdbool.h>
-
 #ifndef _LIBC
 # define __builtin_expect(expr, val)   (expr)
 #endif
@@ -47,46 +44,115 @@
 #define STRSTR strstr
 #endif
 
-/* Return the first occurrence of NEEDLE in HAYSTACK.  Return HAYSTACK
-   if NEEDLE is empty, otherwise NULL if NEEDLE is not found in
-   HAYSTACK.  */
-char *
-STRSTR (const char *haystack, const char *needle)
+static inline char *
+strstr2 (const unsigned char *hs, const unsigned char *ne)
 {
-  size_t needle_len; /* Length of NEEDLE.  */
-  size_t haystack_len; /* Known minimum length of HAYSTACK.  */
+  uint32_t h1 = (ne[0] << 16) | ne[1];
+  uint32_t h2 = 0;
+  for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
+      h2 = (h2 << 16) | c;
+  return h1 == h2 ? (char *)hs - 2 : NULL;
+}
 
-  /* Handle empty NEEDLE special case.  */
-  if (needle[0] == '\0')
-    return (char *) haystack;
+static inline char *
+strstr3 (const unsigned char *hs, const unsigned char *ne)
+{
+  uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
+  uint32_t h2 = 0;
+  for (int c = hs[0]; h1 != h2 && c != 0; c = *++hs)
+      h2 = (h2 | c) << 8;
+  return h1 == h2 ? (char *)hs - 3 : NULL;
+}
 
-  /* Skip until we find the first matching char from NEEDLE.  */
-  haystack = strchr (haystack, needle[0]);
-  if (haystack == NULL || needle[1] == '\0')
-    return (char *) haystack;
+static inline char *
+strstr4 (const unsigned char *hs, const unsigned char *ne)
+{
+  uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8) | ne[3];
+  uint32_t h2 = 0;
+  for (int c = hs[0]; c != 0 && h1 != h2; c = *++hs)
+    h2 = (h2 << 8) | c;
+  return h1 == h2 ? (char *)hs - 4 : NULL;
+}
 
-  /* Ensure HAYSTACK length is at least as long as NEEDLE length.
-     Since a match may occur early on in a huge HAYSTACK, use strnlen
+/* Number of bits used to index shift table.  */
+#define SHIFT_TABLE_BITS 6
+
+/* Extremely fast strstr algorithm with guaranteed linear-time performance.
+   Small needles up to size 4 use a dedicated linear search.  Longer needles
+   up to size 254 use Sunday's Quick-Search algorithm.  Due to its simplicity
+   it has the best average performance of string matching algorithms on almost
+   all inputs.  It uses a bad-character shift table to skip past mismatches.
+   By limiting the needle length to 254, the shift table can be reduced to 8
+   bits per entry, lowering preprocessing overhead and minimizing cache effects.
+   The limit also implies the worst-case performance is linear.
+   Even larger needles are processed by the linear-time Two-Way algorithm.
+*/
+char *
+STRSTR (const char *haystack, const char *needle)
+{
+  const unsigned char *hs = (const unsigned char *) haystack;
+  const unsigned char *ne = (const unsigned char *) needle;
+
+  /* Handle short needle special cases first.  */
+  if (ne[0] == '\0')
+    return (char *)hs;
+  hs = (const unsigned char *)strchr ((const char*)hs, ne[0]);
+  if (hs == NULL || ne[1] == '\0')
+    return (char*)hs;
+  if (ne[2] == '\0')
+    return strstr2 (hs, ne);
+  if (ne[3] == '\0')
+    return strstr3 (hs, ne);
+  if (ne[4] == '\0')
+    return strstr4 (hs, ne);
+
+  /* Ensure haystack length is at least as long as needle length.
+     Since a match may occur early on in a huge haystack, use strnlen
      and read ahead a few cachelines for improved performance.  */
-  needle_len = strlen (needle);
-  haystack_len = __strnlen (haystack, needle_len + 256);
-  if (haystack_len < needle_len)
+  size_t ne_len = strlen ((const char*)ne);
+  size_t hs_len = strnlen ((const char*)hs, ne_len | 512);
+  if (hs_len < ne_len)
     return NULL;
 
-  /* Check whether we have a match.  This improves performance since we avoid
-     the initialization overhead of the two-way algorithm.  */
-  if (memcmp (haystack, needle, needle_len) == 0)
-    return (char *) haystack;
-
-  /* Perform the search.  Abstract memory is considered to be an array
-     of 'unsigned char' values, not an array of 'char' values.  See
-     ISO C 99 section 6.2.6.1.  */
-  if (needle_len < LONG_NEEDLE_THRESHOLD)
-    return two_way_short_needle ((const unsigned char *) haystack,
-				 haystack_len,
-				 (const unsigned char *) needle, needle_len);
-  return two_way_long_needle ((const unsigned char *) haystack, haystack_len,
-			      (const unsigned char *) needle, needle_len);
+  /* Check whether we have a match.  This improves performance since we
+     avoid initialization overheads.  */
+  if (memcmp (hs, ne, ne_len) == 0)
+    return (char *) hs;
+
+  /* Use the Quick-Search algorithm for needle lengths less than 255.  */
+  if (__glibc_likely (ne_len < 255))
+    {
+      uint8_t shift[1 << SHIFT_TABLE_BITS];
+      const unsigned char *end = hs + hs_len - ne_len;
+
+      /* Initialize bad character shift hash table.  */
+      memset (shift, ne_len + 1, sizeof (shift));
+      for (int i = 0; i < ne_len; i++)
+	shift[ne[i] % sizeof (shift)] = ne_len - i;
+
+      do
+	{
+	  hs--;
+
+	  /* Search by skipping past bad characters.  */
+	  size_t tmp = shift[hs[ne_len] % sizeof (shift)];
+	  for (hs += tmp; hs <= end; hs += tmp)
+	    {
+	      tmp = shift[hs[ne_len] % sizeof (shift)];
+	      if (memcmp (hs, ne, ne_len) == 0)
+		return (char*)hs;
+	    }
+	  if (end[ne_len] == '\0')
+	    return NULL;
+	  end += strnlen ((const char*)end + ne_len, 2048);
+	}
+      while (hs <= end);
+
+      return NULL;
+    }
+
+  /* Use Two-Way algorithm for very long needles.  */
+  return two_way_long_needle (hs, hs_len, ne, ne_len);
 }
 libc_hidden_builtin_def (strstr)