[PATCH] Improve strstr performance of short needles

[Wilco Dijkstra]

Improve strstr performance for the common case of short needles. For a single
character strchr is best, for 2-4 characters a small loop is fastest. For these
the speedup over the Two-Way algorithm is ~10 times on large strings.

Newlib builds, the new code passes GLIBC testsuite. OK for commit?

---

diff --git a/newlib/libc/string/strstr.c b/newlib/libc/string/strstr.c
index 580ad62720477a1a34d79e149962e9af0a13548d..925dc2e459cebed1266bd930567ffb5df5adc5bc 100644
--- a/newlib/libc/string/strstr.c
+++ b/newlib/libc/string/strstr.c
@@ -38,12 +38,11 @@ QUICKREF
# include "str-two-way.h"
#endif

+#if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
char *
strstr (const char *searchee,
const char *lookfor)
{
-#if defined(PREFER_SIZE_OVER_SPEED) || defined(__OPTIMIZE_SIZE__)
-
/* Less code size, but quadratic performance in the worst case. */
if (*searchee == 0)
{
@@ -77,6 +76,52 @@ strstr (const char *searchee,

#else /* compilation for speed */

+static inline char *
+strstr2 (const char *hs, const char *ne)
+{
+ uint32_t h1 = (ne[0] << 16) | ne[1];
+ uint32_t h2 = 0;
+ int c = hs[0];
+ while (h1 != h2 && c != 0)
+ {
+ h2 = (h2 << 16) | c;
+ c = *++hs;
+ }
+ return h1 == h2 ? (char *)hs - 2 : NULL;
+}
+
+static inline char *
+strstr3 (const char *hs, const char *ne)
+{
+ uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8);
+ uint32_t h2 = 0;
+ int c = hs[0];
+ while (h1 != h2 && c != 0)
+ {
+ h2 = (h2 | c) << 8;
+ c = *++hs;
+ }
+ return h1 == h2 ? (char *)hs - 3 : NULL;
+}
+
+static inline char *
+strstr4 (const char *hs, const char *ne)
+{
+ uint32_t h1 = (ne[0] << 24) | (ne[1] << 16) | (ne[2] << 8) | ne[3];
+ uint32_t h2 = 0;
+ int c = hs[0];
+ while (h1 != h2 && c != 0)
+ {
+ h2 = (h2 << 8) | c;
+ c = *++hs;
+ }
+ return h1 == h2 ? (char *)hs - 4 : NULL;
+}
+
+char *
+strstr (const char *searchee,
+ const char *lookfor)
+{
/* Larger code size, but guaranteed linear performance. */
const char *haystack = searchee;
const char *needle = lookfor;
@@ -84,6 +129,18 @@ strstr (const char *searchee,
size_t haystack_len; /* Known minimum length of HAYSTACK. */
int ok = 1; /* True if NEEDLE is prefix of HAYSTACK. */

+ /* Handle short needle special cases first. */
+ if (needle[0] == '\0')
+ return (char *) haystack;
+ if (needle[1] == '\0')
+ return strchr (haystack, needle[0]);
+ if (needle[2] == '\0')
+ return strstr2 (haystack, needle);
+ if (needle[3] == '\0')
+ return strstr3 (haystack, needle);
+ if (needle[4] == '\0')
+ return strstr4 (haystack, needle);
+
/* Determine length of NEEDLE, and in the process, make sure
HAYSTACK is at least as long (no point processing all of a long
NEEDLE if HAYSTACK is too short). */

[Corinna Vinschen]

Post by Wilco Dijkstra
Improve strstr performance for the common case of short needles. For a single
character strchr is best, for 2-4 characters a small loop is fastest. For these
the speedup over the Two-Way algorithm is ~10 times on large strings.
Newlib builds, the new code passes GLIBC testsuite. OK for commit?

Pushed with a small tweak.


Thanks,
Corinna