[PATCH] Avoid double bounds check at the start of the loop

[PATCH] Avoid double bounds check at the start of the loop

[Wilco Dijkstra]

Hi James,

So the change you're proposing is correct, but did you benchmark it?
The code is written like it is for a good reason: the checks at the start of
the loop avoid one taken branch per loop iteration. So eventhough there
is an unnecessary branch in the very first iteration, performance actually
improves significantly by doing it this way.

Cheers,
Wilco

[PATCH] Avoid double bounds check at the start of the loop

[James Tirta Halim]

---
 string/memmem.c | 4 ++--
 string/strstr.c | 2 ++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/string/memmem.c b/string/memmem.c
index 13df3bcdd4..dea3e5a29c 100644
--- a/string/memmem.c
+++ b/string/memmem.c
@@ -92,7 +92,7 @@ __memmem (const void *haystack, size_t hs_len,
   shift1 = m1 - shift[hash2 (ne + m1)];
   shift[hash2 (ne + m1)] = m1;
 
-  for ( ; hs <= end; )
+  do
     {
       /* Skip past character pairs not in the needle.  */
       do
@@ -121,7 +121,7 @@ __memmem (const void *haystack, size_t hs_len,
 
       /* Skip based on matching the hash of the needle end.  */
       hs += shift1;
-    }
+    } while (hs <= end);
   return NULL;
 }
 libc_hidden_def (__memmem)
diff --git a/string/strstr.c b/string/strstr.c
index 23618e2eb2..7ec812b931 100644
--- a/string/strstr.c
+++ b/string/strstr.c
@@ -121,6 +121,7 @@ STRSTR (const char *haystack, const char *needle)
   shift1 = m1 - shift[hash2 (ne + m1)];
   shift[hash2 (ne + m1)] = m1;
 
+  goto start;
   while (1)
     {
       if (__glibc_unlikely (hs > end))
@@ -130,6 +131,7 @@ STRSTR (const char *haystack, const char *needle)
 	    return NULL;
 	}
 
+start:
       /* Skip past character pairs not in the needle.  */
       do
 	{
-- 
2.42.0