[glibc/ibm/2.28/master] Speedup first memmem match

[Tulio Magno Quites Machado Filho <tuliom@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=d6ccf2f45c5e09d3a36321ba695325589e8940fe

commit d6ccf2f45c5e09d3a36321ba695325589e8940fe
Author: Rajalakshmi Srinivasaraghavan <raji@linux.vnet.ibm.com>
Date:   Tue Aug 28 12:42:19 2018 +0530

    Speedup first memmem match
    
    As done in commit 284f42bc778e487dfd5dff5c01959f93b9e0c4f5, memcmp
    can be used after memchr to avoid the initialization overhead of the
    two-way algorithm for the first match.  This has shown improvement
    >40% for first match.
    
    (cherry picked from commit c8dd67e7c958de04c3783cbea7c384431707b5f8)

Diff:
---
 ChangeLog       | 4 ++++
 string/memmem.c | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 1455310..182e0ba 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2019-09-13  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>
+
+	* string/memmem.c: Use memcmp for first match.
+
 2019-09-13  Wilco Dijkstra  <wdijkstr@arm.com>
 
 	* string/strcasestr.c (STRCASESTR): Simplify and speedup first match.
diff --git a/string/memmem.c b/string/memmem.c
index 43efaa3..d72b824 100644
--- a/string/memmem.c
+++ b/string/memmem.c
@@ -70,6 +70,10 @@ __memmem (const void *haystack_start, size_t haystack_len,
       haystack_len -= haystack - (const unsigned char *) haystack_start;
       if (haystack_len < needle_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 (void *) haystack;
       return two_way_short_needle (haystack, haystack_len, needle, needle_len);
     }
   else