[PATCH] Improve stpncpy performance

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

Like strncpy, this patch improves stpncpy performance by using strnlen/memcpy/memset rather than a
byte loop. Performance on bench-stpncpy is ~2x faster on average.

ChangeLog:

2015-01-12 Wilco Dijkstra wdijkstr@arm.com

	* string/stpncpy.c (stpncpy): Improve performance using
	__strnlen/memcpy/memset.

---
 string/stpncpy.c | 59 ++++++--------------------------------------------------
 1 file changed, 6 insertions(+), 53 deletions(-)

diff --git a/string/stpncpy.c b/string/stpncpy.c
index fad747e..50521aa 100644
--- a/string/stpncpy.c
+++ b/string/stpncpy.c
@@ -15,7 +15,7 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-/* This is almost copied from strncpy.c, written by Torbjorn Granlund.  */
+#include <stddef.h>
 
 #ifdef HAVE_CONFIG_H
 # include <config.h>
@@ -41,59 +41,12 @@ weak_alias (__stpncpy, stpncpy)
 char *
 STPNCPY (char *dest, const char *src, size_t n)
 {
-  char c;
-  char *s = dest;
-
-  if (n >= 4)
-    {
-      size_t n4 = n >> 2;
-
-      for (;;)
-	{
-	  c = *src++;
-	  *dest++ = c;
-	  if (c == '\0')
-	    break;
-	  c = *src++;
-	  *dest++ = c;
-	  if (c == '\0')
-	    break;
-	  c = *src++;
-	  *dest++ = c;
-	  if (c == '\0')
-	    break;
-	  c = *src++;
-	  *dest++ = c;
-	  if (c == '\0')
-	    break;
-	  if (--n4 == 0)
-	    goto last_chars;
-	}
-      n -= dest - s;
-      goto zero_fill;
-    }
-
- last_chars:
-  n &= 3;
-  if (n == 0)
+  size_t size = __strnlen (src, n);
+  memcpy (dest, src, size);
+  dest += size;
+  if (size == n)
     return dest;
-
-  for (;;)
-    {
-      c = *src++;
-      --n;
-      *dest++ = c;
-      if (c == '\0')
-	break;
-      if (n == 0)
-	return dest;
-    }
-
- zero_fill:
-  while (n-- > 0)
-    dest[n] = '\0';
-
-  return dest - 1;
+  return memset (dest, '\0', n - size);
 }
 #ifdef weak_alias
 libc_hidden_def (__stpncpy)
-- 
1.9.1