linux-kernel - Re: optimalisation for strlcpy (lib/string.c)

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Message-ID: <20061210220326.GH30197@vanheusden.com>
Date:	Sun, 10 Dec 2006 23:03:27 +0100
From:	Folkert van Heusden <folkert@...heusden.com>
To:	Eyal Lebedinsky <eyal@...l.emu.id.au>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: optimalisation for strlcpy (lib/string.c)

> > by this:
> > size_t strlcpy(char *dest, const char *src, size_t size)
> > {
> >         char *tmp = dest;
> > 
> >         for(;;)
> >         {
> >                 *dest = *src;
> >                 if (!*src)
> >                         break;
> > 
> >                 if (--size == 0)
> >                         break;
> > 
> >                 dest++;
> >                 src++;
> >         }
> > 
> >         *dest = 0x00;
> > 
> >         return dest - tmp;
> > }
> > 
> > I've tested the speed difference with this:
> > http://www.vanheusden.com/misc/kernel-strlcpy-opt-test.c
> > and the speed difference is quite a bit on a P4: 28% faster.
> 
> The two do not do exactly the same. The first one handles 'size == 0'
> (should not happen?) safely while the other does not.

Ok, small change:
diff -uNrBbd lib/string.c string-new.c
--- lib/string.c        2006-11-04 02:33:58.000000000 +0100
+++ string-new.c        2006-12-10 23:02:26.000000000 +0100
@@ -121,14 +122,27 @@
  */
 size_t strlcpy(char *dest, const char *src, size_t size)
 {
-       size_t ret = strlen(src);
+        char *tmp = dest;

-       if (size) {
-               size_t len = (ret >= size) ? size - 1 : ret;
-               memcpy(dest, src, len);
-               dest[len] = '\0';
+       if (likeley(size > 0))
+       {
+               for(;;)
+               {
+                       *dest = *src;
+                       if (unlikely(!*src))
+                               break;
+
+                       if (unlikely(--size == 0))
+                               break;
+
+                       dest++;
+                       src++;
        }
-       return ret;
+       }
+
+       *dest = 0x00;
+
+       return dest - tmp;
 }
 EXPORT_SYMBOL(strlcpy);
 #endif


Folkert van Heusden

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