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

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Message-ID: <457C80F0.2080902@eyal.emu.id.au>
Date:	Mon, 11 Dec 2006 08:49:36 +1100
From:	Eyal Lebedinsky <eyal@...l.emu.id.au>
To:	Folkert van Heusden <folkert@...heusden.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: optimalisation for strlcpy (lib/string.c)

Folkert van Heusden wrote:
> Hi,
> 
> Like the other patch (by that other person), I think it is faster to not
> do a strlen first.
> E.g. replace this:
> ize_t strlcpy(char *dest, const char *src, size_t size)
> {
>         size_t ret = strlen(src);
> 
>         if (size) {
>                 size_t len = (ret >= size) ? size - 1 : ret;
>                 memcpy(dest, src, len);
>                 dest[len] = '\0';
>         }
>         return ret;
> }
> 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;
> }
> patch:
> 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 22:22:08.000000000 +0100
> @@ -121,14 +121,24 @@
>   */
>  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';
> +        for(;;)
> +        {
> +                *dest = *src;
> +                if (!*src)
> +                        break;
> +
> +                if (--size == 0)
> +                        break;
> +
> +                dest++;
> +                src++;
>         }
> -       return ret;
> +
> +        *dest = 0x00;
> +
> +        return dest - tmp;
>  }
>  EXPORT_SYMBOL(strlcpy);
>  #endif
> 
> 
> 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.
> 
> 
> Signed-off by: Folkert van Heusden <folkert@...heusden.com>
> 
> 
> Folkert van Heusden

The two do not do exactly the same. The first one handles 'size == 0'
(should not happen?) safely while the other does not.

-- 
Eyal Lebedinsky (eyal@...l.emu.id.au) <http://samba.org/eyal/>
	attach .zip as .dat
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