linux-kernel - Re: [PATCH] un-improve strrchr()

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Message-ID: <1435514515.9587.46.camel@perches.com>
Date:	Sun, 28 Jun 2015 11:01:55 -0700
From:	Joe Perches <joe@...ches.com>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux@...musvillemoes.dk
Subject: Re: [PATCH] un-improve strrchr()

On Sun, 2015-06-28 at 19:44 +0300, Alexey Dobriyan wrote:
> Commit 8da53d4595a53fb9a3380dd4d1c9bc24c7c9aab8
> ("lib/string.c: improve strrchr()") changed strrchr() implementation
> from "rewind to the end and search backwards" to "search forward"
> optimizing for characher not found case. However, common case is exactly
> the opposite: string is absolute pathname, c is '/' always to be found.
> 
> Previous code did 1 branch per character + 1 branch for every character
> in the last path component. Current code does 2 branches per characher
> regardless.

Are you comparing total cycles of all of the branches
in the called functions too?

As written the current version removes the strlen call.

> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -313,12 +313,13 @@ EXPORT_SYMBOL(strchrnul);
>   */
>  char *strrchr(const char *s, int c)
>  {
> -	const char *last = NULL;
> +	const char *p = s + strlen(s);
> +
>  	do {
> -		if (*s == (char)c)
> -			last = s;
> -	} while (*s++);
> -	return (char *)last;
> +		if (*p == (char)c)
> +			return (char *)p;
> +	} while (--p >= s);
> +	return NULL;
>  }
>  EXPORT_SYMBOL(strrchr);
>  #endif


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