Re: [PATCH] Optimize prefixcmp()

From: Pierre Habouzit <madcoder@debian.org>
Date: Dec 30, 2007, 13:55 UTC
Message-ID: <20071230135557.GA25917@artemis.madism.org>
In-Reply-To: <e5bfff550712300502p543680b9jbeb9469a5a970f0@mail.gmail.com>

On Sun, Dec 30, 2007 at 01:02:28PM +0000, Marco Costalba wrote:

Show 31 quoted lines

> Subject: [PATCH] Certain codepaths (notably "git log --pretty=format...") use
> 
> prefixcmp() extensively, with very short prefixes.  In those cases,
> calling strlen() is a wasteful operation, so avoid it.
> 
> Initial patch by Johannes Schindelin.
> 
> Signed-off-by: Marco Costalba <mcostalba@gmail.com>
> ---
>  git-compat-util.h |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 79eb10e..843a8f5 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -398,7 +398,16 @@ static inline int sane_case(int x, int high)
> 
>  static inline int prefixcmp(const char *str, const char *prefix)
>  {
> -	return strncmp(str, prefix, strlen(prefix));
> +	do {
> +		if (*str != *prefix)
> +			return *(unsigned const char *)prefix - *(unsigned const char *)str;
> +
> +		if (!*(++prefix))
> +			return 0;
> +
> +		str++;
> +
> +	} while (1);

  This code doesn't work if prefix is "". You want something like:

    for (; *prefix; prefix++, str++) {
        if (*str != *prefix)
            return *(unsigned const char *)prefix - *(unsigned const char *)str;
    }
    return 0;

-- 
·O·  Pierre Habouzit
··O                                                madcoder@debian.org
OOO                                                http://www.madism.org