From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Kastrup Subject: Re: [PATCH v3] skip_prefix: rewrite so that prefix is scanned once Date: Tue, 04 Mar 2014 01:09:39 +0100 Message-ID: <877g8akenw.fsf@fencepost.gnu.org> References: <1393816384-3300-1-git-send-email-siddharth98391@gmail.com> Mime-Version: 1.0 Content-Type: text/plain Cc: Junio C Hamano , Siddharth Goel , Git Mailing List , Eric Sunshine To: Duy Nguyen X-From: git-owner@vger.kernel.org Tue Mar 04 01:09:49 2014 Return-path: Envelope-to: gcvg-git-2@plane.gmane.org Received: from vger.kernel.org ([209.132.180.67]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1WKcva-0002Uo-55 for gcvg-git-2@plane.gmane.org; Tue, 04 Mar 2014 01:09:46 +0100 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755762AbaCDAJl (ORCPT ); Mon, 3 Mar 2014 19:09:41 -0500 Received: from fencepost.gnu.org ([208.118.235.10]:35070 "EHLO fencepost.gnu.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755711AbaCDAJl (ORCPT ); Mon, 3 Mar 2014 19:09:41 -0500 Received: from localhost ([127.0.0.1]:34110 helo=lola) by fencepost.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WKcvU-0001Uv-3H; Mon, 03 Mar 2014 19:09:40 -0500 Received: by lola (Postfix, from userid 1000) id C039DE065F; Tue, 4 Mar 2014 01:09:39 +0100 (CET) In-Reply-To: (Duy Nguyen's message of "Tue, 4 Mar 2014 06:37:25 +0700") User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux) Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Archived-At: Duy Nguyen writes: > On Tue, Mar 4, 2014 at 5:43 AM, Junio C Hamano wrote: >> diff --git a/git-compat-util.h b/git-compat-util.h >> index cbd86c3..68ffaef 100644 >> --- a/git-compat-util.h >> +++ b/git-compat-util.h >> @@ -357,8 +357,14 @@ extern int suffixcmp(const char *str, const char *suffix); >> >> static inline const char *skip_prefix(const char *str, const char *prefix) >> { >> - size_t len = strlen(prefix); >> - return strncmp(str, prefix, len) ? NULL : str + len; > > Just a note. gcc does optimize strlen("abcdef") to 6, and with that > information at compile time built-in strncmp might do better. Indeed, most (but not all) of the calls have a constant string as prefix. However, strncmp in each iteration checks for both *str as well as *prefix to be different from '\0' independently (and it appears unlikely to me that the optimizer will figure out that it's unnecessary for either) _and_ compares them for equality so it's not likely to be faster than the open-coded loop. One could, however, use memcmp instead of strncmp. I'm just not sure whether memcmp is guaranteed not to peek beyond the first mismatching byte even if the count would allow for more. It could lead to undefined behavior if the first mismatching byte would be the ending NUL byte of str. -- David Kastrup