From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-3.8 required=3.0 tests=AWL,BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI shortcircuit=no autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 5B23F1F954 for ; Wed, 22 Aug 2018 17:04:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727282AbeHVU3x (ORCPT ); Wed, 22 Aug 2018 16:29:53 -0400 Received: from mail-qt0-f172.google.com ([209.85.216.172]:40288 "EHLO mail-qt0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726852AbeHVU3x (ORCPT ); Wed, 22 Aug 2018 16:29:53 -0400 Received: by mail-qt0-f172.google.com with SMTP id h4-v6so2926994qtj.7 for ; Wed, 22 Aug 2018 10:04:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-transfer-encoding:content-language; bh=MyR6wxpeWrBH0uUkWH5elv+KEo0TAJlHFzfzaKRNRI0=; b=q5X9xG/42wgPM/EBh8Bj348ZT4a7a5wRb/K+XiUDw9B7LJJ6fZ346GfkHFiCJylsfD LmGxH2Zv4MjfhMKWhQZu6V32m8+wmQEonsUqsB2zP9l6H/1cnfy6Ac++3SV/WFzGagp4 RBEOdFLu+IWKl4Jgtyf6+U/iX7nFOVDcGu9hkq+hIkepC618oYISkzqmkGbpoA0AgM8k e3a1wex20EI4Iqvnm5PhO4kTOKaHeO9svIp5nqCnAggkhoER8EJrBJpcdoUYAN02oeM9 NKIy5jGfo3CQkm+F36cHnHPc/+5Y3QvqF3JEuM2sF4Po05xdlrif14sXLh6gehTr0WL+ lBPA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-transfer-encoding :content-language; bh=MyR6wxpeWrBH0uUkWH5elv+KEo0TAJlHFzfzaKRNRI0=; b=nECxCmrYN2cO0MzTYHArFube12KjxuNoE2mQwq6Dzz51QerwYJ3IN6vF9mr+zxupim Z5vmOb0jJ0KhyTVTgJNOY0HRlQc6KT3oJsIXmQFOV0UIOrqm1e3SV90XmsCRQBiimkva h14ovKbaU1rVRNCi/4wzCaevcjPY98hXLXT9lOv/ChxGqpcPoxGdFF/TXe8AijFmB35K rFyATAFTX/oNxDD3i8mCrUOoymB+yYc/M0do9UfDHlC8be8odONt3YXMgmefz653BqoP 4/I9DLohwppWSY+i2qmMKKBQJKWKMitQp4fMREeXrtZFoUW+APQ9ufVHXH5a/611BKoI lZ8Q== X-Gm-Message-State: AOUpUlG2wyA4YaHDD4grkmZ394lp1BhWP149EKmhgK+ciS+hP1J3REtI YdLfT128riESFg1z4DnlbUq6i8SN X-Google-Smtp-Source: AA+uWPxzomKH/gMIq9OajKlkATB0sfWRbqJ4sG7xt9crU2fJplndJIXG+e1Jf6DGzrDTtnBbx2wzpg== X-Received: by 2002:ac8:198c:: with SMTP id u12-v6mr10047906qtj.86.1534957451092; Wed, 22 Aug 2018 10:04:11 -0700 (PDT) Received: from [10.0.1.17] ([98.122.163.216]) by smtp.gmail.com with ESMTPSA id p57-v6sm1373721qtj.42.2018.08.22.10.04.09 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 22 Aug 2018 10:04:10 -0700 (PDT) Subject: Re: [ANNOUNCE] Git v2.19.0-rc0 To: Duy Nguyen Cc: Jeff King , =?UTF-8?B?w4Z2YXIgQXJuZmrDtnLDsCBCamFybWFz?= =?UTF-8?Q?on?= , "brian m. carlson" , Junio C Hamano , Git Mailing List References: <20180821212923.GB24431@sigill.intra.peff.net> <20180822004815.GA535143@genre.crustytoothpaste.net> <20180822030344.GA14684@sigill.intra.peff.net> <20180822053626.GB535143@genre.crustytoothpaste.net> <20180822060735.GA13195@sigill.intra.peff.net> <20180822151703.GB32630@sigill.intra.peff.net> <20180822162609.GA11904@sigill.intra.peff.net> <7ea416cf-b043-1274-e161-85a8780b8e1c@gmail.com> From: Derrick Stolee Message-ID: <387295e3-3112-ed09-d988-3d8156e3c7ce@gmail.com> Date: Wed, 22 Aug 2018 13:04:08 -0400 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On 8/22/2018 12:58 PM, Duy Nguyen wrote: > On Wed, Aug 22, 2018 at 6:49 PM Derrick Stolee wrote: >> On 8/22/2018 12:26 PM, Jeff King wrote: >>> On Wed, Aug 22, 2018 at 06:14:24PM +0200, Duy Nguyen wrote: >>> >>>> On Wed, Aug 22, 2018 at 6:08 PM Duy Nguyen wrote: >>>>> On Wed, Aug 22, 2018 at 6:03 PM Jeff King wrote: >>>>>> On Wed, Aug 22, 2018 at 07:14:42AM -0400, Derrick Stolee wrote: >>>>>> >>>>>>> The other thing I was going to recommend (and I'll try to test this out >>>>>>> myself later) is to see if 'the_hash_algo->rawsz' is being treated as a >>>>>>> volatile variable, since it is being referenced through a pointer. Perhaps >>>>>>> storing the value locally and then casing on it would help? >>>>>> I tried various sprinkling of "const" around the declarations to make it >>>>>> clear that the values wouldn't change once we saw them. But I couldn't >>>>>> detect any difference. At most I think that would let us hoist the "if" >>>>>> out of the loop, but gcc still seems unwilling to expand the memcmp when >>>>>> there are other branches. >>>>>> >>>>>> I think if that's the thing we want to have happen, we really do need to >>>>>> just write it out on that branch rather than saying "memcmp". >>>>> This reminds me of an old discussion about memcpy() vs doing explicit >>>>> compare loop with lots of performance measurements.. >>>> Ah found it. Not sure if it is still relevant in light of multiple hash support >>>> >>>> https://public-inbox.org/git/20110427225114.GA16765@elte.hu/ >>> Yes, that was what I meant. We actually did switch to that hand-rolled >>> loop, but later we went back to memcmp in 0b006014c8 (hashcmp: use >>> memcmp instead of open-coded loop, 2017-08-09). >> Looking at that commit, I'm surprised the old logic was just a for loop, instead of a word-based approach, such as the following: > Might work on x86 but it breaks on cpu architectures with stricter > alignment. I don't think we have a guarantee that object_id is always > 8 byte aligned. You (and Peff) are probably correct here, which is unfortunate. I'm not familiar with alignment constraints, but assume that such a word-based approach is best. Thanks, -Stolee