From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 69844 invoked by alias); 3 Oct 2016 12:26:10 -0000 Mailing-List: contact gcc-patches-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-patches-owner@gcc.gnu.org Received: (qmail 69834 invoked by uid 89); 3 Oct 2016 12:26:09 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_PASS autolearn=ham version=3.3.2 spammy= X-HELO: mail-qt0-f194.google.com Received: from mail-qt0-f194.google.com (HELO mail-qt0-f194.google.com) (209.85.216.194) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 03 Oct 2016 12:26:08 +0000 Received: by mail-qt0-f194.google.com with SMTP id m5so1673610qtb.0 for ; Mon, 03 Oct 2016 05:26:08 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:sender:subject:to:references:cc:from:message-id :date:user-agent:mime-version:in-reply-to:content-transfer-encoding; bh=7NGlmKOMHyquWlfE5DbPl41Tkci7HkwSbuTotvyuyuQ=; b=DN7sCzUlrbok4VaV7DMdfDDga/yt2btVa27oxHxpNRZFpZi9qPjjhBUdfE3w2Mbi9A ZXoeF3c7a/z9BHSrDYRZ4C/KB1keonW/3hJctCSY8aWO84/iaYCTLxgypTzjH7eyFW67 ZcnGbpbZGrxQz9xkzBI6infWmWMmw2JoLp6iI63PsObJRT/fBo318MX/NdxUwwgkNbiY te6m23D8gBOIGSbjDd2fwjijAg9coqzWu33skd2V+6vBK7E7s4FT1/ECcW+xb2k0dwRu ixDRk/CTAxrqTgo20uNT6XjmEgaGAn3CpQpaOHffxMfByjNvShuzinw0ErqQ9Y+uKtjw +nGQ== X-Gm-Message-State: AA6/9Rl71qdzlmh+m1//iTEPJPB9bRTS1ocTbv5CFqjn7GievsEzK/QXTsCVGJ7fVhhsnA== X-Received: by 10.200.50.78 with SMTP id y14mr4209610qta.57.1475497567047; Mon, 03 Oct 2016 05:26:07 -0700 (PDT) Received: from ?IPv6:2601:181:c003:1930:3fe6:c217:b86a:6e86? ([2601:181:c003:1930:3fe6:c217:b86a:6e86]) by smtp.googlemail.com with ESMTPSA id o128sm16853455qkf.17.2016.10.03.05.26.05 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 03 Oct 2016 05:26:05 -0700 (PDT) Subject: Re: [PATCH] Set -fprofile-update=atomic when -pthread is present To: =?UTF-8?Q?Martin_Li=c5=a1ka?= , Jeff Law , Andi Kleen References: <87f2bc4f-c4df-eadd-aec6-a937ed0ccaba@acm.org> <1253ac69-3301-f185-e43a-a34cadf8f51e@suse.cz> <67fda6d2-9b3e-a0d1-effc-34e1115030b2@acm.org> <1ff3cc75-7cee-79f3-395b-ef7a4d286a3d@acm.org> <04a05835-4666-4d7d-c1a9-d4bcc4ea924a@suse.cz> <87k2fpdatl.fsf@tassilo.jf.intel.com> <6f8b1905-818b-bfff-1bf3-5ba04f3b4b64@suse.cz> <20160818155130.GE5871@two.firstfloor.org> <631cf1bd-8ae9-f07b-5672-5084b699f650@redhat.com> <2c750f4c-c96c-2b22-43ae-53bbebf18af8@suse.cz> Cc: gcc-patches@gcc.gnu.org, jh@suse.cz From: Nathan Sidwell Message-ID: <73aa44d7-5287-e4b8-6188-a87d52d3d6b9@acm.org> Date: Mon, 03 Oct 2016 12:26:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0 MIME-Version: 1.0 In-Reply-To: <2c750f4c-c96c-2b22-43ae-53bbebf18af8@suse.cz> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 8bit X-SW-Source: 2016-10/txt/msg00062.txt.bz2 On 10/03/16 08:13, Martin Liška wrote: > On 08/18/2016 05:53 PM, Jeff Law wrote: >> On 08/18/2016 09:51 AM, Andi Kleen wrote: >>>> I'd prefer to make updates atomic in multi-threaded applications. >>>> The best proxy we have for that is -pthread. >>>> >>>> Is it slower, most definitely, but odds are we're giving folks >>>> garbage data otherwise, which in many ways is even worse. >>> >>> It will likely be catastrophically slower in some cases. >>> >>> Catastrophically as in too slow to be usable. >>> >>> An atomic instruction is a lot more expensive than a single increment. Also >>> they sometimes are really slow depending on the state of the machine. >> And for those cases there's a way to override. >> >> The default should be set for correctness. >> >> jeff > > I would to somehow resolve the discussion related to default value selection. > Is the prevailing consensus that we should set -fprofile-update=atomic when > -pthread is set? If so, I'll prepare a patch. I tend to do it this way. This is my preference. nathan