From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [216.205.24.124]) by sourceware.org (Postfix) with ESMTP id 1688C384A012 for ; Fri, 16 Jul 2021 16:54:50 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 1688C384A012 Received: from mail-wr1-f70.google.com (mail-wr1-f70.google.com [209.85.221.70]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-471-mHAzwa4qNb6ngocm03zZQg-1; Fri, 16 Jul 2021 12:54:43 -0400 X-MC-Unique: mHAzwa4qNb6ngocm03zZQg-1 Received: by mail-wr1-f70.google.com with SMTP id m4-20020adffa040000b02901404c442853so5082963wrr.12 for ; Fri, 16 Jul 2021 09:54:42 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=lUmGfaBZXdIkz8YKWY5dqu79EKPZqOVmEkgzvymfPWk=; b=OblPOyJ1KdqgPtF+bQK/jMrrE2empavO3Vx8xJpmzJHDMe43bbYCNlG9EQDGTinFwY UbADTU+BTx7SZWeBI6DSuortlLENfKDaMlZHheBpvVouQCK79z5MuAmUKvI7g13aW9b2 5tU6JKf3o3k4UlkQoJfIb5kcf4/f6Qi4tdvEi5Ajw6eZqu7EONX0/fB/IGL1WOfDhPmD rqi3kEbRF6X80gMdNWPkq0HQfmdrYwnLKNyGLELmZ0VXbVBchi3X4ybM5+jqx9YcH5LU jSHSuquxM/CteNTtHHH3OvzkA5VUVenMq0xUpmYqlfuhgIW37ACYx9xrOAKXh/L/p8qJ HCew== X-Gm-Message-State: AOAM532XhY0nltv1P0vUVBtiofNyxBOowpS+nCKfF0iLgxTipDtblOh3 f45zyHsiyWV+V0ux8tMm8tjrT5gFb81/w2Tvb+PVJ74bSmt0Y91nfOUtcJ/YZWbwwsta7PGVA2c RgoZqOtzzjXUtFnoKh58h60xHRulY6Oo= X-Received: by 2002:a05:6000:1375:: with SMTP id q21mr13635296wrz.147.1626454482033; Fri, 16 Jul 2021 09:54:42 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzy2FHh2fFukEQGDDLV5lE149htl68mCVoT9VFOracLH20pjNUgoGcmpTMdl7zX9ajMmpE5oVbhf9+kvAx3Ew8= X-Received: by 2002:a05:6000:1375:: with SMTP id q21mr13635274wrz.147.1626454481766; Fri, 16 Jul 2021 09:54:41 -0700 (PDT) MIME-Version: 1.0 References: <20210716023656.670004-1-jason@redhat.com> <2136759.qKCeTcHjAi@excalibur> In-Reply-To: From: Jonathan Wakely Date: Fri, 16 Jul 2021 17:54:30 +0100 Message-ID: Subject: Re: [PATCH] c++: implement C++17 hardware interference size To: Jason Merrill Cc: Matthias Kretz , "Richard Earnshaw (lists)" , "libstdc++" , gcc-patches List , GNU C Library X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-6.4 required=5.0 tests=BAYES_00, DKIMWL_WL_HIGH, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, RCVD_IN_DNSWL_LOW, RCVD_IN_MSPIKE_H4, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP autolearn=unavailable autolearn_force=no version=3.4.4 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on server2.sourceware.org X-BeenThere: libstdc++@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++ mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 16 Jul 2021 16:54:51 -0000 On Fri, 16 Jul 2021 at 16:33, Jason Merrill wrote: > Adjusting them based on tuning would certainly simplify a significant use > case, perhaps the only reasonable use. Cases more concerned with ABI > stability probably shouldn't use them at all. And that would mean not > needing to worry about the impossible task of finding the right values for > an entire architecture. But it would be quite a significant change in behaviour if -mtune started affecting ABI, wouldn't it? > I'm thinking about warning by default for any use of the variables without > explicitly specifying their values on the command line. Users could disable > the warning if they're happy using whatever the defaults happen to be. I like that suggestion. Maybe the warning could suggest optimal values based on the current -mtune flag. That way -mtune wouldn't need to alter ABI, but by combining -mtune with explicit values for the variables you get the best performance. And -mtune without overriding the default values preserves ABI.