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 [170.10.133.124]) by sourceware.org (Postfix) with ESMTP id 2557B399CC24 for ; Fri, 16 Jul 2021 16:54:46 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 2557B399CC24 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-590--2c8uG7dPuC5jtXbPpSAWQ-1; Fri, 16 Jul 2021 12:54:43 -0400 X-MC-Unique: -2c8uG7dPuC5jtXbPpSAWQ-1 Received: by mail-wr1-f70.google.com with SMTP id 32-20020adf82a30000b029013b21c75294so5075648wrc.14 for ; Fri, 16 Jul 2021 09:54:43 -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=i1nxkE3WLfDt/ZZ2fQIZivdLG95KMIKWdLt/OzrMAS9hK97Ew4G3rWuv2QdpuZr/PN prskNTHKs0DbahWhvudxfjy8faZwwMhuJfTpepr6G3JHmojXYsiNwT1pJ9zl985Wzsw6 NPYZJ2bLdCZEikEcyW11bxvaVlQSSGNRA2E5qHZnklKn7u5+f/Sfz+1TouPZkHt5ouNT rDckD2Q96a3/Sq9DJ605fR7AnHSRqTaugQ7l4zTwNQW6Pb+kdt5GsCItNlGouYVrK0F5 +sXUGm+B1oQ0hOz4MduJXJSa4R92fEj/z5Q4y5G5/+MEnfU9ent3dIlJl1bKULMKDjd7 i4BQ== X-Gm-Message-State: AOAM530cxAoi1lQcyJ5EzAEFN2IbrzizI5zMBDpmjtUkIZrYaaEZDBY4 v0ONpUtLa6v1BbonXAB3eC3nufz2+u53mVmw9RdO+ieqiyQqLWsL0pIm09GgbqDAjSR2F/MYUuk znmHFgfysOHUBfLHgCVq91k6m8xZps+yVEg== X-Received: by 2002:a05:6000:1375:: with SMTP id q21mr13635301wrz.147.1626454482097; 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: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 16 Jul 2021 16:54:47 -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.