From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 30394C47082 for ; Mon, 7 Jun 2021 14:21:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 12F1261029 for ; Mon, 7 Jun 2021 14:21:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230253AbhFGOXH (ORCPT ); Mon, 7 Jun 2021 10:23:07 -0400 Received: from gate.crashing.org ([63.228.1.57]:33204 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230211AbhFGOXE (ORCPT ); Mon, 7 Jun 2021 10:23:04 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 157EGxr9002306; Mon, 7 Jun 2021 09:16:59 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 157EGxho002305; Mon, 7 Jun 2021 09:16:59 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 7 Jun 2021 09:16:58 -0500 From: Segher Boessenkool To: Peter Zijlstra Cc: Linus Torvalds , Alan Stern , "Paul E. McKenney" , Will Deacon , Andrea Parri , Boqun Feng , Nick Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Linux Kernel Mailing List , linux-toolchains@vger.kernel.org, linux-arch Subject: Re: [RFC] LKMM: Add volatile_if() Message-ID: <20210607141658.GE18427@gate.crashing.org> References: <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <20210606115336.GS18427@gate.crashing.org> <20210606184021.GY18427@gate.crashing.org> <20210606195242.GA18427@gate.crashing.org> <20210606202616.GC18427@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Mon, Jun 07, 2021 at 12:52:15PM +0200, Peter Zijlstra wrote: > On Sun, Jun 06, 2021 at 03:26:16PM -0500, Segher Boessenkool wrote: > > I am saying that if you depend on that some C code you write will result > > in some particular machine code, without actually *forcing* the compiler > > to output that exact machine code, then you will be disappointed. Maybe > > not today, and maybe it will take years, if you are lucky. > > > > (s/forcing/instructing/ of course, compilers have feelings too!) > > And hence the request for a language extension. Both compilers have a > vast array of language extensions that are outside of the C spec (thank > you!), so can we please get one more? I don't see why not? It will need to be well-defined, so that it *can* be implemented. And ideally it will be useful for other applications as well. Finally, it should "play nice" with other extensions and language features. Segher