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 90A5BC4743D for ; Fri, 4 Jun 2021 19:57:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 72BD8613EC for ; Fri, 4 Jun 2021 19:57:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229854AbhFDT7F (ORCPT ); Fri, 4 Jun 2021 15:59:05 -0400 Received: from gate.crashing.org ([63.228.1.57]:55270 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230435AbhFDT7E (ORCPT ); Fri, 4 Jun 2021 15:59:04 -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 154Jr3qC010194; Fri, 4 Jun 2021 14:53:03 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 154Jr1JO010190; Fri, 4 Jun 2021 14:53:01 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Fri, 4 Jun 2021 14:53:01 -0500 From: Segher Boessenkool To: "Paul E. McKenney" Cc: Peter Zijlstra , Linus Torvalds , will@kernel.org, stern@rowland.harvard.edu, parri.andrea@gmail.com, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, linux-kernel@vger.kernel.org, linux-toolchains@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: [RFC] LKMM: Add volatile_if() Message-ID: <20210604195301.GM18427@gate.crashing.org> References: <20210604153518.GD18427@gate.crashing.org> <20210604164047.GH18427@gate.crashing.org> <20210604185526.GW4397@paulmck-ThinkPad-P17-Gen-1> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210604185526.GW4397@paulmck-ThinkPad-P17-Gen-1> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Fri, Jun 04, 2021 at 11:55:26AM -0700, Paul E. McKenney wrote: > On Fri, Jun 04, 2021 at 11:40:47AM -0500, Segher Boessenkool wrote: > > My point is that you ask compiler developers to paint themselves into a > > corner if you ask them to change such fundamental C syntax. > > Once we have some experience with a language extension, the official > syntax for a standardized version of that extension can be bikeshedded. > Committees being what they are, what we use in the meantime will > definitely not be what is chosen, so there is not a whole lot of point > in worrying about the exact syntax in the meantime. ;-) I am only saying that it is unlikely any compiler that is used in production will want to experiment with "volatile if". > > I would love to see something that meshes well with the rest of C. But > > there is no 1-1 translation from C code to machine code (not in either > > direction), so anything that more or less depends on that will always > > be awkward. If you can actually express the dependency in your source > > code that will get us 95% to where we want to be. ^^^ > > > Data dependencies, control dependencies and address dependencies, C > > > doesn't really like them, we rely on them. It would be awesome if we can > > > fix this. > > > > Yes. The problem is that C is a high-level language. All C semantics > > are expressed on a an "as-if" level, never as "do this, then that" -- > > well, of course that *is* what it says, it's an imperative language just > > like most, but that is just how you *think* about things on a conceptual > > level, there is nothing that says the machine code has to do the same > > thing in the same order as you wrote! > > Which is exactly why these conversations are often difficult. There is > a tension between pushing the as-if rule as far as possible within the > compiler on the one hand and allowing developers to write code that does > what is needed on the other. ;-) There is a tension between what users expect from the compiler and what actually is promised. The compiler is not pushing the as-if rule any further than it always has: it just becomes better at optimising over time. The as-if rule is and always has been absolute. What is needed to get any progress is for user expectations to be feasible and not contradict existing requirements. See "^^^" above. Segher