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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C76B3C433F5 for ; Mon, 31 Jan 2022 22:21:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233042AbiAaWVe (ORCPT ); Mon, 31 Jan 2022 17:21:34 -0500 Received: from gate.crashing.org ([63.228.1.57]:40799 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232909AbiAaWVd (ORCPT ); Mon, 31 Jan 2022 17:21:33 -0500 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 20VMDZ8D023795; Mon, 31 Jan 2022 16:13:35 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 20VMDXsA023794; Mon, 31 Jan 2022 16:13:33 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 31 Jan 2022 16:13:33 -0600 From: Segher Boessenkool To: Nick Desaulniers Cc: apinski@marvell.com, Josh Poimboeuf , Borislav Petkov , Vasily Gorbik , Linus Torvalds , Ingo Molnar , Dave Hansen , Thomas Gleixner , Peter Zijlstra , Luc Van Oostenryck , x86@kernel.org, llvm@lists.linux.dev, linux-sparse@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot , Nathan Chancellor , linux-toolchains@vger.kernel.org Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__ Message-ID: <20220131221333.GR614@gate.crashing.org> References: <20220114010526.1776605-1-ndesaulniers@google.com> <20220118192256.jzk5dnceeusq7x7u@treble> <20220118230120.pivvson7qekfiqic@treble> <20220125233128.GT614@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-kernel@vger.kernel.org On Mon, Jan 31, 2022 at 12:45:20PM -0800, Nick Desaulniers wrote: > On Tue, Jan 25, 2022 at 3:34 PM Segher Boessenkool > wrote: > > > > Hi! > > > > On Mon, Jan 24, 2022 at 03:26:36PM -0800, Nick Desaulniers wrote: > > > I'm more confident that we can remove the `volatile` keyword (I was > > > thinking about adding a new diagnostic to clang to warn that volatile > > > is redundate+implied for asm goto or inline asm that doesn't have > > > outputs) though that's not the problem here and will probably generate > > > some kernel wide cleanup before we could enable such a flag. > > > > Its main value is that it would discourage users from thinking volatile > > is magic. Seriously worth some pain! > > https://reviews.llvm.org/D118297 > PTAL "" Really the volatile asm-qualifier exists only to signal that an asm statement should not be DCE'd (when it has outputs but they are unused), CSE'd, or LICM'd. It is not a general compiler barrier. It means that the asm has a side effect (one unknown to the compiler), so it must be executed in the real machine just where it would be in the abstract machine. It *can* be CSEd, it *can* be DCEd, it can even be optimised by LICM in certain cases: but it has to be executed as often (and in the same order etc.) in the resulting machine code as it would be if you single-stepped through the source code by hand. Those are fine examples if you add "in most cases" (and that they are just examples, it's not an exhaustive list). Thanks, Segher