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 3D908C4743D for ; Fri, 4 Jun 2021 16:57:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1C80C613B4 for ; Fri, 4 Jun 2021 16:57:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230245AbhFDQ7G (ORCPT ); Fri, 4 Jun 2021 12:59:06 -0400 Received: from gate.crashing.org ([63.228.1.57]:36210 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229864AbhFDQ7G (ORCPT ); Fri, 4 Jun 2021 12:59:06 -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 154GqohH027719; Fri, 4 Jun 2021 11:52:50 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 154GqmG1027681; Fri, 4 Jun 2021 11:52:48 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Fri, 4 Jun 2021 11:52:42 -0500 From: Segher Boessenkool To: Peter Zijlstra Cc: Linus Torvalds , Will Deacon , "Paul E. McKenney" , Alan Stern , 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: <20210604165242.GI18427@gate.crashing.org> References: 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-toolchains@vger.kernel.org On Fri, Jun 04, 2021 at 06:37:22PM +0200, Peter Zijlstra wrote: > On Fri, Jun 04, 2021 at 09:30:01AM -0700, Linus Torvalds wrote: > > Why is "volatile_if()" not just > > > > #define barier_true() ({ barrier(); 1; }) > > > > #define volatile_if(x) if ((x) && barrier_true()) > > > > because that should essentially cause the same thing - the compiler > > should be *forced* to create one conditional branch (because "barrier" > > is an asm that can't be done on the false side, so it can't do it with > > arithmetic or other games), and after that we're done. > > > > No need for per-architecture "asm goto" games. No new memory barriers. > > No actual new code generation (except for the empty asm volatile that > > is a barrier). > > Because we weren't sure compilers weren't still allowed to optimize the > branch away. barrier_true is a volatile asm, so it should be executed on the real machine exactly as often as on the abstract machine (and in order with other side effects). And the && short-circuits, so you will always have the same effect as a branch. But there of course is nothing that forces there to be a branch (as a silly example, the compiler could convert some control flow to go via computed return addresses). Segher