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 4E434C47096 for ; Sun, 6 Jun 2021 13:45:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1C9CE613EF for ; Sun, 6 Jun 2021 13:45:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230104AbhFFNrY (ORCPT ); Sun, 6 Jun 2021 09:47:24 -0400 Received: from netrider.rowland.org ([192.131.102.5]:33713 "HELO netrider.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S230099AbhFFNrY (ORCPT ); Sun, 6 Jun 2021 09:47:24 -0400 Received: (qmail 1736478 invoked by uid 1000); 6 Jun 2021 09:45:32 -0400 Date: Sun, 6 Jun 2021 09:45:32 -0400 From: Alan Stern To: Segher Boessenkool Cc: "Paul E. McKenney" , Linus Torvalds , Peter Zijlstra , 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: <20210606134532.GA1736178@rowland.harvard.edu> References: <20210604205600.GB4397@paulmck-ThinkPad-P17-Gen-1> <20210604214010.GD4397@paulmck-ThinkPad-P17-Gen-1> <20210605145739.GB1712909@rowland.harvard.edu> <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <20210606115336.GS18427@gate.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210606115336.GS18427@gate.crashing.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Sun, Jun 06, 2021 at 06:53:36AM -0500, Segher Boessenkool wrote: > On Sat, Jun 05, 2021 at 09:29:03PM -0400, Alan Stern wrote: > > Interesting. And changing one of the branches from barrier() to __asm__ > > __volatile__("nop": : :"memory") also causes a branch to be emitted. So > > even though the compiler doesn't "look inside" assembly code, it does > > compare two pieces at least textually and apparently assumes if they are > > identical then they do the same thing. > > And that is a simple fact, since the same assembler code (at the same > spot in the program) will do the same thing no matter how that ended up > there. Sure. But the same assembler code at two different spots in the program might not do the same thing. (Think of code that stores the current EIP register's value into a variable.) So while de-duplicating such code may be allowed, it will give rise to observable results at execution time. Alan > And the compiler always is allowed to duplicate, join, delete, you name > it, inline assembler code. The only thing that it cares about is > semantics of the code, just like for any other code. > > > Segher