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=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 7C760C47083 for ; Fri, 4 Jun 2021 16:14:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5BBF9613FF for ; Fri, 4 Jun 2021 16:14:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230320AbhFDQQM (ORCPT ); Fri, 4 Jun 2021 12:16:12 -0400 Received: from gate.crashing.org ([63.228.1.57]:40701 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229746AbhFDQQL (ORCPT ); Fri, 4 Jun 2021 12:16:11 -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 154G9uPx023147; Fri, 4 Jun 2021 11:09:57 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 154G9uR9023146; Fri, 4 Jun 2021 11:09:56 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Fri, 4 Jun 2021 11:09:55 -0500 From: Segher Boessenkool To: Peter Zijlstra Cc: Linus Torvalds , will@kernel.org, paulmck@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: <20210604160955.GG18427@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-kernel@vger.kernel.org Hi! On Fri, Jun 04, 2021 at 12:12:07PM +0200, Peter Zijlstra wrote: > With optimizing compilers becoming more and more agressive and C so far > refusing to acknowledge the concept of control-dependencies even while > we keep growing the amount of reliance on them, things will eventually > come apart. Yes, C is still not a portable assembler. > There have been talks with toolchain people on how to resolve this; one > suggestion was allowing the volatile qualifier on branch statements like > 'if', but so far no actual compiler has made any progress on this. "if" is not a "branch statement". > --- a/arch/powerpc/include/asm/barrier.h > +++ b/arch/powerpc/include/asm/barrier.h > @@ -80,6 +80,19 @@ do { \ > ___p1; \ > }) > > +#ifndef __ASSEMBLY__ > +/* Guarantee a conditional branch that depends on @cond. */ > +static __always_inline bool volatile_cond(bool cond) > +{ > + asm_volatile_goto("and. %0,%0,%0; bne %l[l_yes]" > + : : "r" (cond) : "cc", "memory" : l_yes); > + return false; > +l_yes: > + return true; > +} > +#define volatile_cond volatile_cond > +#endif "cmpwi" is ever so slightly better than "and.". And you can write "cr0" instead of "cc" more explicitely (it means the same thing though). I didn't find a description of the expected precise semantics anywhere in this patch. This however is the most important thing required here! Segher