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=-6.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 74435C48BC2 for ; Sun, 6 Jun 2021 23:37:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5550B60FF0 for ; Sun, 6 Jun 2021 23:37:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230173AbhFFXjW (ORCPT ); Sun, 6 Jun 2021 19:39:22 -0400 Received: from mail.kernel.org ([198.145.29.99]:39410 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229885AbhFFXjU (ORCPT ); Sun, 6 Jun 2021 19:39:20 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id B127B6142A; Sun, 6 Jun 2021 23:37:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1623022649; bh=mph3swxlAwxy+A0hDtsDzqXJcz/qyhwentRzpRx82hI=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=peM/WjT6Y4rssJJS4bgPTd5qp8rCmIJjLQkBGkAOdC7AcL3CGRYqKpQLGykkGyeTz t2x+ek9wo5iQTzy+0u1W97qEeYpmdDox7UbItGgBakkse4wpPYoEwj/QmZEJuMWOun d8IRDry9mRlbWapaJ1PgZwePaIKXrzv3ZCZgDAtbsfvm/Qtn34StkqUGmaCEsH4nE1 5Ueb/R65Oa/tMygrFGwe852PugFWWXDiyVGasUh3vur5byi0T7zP5FlXsex8ZXkqjX sNEG0APaFCZ+GAE+R/eR/RBwr7+bF21SmfhcJVbaiOHRBXUuVYsmF34i/eKg5LiJj5 h+vh5IqXtlmjg== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 823755C014A; Sun, 6 Jun 2021 16:37:29 -0700 (PDT) Date: Sun, 6 Jun 2021 16:37:29 -0700 From: "Paul E. McKenney" To: Segher Boessenkool Cc: Linus Torvalds , Alan Stern , 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: <20210606233729.GN4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210605145739.GB1712909@rowland.harvard.edu> <20210606001418.GH4397@paulmck-ThinkPad-P17-Gen-1> <20210606012903.GA1723421@rowland.harvard.edu> <20210606115336.GS18427@gate.crashing.org> <20210606184021.GY18427@gate.crashing.org> <20210606195242.GA18427@gate.crashing.org> <20210606202616.GC18427@gate.crashing.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210606202616.GC18427@gate.crashing.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jun 06, 2021 at 03:26:16PM -0500, Segher Boessenkool wrote: > On Sun, Jun 06, 2021 at 01:11:53PM -0700, Linus Torvalds wrote: > > On Sun, Jun 6, 2021 at 12:56 PM Segher Boessenkool > > wrote: > > > > > > Yes, I know. But it is literally the *only* way to *always* get a > > > conditional branch: by writing one. > > > > The thing is, I don't actually believe you. > > Fortune favours the bold! > > > The barrier() thing can work - all we need to do is to simply make it > > impossible for gcc to validly create anything but a conditional > > branch. > > And the only foolproof way of doing that is by writing a branch. > > > If either side of the thing have an asm that cannot be combined, gcc > > simply doesn't have any choice in the matter. There's no other valid > > model than a conditional branch around it (of some sort - doing an > > indirect branch that has a data dependency isn't wrong either, it just > > wouldn't be something that a sane compiler would generate because it's > > obviously much slower and more complicated). > > Or push something to the stack and return. Or rewrite the whole thing > as an FSM. Or or or. > > (And yes, there are existing compilers that can do both of these things > on some code). > > > We are very used to just making the compiler generate the code we > > need. That is, fundamentally, what any use of inline asm is all about. > > We want the compiler to generate all the common cases and all the > > regular instructions. > > > > The conditional branch itself - and the instructions leading up to it > > - are exactly those "common regular instructions" that we'd want the > > compiler to generate. That is in fact more true here than for most > > inline asm, exactly because there are so many different possible > > combinations of conditional branches (equal, not equal, less than,..) > > and so many ways to generate the code that generates the condition. > > > > So we are much better off letting the compiler do all that for us - > > it's very much what the compiler is good at. > > Yes, exactly. > > I am saying that if you depend on that some C code you write will result > in some particular machine code, without actually *forcing* the compiler > to output that exact machine code, then you will be disappointed. Maybe > not today, and maybe it will take years, if you are lucky. > > (s/forcing/instructing/ of course, compilers have feelings too!) OK, I will bite... What would you suggest as a way of instructing the compiler to emit the conditional branch that we are looking for? Thanx, Paul