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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 64E51C433EF for ; Wed, 29 Sep 2021 17:47:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 475C761440 for ; Wed, 29 Sep 2021 17:47:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343673AbhI2RtX (ORCPT ); Wed, 29 Sep 2021 13:49:23 -0400 Received: from gate.crashing.org ([63.228.1.57]:47517 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245131AbhI2RtX (ORCPT ); Wed, 29 Sep 2021 13:49:23 -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 18THfmdA001474; Wed, 29 Sep 2021 12:41:48 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 18THfk9G001468; Wed, 29 Sep 2021 12:41:46 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Wed, 29 Sep 2021 12:41:46 -0500 From: Segher Boessenkool To: Florian Weimer Cc: Mathieu Desnoyers , will@kernel.org, paulmck@kernel.org, Peter Zijlstra , linux-kernel@vger.kernel.org, Linus Torvalds , 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-toolchains@vger.kernel.org, linux-arch@vger.kernel.org Subject: Re: [RFC PATCH] LKMM: Add ctrl_dep() macro for control dependency Message-ID: <20210929174146.GF22689@gate.crashing.org> References: <20210928211507.20335-1-mathieu.desnoyers@efficios.com> <87lf3f7eh6.fsf@oldenburg.str.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87lf3f7eh6.fsf@oldenburg.str.redhat.com> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org Hi! On Wed, Sep 29, 2021 at 02:28:37PM +0200, Florian Weimer wrote: > If you need a specific instruction emitted, you need a compiler > intrinsic or inline assembly. Not an intrinsic. Builtins (like almost all other code) do not say "generate this particular machine code", they say "generate code that does ". That is one reason why builtins are more powerful than inline assembler (another related reason is that they tell the compiler exactly what behaviour is expected). > I don't think it's possible to piggy-back this on something else. Unless we get a description of what this does in term of language semantics (instead of generated machine code), there is no hope, even. Segher