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=-2.2 required=3.0 tests=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 5BEDDC3A59B for ; Mon, 19 Aug 2019 07:58:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 38A202085A for ; Mon, 19 Aug 2019 07:58:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726776AbfHSH6P (ORCPT ); Mon, 19 Aug 2019 03:58:15 -0400 Received: from gate.crashing.org ([63.228.1.57]:42491 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725790AbfHSH6P (ORCPT ); Mon, 19 Aug 2019 03:58:15 -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 x7J7vnxs028798; Mon, 19 Aug 2019 02:57:49 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x7J7vmIb028797; Mon, 19 Aug 2019 02:57:48 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 19 Aug 2019 02:57:48 -0500 From: Segher Boessenkool To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc: optimise WARN_ON() Message-ID: <20190819075748.GY31406@gate.crashing.org> References: <20190817090442.C5FEF106613@localhost.localdomain> <20190818120135.GV31406@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 19, 2019 at 07:40:42AM +0200, Christophe Leroy wrote: > Le 18/08/2019 à 14:01, Segher Boessenkool a écrit : > >On Sat, Aug 17, 2019 at 09:04:42AM +0000, Christophe Leroy wrote: > >>Unlike BUG_ON(x), WARN_ON(x) uses !!(x) as the trigger > >>of the t(d/w)nei instruction instead of using directly the > >>value of x. > >> > >>This leads to GCC adding unnecessary pair of addic/subfe. > > > >And it has to, it is passed as an "r" to an asm, GCC has to put the "!!" > >value into a register. > > > >>By using (x) instead of !!(x) like BUG_ON() does, the additional > >>instructions go away: > > > >But is it correct? What happens if you pass an int to WARN_ON, on a > >64-bit kernel? > > On a 64-bit kernel, an int is still in a 64-bit register, so there would > be no problem with tdnei, would it ? an int 0 is the same as an long 0, > right ? The top 32 bits of a 64-bit register holding an int are undefined. Take as example int x = 42; x = ~x; which may put ffff_ffff_ffff_ffd5 into the reg, not 0000_0000_ffff_ffd5 as you might expect or want. For tw instructions this makes no difference (they only look at the low 32 bits anyway); for td insns, it does. > It is on 32-bit kernel that I see a problem, if one passes a long long > to WARN_ON(), the forced cast to long will just drop the upper size of > it. So as of today, BUG_ON() is buggy for that. Sure, it isn't defined what types you can pass to that macro. Another thing that makes inline functions much saner to use. > >(You might want to have 64-bit generate either tw or td. But, with > >your __builtin_trap patch, all that will be automatic). > > Yes I'll discard this patch and focus on the __builtin_trap() one which > should solve most issues. But see my comment there about the compiler knowing all code after an unconditional trap is dead. Segher