From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756778Ab2INVcn (ORCPT ); Fri, 14 Sep 2012 17:32:43 -0400 Received: from mail.skyhub.de ([78.46.96.112]:41391 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756132Ab2INVcl (ORCPT ); Fri, 14 Sep 2012 17:32:41 -0400 Date: Fri, 14 Sep 2012 23:32:39 +0200 From: Borislav Petkov To: Linus Torvalds Cc: mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, JBeulich@suse.com, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [tip:x86/asm] x86: Prefer TZCNT over BFS Message-ID: <20120914213239.GB29307@liondog.tnic> Mail-Followup-To: Borislav Petkov , Linus Torvalds , mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, JBeulich@suse.com, tglx@linutronix.de, linux-tip-commits@vger.kernel.org References: <504DEA1B020000780009A277@nat28.tlf.novell.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 14, 2012 at 02:14:57PM -0700, Linus Torvalds wrote: > On Thu, Sep 13, 2012 at 11:23 PM, tip-bot for Jan Beulich > wrote: > > > > x86: Prefer TZCNT over BFS > > This patch is insane. > > > For the moment, only do this when the respective generic-CPU > > option is selected (as there are no specific-CPU options > > covering the CPUs supporting TZCNT), and don't do that when size > > optimization was requested. > > This is pure garbage. > > Anybody who thinks this: > > > +#if (defined(CONFIG_X86_GENERIC) || defined(CONFIG_GENERIC_CPU)) \ > > + && !defined(CONFIG_CC_OPTIMIZE_FOR_SIZE) > > is a good idea should be shot. Don't do it. > > Introduce a new CONFIG variable with a sane name, for chrissake, the > same way we have CONFIG_X86_XADD etc. It would be logical to call it > X86_TZCNT, wouldn't it? > > And then add sane rules for that in the x86 config file. And no, the > above is *NOT* a sane rule at all. If I read that right, it will > enable TZCNT even for old 32-bit CPU's, for example. That's just > f*cking insane. > > Stop this kind of idiocy. The code looks bad, and the logic is pure shit too. And the other important question is, is this even worth the complexity? I mean "may execute that faster than 'bsf ...'" doesn't mean a lot so can anyone remind me again why we're doing this? Any hot paths I've missed? Thanks. -- Regards/Gruss, Boris.