From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755373Ab2IRGk4 (ORCPT ); Tue, 18 Sep 2012 02:40:56 -0400 Received: from nat28.tlf.novell.com ([130.57.49.28]:60456 "EHLO nat28.tlf.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752110Ab2IRGky convert rfc822-to-8bit (ORCPT ); Tue, 18 Sep 2012 02:40:54 -0400 Message-Id: <50583394020000780009BF8B@nat28.tlf.novell.com> X-Mailer: Novell GroupWise Internet Agent 12.0.0 Date: Tue, 18 Sep 2012 07:40:52 +0100 From: "Jan Beulich" To: "H. Peter Anvin" Cc: "Ingo Molnar" , , "Linus Torvalds" , Subject: Re: [tip:x86/asm] x86: Prefer TZCNT over BFS References: <504DEA1B020000780009A277@nat28.tlf.novell.com> <5056F04B020000780009BAF9@nat28.tlf.novell.com> <20120917100046.GA32463@gmail.com> <50575855.8040308@zytor.com> In-Reply-To: <50575855.8040308@zytor.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8BIT Content-Disposition: inline Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >>> On 17.09.12 at 19:05, "H. Peter Anvin" wrote: > Honestly, I don't see a reason to not do this unconditionally. On > anything but (pre-686-era) extremely old CPUs the cost of the extra > prefix is zero. On the really old CPUs the cost of the BSF instruction > will dwarf the penalty cycle for the extra prefix, so the whole > mechanism around doing it conditionally seems pointless. I certainly don't mind re-submitting with the whole thing made unconditional (implying that the prior version would be dropped in any case). Jan