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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 1B041C388F7 for ; Thu, 22 Oct 2020 16:43:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AF2A524641 for ; Thu, 22 Oct 2020 16:43:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2897715AbgJVQnN (ORCPT ); Thu, 22 Oct 2020 12:43:13 -0400 Received: from gate.crashing.org ([63.228.1.57]:45828 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2503628AbgJVQnN (ORCPT ); Thu, 22 Oct 2020 12:43:13 -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 09MGbfEF008755; Thu, 22 Oct 2020 11:37:41 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 09MGbdTR008754; Thu, 22 Oct 2020 11:37:39 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 22 Oct 2020 11:37:39 -0500 From: Segher Boessenkool To: Christophe Leroy Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , jakub@redhat.com, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64() Message-ID: <20201022163739.GU2672@gate.crashing.org> References: <348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <348c2d3f19ffcff8abe50d52513f989c4581d000.1603375524.git.christophe.leroy@csgroup.eu> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 22, 2020 at 02:05:46PM +0000, Christophe Leroy wrote: > fls() and fls64() are using __builtin_ctz() and _builtin_ctzll(). > On powerpc, those builtins trivially use ctlzw and ctlzd power > instructions. > > Allthough those instructions provide the expected result with > input argument 0, __builtin_ctz() and __builtin_ctzll() are > documented as undefined for value 0. > When the input of fls(x) is a constant, just check x for nullity and > return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction > directly. That looks good :-) Acked-by: Segher Boessenkool Segher