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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC9A5ECAAD3 for ; Wed, 31 Aug 2022 07:58:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231161AbiHaH6Z (ORCPT ); Wed, 31 Aug 2022 03:58:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54044 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229800AbiHaH6T (ORCPT ); Wed, 31 Aug 2022 03:58:19 -0400 Received: from smtp.smtpout.orange.fr (smtp-29.smtpout.orange.fr [80.12.242.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4209D42AC6 for ; Wed, 31 Aug 2022 00:58:16 -0700 (PDT) Received: from YC20090004.ad.ts.tri-ad.global ([103.175.111.222]) by smtp.orange.fr with ESMTPA id TIbgoo5DzXFXxTIbrogpIH; Wed, 31 Aug 2022 09:58:14 +0200 X-ME-Helo: YC20090004.ad.ts.tri-ad.global X-ME-Auth: bWFpbGhvbC52aW5jZW50QHdhbmFkb28uZnI= X-ME-Date: Wed, 31 Aug 2022 09:58:14 +0200 X-ME-IP: 103.175.111.222 From: Vincent Mailhol To: Borislav Petkov Cc: Nick Desaulniers , Thomas Gleixner , Ingo Molnar , x86@kernel.org, Peter Zijlstra , Dave Hansen , "H . Peter Anvin" , Nathan Chancellor , Tom Rix , linux-kernel@vger.kernel.org, llvm@lists.linux.dev, David Howells , Jan Beulich , Christophe Jaillet , Joe Perches , Josh Poimboeuf , Vincent Mailhol Subject: [PATCH v6 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions Date: Wed, 31 Aug 2022 16:57:40 +0900 Message-Id: <20220831075742.295-1-mailhol.vincent@wanadoo.fr> X-Mailer: git-send-email 2.35.1 In-Reply-To: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The compilers provide some builtin expression equivalent to the ffs(), __ffs() and ffz() functions of the kernel. The kernel uses optimized assembly which produces better code than the builtin functions. However, such assembly code can not be folded when used with constant expressions. This series relies on __builtin_constant_p to select the optimal solution: * use kernel assembly for non constant expressions * use compiler's __builtin function for constant expressions. ** Statistics ** Patch 1/2 optimizes 26.7% of ffs() calls and patch 2/2 optimizes 27.9% of __ffs() and ffz() calls (details of the calculation in each patch). ** Changelog ** v5 -> v6: * Rename variable___ffs() into variable__ffs() (two underscores instead of three) v4 -> v5: * (no changes on code, only commit comment was modified) * Rewrite the commit log: - Use two spaces instead of `| ' to indent code snippets. - Do not use `we'. - Do not use `this patch' in the commit description. Instead, use imperative tone. Link: https://lore.kernel.org/all/YvUZVYxbOMcZtR5G@zn.tnic/ v3 -> v4: * (no changes on code, only commit comment was modified) * Remove note and link to Nick's message in patch 1/2, c.f.: Link: https://lore.kernel.org/all/CAKwvOdnnDaiJcV1gr9vV+ya-jWxx7+2KJNTDThyFctVDOgt9zQ@mail.gmail.com/ * Add Reviewed-by: Nick Desaulniers tag in patch 2/2. v2 -> v3: * Redacted out the instructions after ret and before next function in the assembly output. * Added a note and a link to Nick's message on the constant propagation missed-optimization in clang: Link: https://lore.kernel.org/all/CAKwvOdnH_gYv4qRN9pKY7jNTQK95xNeH1w1KZJJmvCkh8xJLBg@mail.gmail.com/ * Fix copy/paste typo in statistics of patch 1/2. Number of occurences before patches are 1081 and not 3607 (percentage reduction of 26.7% remains correct) * Rename the functions as follow: - __varible_ffs() -> variable___ffs() - __variable_ffz() -> variable_ffz() * Add Reviewed-by: Nick Desaulniers tag in patch 1/2. Vincent Mailhol (2): x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions arch/x86/include/asm/bitops.h | 64 +++++++++++++++++++++-------------- 1 file changed, 38 insertions(+), 26 deletions(-) -- 2.35.1