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 36333C433EF for ; Wed, 5 Jan 2022 10:09:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234958AbiAEKJB (ORCPT ); Wed, 5 Jan 2022 05:09:01 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39198 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229770AbiAEKJA (ORCPT ); Wed, 5 Jan 2022 05:09:00 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 43258C061761 for ; Wed, 5 Jan 2022 02:09:00 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 25659616AE for ; Wed, 5 Jan 2022 10:08:59 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 63228C36AEB; Wed, 5 Jan 2022 10:08:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1641377338; bh=vszags4f61XmoqMHQ0WsiGlgt8KGGLOuKDImhaLeyTY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=RU0JUlYlesjtlv2BlLOmFSW5M8+4Az9HLihf6DKSpCkSLoZxrrJ2SoVfqYHGc2Ets jZzBWTvCSiLZikJDJkWgH+q5TBZS3S0OXurxX0d8F9aNaSqkN4DwOsNYTb+lXmWKcW jOFEfpjnkw1GBRyFbOGn175FDrwGnlr8ZGNN4sfQ= Date: Wed, 5 Jan 2022 11:08:54 +0100 From: Greg KH To: cgel.zte@gmail.com Cc: arnd@arndb.de, chi.minghao@zte.com.cn, fkassabri@habana.ai, linux-kernel@vger.kernel.org, obitton@habana.ai, ogabbay@kernel.org, osharabi@habana.ai, ynudelman@habana.ai, zealci@zte.com.cn Subject: Re: [PATCH misc-next] drivers/misc: remove redundant rc variable Message-ID: References: <20220105092229.617027-1-chi.minghao@zte.com.cn> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220105092229.617027-1-chi.minghao@zte.com.cn> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 05, 2022 at 09:22:29AM +0000, cgel.zte@gmail.com wrote: > I found that there are indeed some optimizations > after removing the redundancy. > > For example: > > int foo(int a, int b) > { > int ret; > ret = a>b ? a: b; > return ret; > } > int main() > { > int val; > val = foo(7,2); > return 0; > } > > and > > int foo(int a, int b) > { > return a>b ? a: b; > } > int main() > { > int val; > val = foo(7,2); > return 0; > } > > The corresponding disassembly code is as follows： > > 1129: f3 0f 1e fa endbr64 > 112d: 55 push %rbp > 112e: 48 89 e5 mov %rsp,%rbp > 1131: 89 7d fc mov %edi,-0x4(%rbp) > 1134: 89 75 f8 mov %esi,-0x8(%rbp) > 1137: 8b 45 fc mov -0x4(%rbp),%eax > 113a: 39 45 f8 cmp %eax,-0x8(%rbp) > 113d: 0f 4d 45 f8 cmovge -0x8(%rbp),%eax > 1141: 5d pop %rbp > 1142: c3 retq > > and > > 1129: f3 0f 1e fa endbr64 > 112d: 55 push %rbp > 112e: 48 89 e5 mov %rsp,%rbp > 1131: 89 7d ec mov %edi,-0x14(%rbp) > 1134: 89 75 e8 mov %esi,-0x18(%rbp) > 1137: 8b 45 ec mov -0x14(%rbp),%eax > 113a: 39 45 e8 cmp %eax,-0x18(%rbp) > 113d: 0f 4d 45 e8 cmovge -0x18(%rbp),%eax > 1141: 89 45 fc mov %eax,-0x4(%rbp) > 1144: 8b 45 fc mov -0x4(%rbp),%eax > 1147: 5d pop %rbp > 1148: c3 retq > > After removing the redundancy, the compiler does > have some optimizations Please look at the actual output of the kernel files, the build options that the kernel enforces should not cause a difference here. thanks, greg k-h