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 29DD4C433EF for ; Wed, 5 Jan 2022 10:09:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239228AbiAEKJt (ORCPT ); Wed, 5 Jan 2022 05:09:49 -0500 Received: from ams.source.kernel.org ([145.40.68.75]:41548 "EHLO ams.source.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229770AbiAEKJq (ORCPT ); Wed, 5 Jan 2022 05:09:46 -0500 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 ams.source.kernel.org (Postfix) with ESMTPS id 50733B81964 for ; Wed, 5 Jan 2022 10:09:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7424BC36AEB; Wed, 5 Jan 2022 10:09:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1641377384; bh=D0+qyRmDko+O3mn6+d0pVvsip0kFrK7s/RqQWhXS+oY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=dUESPgrChmBJ759mlrX5FZop/jACBWVlP0LVXyMC2U6GrUHV4PI8qdAXeA3JhwquQ vVgo3KK2PjUryXUTjSdZF9IQ2Ka7FGSrFxz0qWaSXxcsALUUvuQvWH6kbHQXAp3MiY wQPaUGB+dp6EE5BO1MIog3tPZAGT0vzTaGw4qwoo= Date: Wed, 5 Jan 2022 11:09:41 +0100 From: Greg KH To: cgel.zte@gmail.com Cc: arve@android.com, chi.minghao@zte.com.cn, christian@brauner.io, hridya@google.com, joel@joelfernandes.org, linux-kernel@vger.kernel.org, maco@android.com, surenb@google.com, tkjos@android.com, zealci@zte.com.cn Subject: Re: 'Re: [PATCH] drivers/android: remove redundant ret variable' Message-ID: References: <20220105091942.616961-1-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: <20220105091942.616961-1-chi.minghao@zte.com.cn> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 05, 2022 at 09:19:42AM +0000, cgel.zte@gmail.com wrote: > I found that there are indeed some optimizations > after removing the redundancy. You stripped off all context, making this impossible to remember what you are responding to :( > > 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 As I said on your other email, look at the kernel built files please, they should be much different from your tiny example above. thanks, greg k-h