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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 D1BDFC33CAF for ; Fri, 17 Jan 2020 01:21:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 971E82072B for ; Fri, 17 Jan 2020 01:21:25 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="sigesBb1" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387958AbgAQBVY (ORCPT ); Thu, 16 Jan 2020 20:21:24 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:59486 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729031AbgAQBVY (ORCPT ); Thu, 16 Jan 2020 20:21:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=jcajjxZ1xWQD9GN1eFNPK4/qoRHMGlsWkodtrglknLA=; b=sigesBb15/hF+QKL8jZxrBOUc nnZIcoGtGP8fXCdmd4UFK1FVFA+krbsrDCI4gNAiETqEt2BgzTsJnFlmQTdXBs7nEI3CTYD/QtLrS 78nB82UNUZc1TBhOoFPbQpN9qHRQwX7PtqmWCBSaWshL2egA1wXxWW6BE9/MeCeNHEjWHSZfN8GqD WJpv4HOdgZ0o5mKmmGh6oaBW3h0JXeVs29rLeC6EYEgsGa8do4sj4jmol+hJHSyiU72+GTC7zm3RX gkBrhYTmlTylxlJu2JWgE2xK76y1QjV+BVBMf+xI6I04LxhRq42PJr5uDXpfNZ0Wd6/+OZu3ivK7+ UEuNsYVfg==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1isGKB-0003RF-3D; Fri, 17 Jan 2020 01:21:23 +0000 Date: Thu, 16 Jan 2020 17:21:23 -0800 From: Matthew Wilcox To: Pavel Begunkov Cc: Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs: optimise kiocb_set_rw_flags() Message-ID: <20200117012123.GA9226@bombadil.infradead.org> References: <7d493d4872b75fc59556a63ee62c43b30c661ff9.1579223790.git.asml.silence@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7d493d4872b75fc59556a63ee62c43b30c661ff9.1579223790.git.asml.silence@gmail.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 17, 2020 at 04:16:41AM +0300, Pavel Begunkov wrote: > kiocb_set_rw_flags() generates a poor code with several memory writes > and a lot of jumps. Help compilers to optimise it. > > Tested with gcc 9.2 on x64-86, and as a result, it its output now is a > plain code without jumps accumulating in a register before a memory > write. Nice! > static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) > { > + int kiocb_flags = 0; > + > if (unlikely(flags & ~RWF_SUPPORTED)) > return -EOPNOTSUPP; > > if (flags & RWF_NOWAIT) { > if (!(ki->ki_filp->f_mode & FMODE_NOWAIT)) > return -EOPNOTSUPP; > - ki->ki_flags |= IOCB_NOWAIT; > + kiocb_flags |= IOCB_NOWAIT; > } > if (flags & RWF_HIPRI) > - ki->ki_flags |= IOCB_HIPRI; > + kiocb_flags |= IOCB_HIPRI; > if (flags & RWF_DSYNC) > - ki->ki_flags |= IOCB_DSYNC; > + kiocb_flags |= IOCB_DSYNC; > if (flags & RWF_SYNC) > - ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC); > + kiocb_flags |= (IOCB_DSYNC | IOCB_SYNC); > if (flags & RWF_APPEND) > - ki->ki_flags |= IOCB_APPEND; > + kiocb_flags |= IOCB_APPEND; > + > + if (kiocb_flags) > + ki->ki_flags |= kiocb_flags; > return 0; > } Might it generate even better code to do ... int kiocb_flags = 0; + if (!flags) + return 0; if (unlikely(flags & ~RWF_SUPPORTED)) return -EOPNOTSUPP; ... - if (kiocb_flags) - ki->ki_flags |= kiocb_flags; + ki->ki_flags |= kiocb_flags;