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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 1AE53C432C3 for ; Sat, 30 Nov 2019 22:05:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E475A2075A for ; Sat, 30 Nov 2019 22:05:05 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="s4A2sJws" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727320AbfK3WFE (ORCPT ); Sat, 30 Nov 2019 17:05:04 -0500 Received: from mail-qt1-f196.google.com ([209.85.160.196]:36189 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727179AbfK3WFE (ORCPT ); Sat, 30 Nov 2019 17:05:04 -0500 Received: by mail-qt1-f196.google.com with SMTP id y10so36782144qto.3; Sat, 30 Nov 2019 14:05:04 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:date:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=LavGqVWdCa+DgEJijwHbAU0oPvPk/rvkw7rn6vGHe94=; b=s4A2sJwsb7TgqnsHw/GTBPDuz7XkDrPomjfcp991BO0a1XcmM8BwZ5Hao4Vcboim29 7EWGGQ5XtYgl4ZzKChRe31qVjEZhiYf8KJCF+36+CtByqBbx+Q/TzamBjKpbace0nrGT ecLpX8RTH6MlIb5o3rCO65H79wgTIAy91QhMXcM78M+KivJ4pKqH2iMOyAb79S8zmi2L Jk9HcQ5V2KQQeeZhrzpwu37Nv4532oxHd00i4gLqxNFIPxnQKxTjSlYa0YeQsmfZFiVx oYeVWdjOcSZKWTflBPlPbqq6FLqPDeteCYwwCHZzlmmEYCRkAekt9HGl5lhvSQ+dhARu bJVA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:from:date:to:cc:subject:message-id :references:mime-version:content-disposition:in-reply-to:user-agent; bh=LavGqVWdCa+DgEJijwHbAU0oPvPk/rvkw7rn6vGHe94=; b=kgwKf9lGvl5gy+SuR5vWdcv90YrN0+6wfXmPTav/6DVWJf3guEeE7TA4DqcZruRR1j t0RIQdSd4CpneKfw0cePK5kgsluTPozcQ0EQVrUZxW9d6Lznwk1PNu8nX1Qo2qxXzt3x cP/aW1rhyARnl0UMHzxn7Xpgrz4sf7RfggkgY6iHXlRbPcFI3RthXdQDct/2SdGRZ2yX vKk40FdSc+Scr4wYvl/LUAMow8tGckpwCAnuatEIzIFEhTtUm8RtimgkoCn1EDDRd/c0 2mkwuIkFj/cqRTU581e7iD6obM2WKTlwQgbg5QfihldTv7ZrX7/02Nlqno/xnqPBxkXc rCHQ== X-Gm-Message-State: APjAAAVlico8Rg9hHbzWLO3pVv8CFuPD6BA0BSAGVAUnpbumoISvYKjG WKGolfnR07+r0VeuZLtwhjA= X-Google-Smtp-Source: APXvYqzTU9c13zW91d8JiG3CkIJUp+ndwSysFJrulNhocQrAsWtPE/0I97hN+dA7VDf4mt/1dO67Eg== X-Received: by 2002:ac8:37b6:: with SMTP id d51mr50374025qtc.261.1575151503469; Sat, 30 Nov 2019 14:05:03 -0800 (PST) Received: from rani.riverdale.lan ([2001:470:1f07:5f3::b55f]) by smtp.gmail.com with ESMTPSA id 24sm4129561qka.32.2019.11.30.14.05.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 30 Nov 2019 14:05:02 -0800 (PST) From: Arvind Sankar X-Google-Original-From: Arvind Sankar Date: Sat, 30 Nov 2019 17:05:00 -0500 To: Pavel Begunkov Cc: Jens Axboe , nivedita@alum.mit.edu, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] block: optimise bvec_iter_advance() Message-ID: <20191130220458.GA297712@rani.riverdale.lan> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Nov 30, 2019 at 11:23:52PM +0300, Pavel Begunkov wrote: > bvec_iter_advance() is quite popular, but compilers fail to do proper > alias analysis and optimise it good enough. The assembly is checked > for gcc 9.2, x86-64. > > - remove @iter->bi_size from min(...), as it's always less than @bytes. > Modify at the beginning and forget about it. > > - the compiler isn't able to collapse memory dependencies and remove > writes in the loop. Help it by explicitely using local vars. > > Signed-off-by: Pavel Begunkov > --- > > v2: simplify code (Arvind Sankar) > Thanks :) Signed-off-by: Arvind Sankar Btw, I discovered that gcc 9.2 doesn't optimize away the second comparison in something like m = min(a,b); return m>a; So the WARN_ONCE bit doesn't get optimized away even in cases like bio_for_each_bvec where it's guaranteed at compile-time to not trigger.