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=-7.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,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 D513BC433DF for ; Thu, 2 Jul 2020 18:01:01 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 342B220870 for ; Thu, 2 Jul 2020 18:01:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="QWmK8WMR" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 342B220870 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19205-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 7457 invoked by uid 550); 2 Jul 2020 18:00:55 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 7419 invoked from network); 2 Jul 2020 18:00:54 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1593712842; bh=Hak91qzk3x/aeXBK1n/XMiTpR5KSRvENrzPRsP83yxM=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=QWmK8WMRCdHSwyVK000d5y6dbgLM+MjgvdkwYsTsrjr5LZBrjrxYWW1nMi6rZxccV IiaPyrs2F2bjvlZIb4WjffkjC0aKcE/DRPRkmEy2Vz5+fS5gilxXEQzxwjlFv3ETvE BcELqK8pbmTnqz6ggKuzaDwzCOlpTPOwyp2kT1xI= Date: Thu, 2 Jul 2020 11:00:42 -0700 From: "Paul E. McKenney" To: David Laight Cc: 'Peter Zijlstra' , Marco Elver , Nick Desaulniers , Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , "linux-pci@vger.kernel.org" , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200702180042.GW9247@paulmck-ThinkPad-P72> References: <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> <20200625085745.GD117543@hirez.programming.kicks-ass.net> <20200630191931.GA884155@elver.google.com> <20200630201243.GD4817@hirez.programming.kicks-ass.net> <20200630203016.GI9247@paulmck-ThinkPad-P72> <20200701091054.GW4781@hirez.programming.kicks-ass.net> <4427b0f825324da4b1640e32265b04bd@AcuMS.aculab.com> <20200701160624.GO9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) On Thu, Jul 02, 2020 at 09:37:26AM +0000, David Laight wrote: > From: Paul E. McKenney > > Sent: 01 July 2020 17:06 > ... > > > Would an asm statement that uses the same 'register' for input and > > > output but doesn't actually do anything help? > > > It won't generate any code, but the compiler ought to assume that > > > it might change the value - so can't do optimisations that track > > > the value across the call. > > > > It might replace the volatile load, but there are optimizations that > > apply to the downstream code as well. > > > > Or are you suggesting periodically pushing the dependent variable > > through this asm? That might work, but it would be easier and > > more maintainable to just mark the variable. > > Marking the variable requires compiler support. > Although what 'volatile register int foo;' means might be interesting. > > So I was thinking that in the case mentioned earlier you do: > ptr += LAUNDER(offset & 1); > to ensure the compiler didn't convert to: > if (offset & 1) ptr++; > (Which is probably a pessimisation - the reverse is likely better.) Indeed, Akshat's prototype follows the "volatile" qualifier in many ways. https://github.com/AKG001/gcc/ Thanx, Paul