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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 EE2F5C0650F for ; Tue, 30 Jul 2019 13:55:28 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 41F98206E0 for ; Tue, 30 Jul 2019 13:55:28 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 41F98206E0 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from lists.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 45ydNr3zpVzDqHR for ; Tue, 30 Jul 2019 23:55:24 +1000 (AEST) Authentication-Results: lists.ozlabs.org; spf=permerror (mailfrom) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Authentication-Results: lists.ozlabs.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 45ydG64NqnzDqVv for ; Tue, 30 Jul 2019 23:49:34 +1000 (AEST) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x6UDmwB5006744; Tue, 30 Jul 2019 08:48:58 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x6UDmu7O006742; Tue, 30 Jul 2019 08:48:56 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Tue, 30 Jul 2019 08:48:56 -0500 From: Segher Boessenkool To: Arnd Bergmann Subject: Re: [PATCH] powerpc: workaround clang codegen bug in dcbz Message-ID: <20190730134856.GO31406@gate.crashing.org> References: <20190729202542.205309-1-ndesaulniers@google.com> <20190729203246.GA117371@archlinux-threadripper> <20190729215200.GN31406@gate.crashing.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.4.2.3i X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kbuild test robot , Nick Desaulniers , Linux Kernel Mailing List , clang-built-linux , Paul Mackerras , Nathan Chancellor , linuxppc-dev Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Jul 30, 2019 at 09:34:28AM +0200, Arnd Bergmann wrote: > Upon a second look, I think the issue is that the "Z" is an input argument > when it should be an output. clang decides that it can make a copy of the > input and pass that into the inline asm. This is not the most efficient > way, but it seems entirely correct according to the constraints. Most dcb* (and all icb*) do not change the memory pointed to. The memory is an input here, logically as well, and that is obvious. > Changing it to an output "=Z" constraint seems to make it work: > > https://godbolt.org/z/FwEqHf > > Clang still doesn't use the optimum form, but it passes the correct pointer. As I said many times already, LLVM does not seem to treat all asm operands as lvalues. That is a bug. And it is critical for memory operands for example, as should be obvious if you look at at for a few seconds (you pass *that* memory, not a copy of it). The thing you pass has an identity. It's an lvalue. This is true for *all* inline asm operands, not just output operands and memory operands, but it is most obvious there. Or, LLVM might have a bug elsewhere. Either way, the asm is fine, and it has worked fine in GCC since forever. Changing this constraint to be an output constraint would just be obfuscation (we could change *all* operands to *everything* to be inout ("+") constraints, and it won't affect correctness, just the reader's sanity). Segher