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=-1.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 762C1C49ED9 for ; Tue, 10 Sep 2019 18:30:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2F8F1216F4 for ; Tue, 10 Sep 2019 18:30:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=ellerman.id.au header.i=@ellerman.id.au header.b="bjPAWTkE" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727023AbfIJSam (ORCPT ); Tue, 10 Sep 2019 14:30:42 -0400 Received: from bilbo.ozlabs.org ([203.11.71.1]:58755 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725875AbfIJSam (ORCPT ); Tue, 10 Sep 2019 14:30:42 -0400 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 46SYW11NV4z9sCJ; Wed, 11 Sep 2019 04:30:36 +1000 (AEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ellerman.id.au; s=201909; t=1568140238; bh=f4U1zO2/pzQQfC6HW1AIvY8jEzHpvz5Va9YInwDeUe4=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=bjPAWTkE2pbHDx9ZvsEug69vxtE8M/T3yz/Qd+vrB8ielWshLQ6SaPq+mGAbac2ct ej7BQXJYmyEQPMX3ETl4iM0BYuRfmYzjTKbXwy4HtOyNGtv4OsKWUBy/cmESiekcao Z834pvArUYWOFG+n9JeXwhoSFnFKZrvnQ2Sho6LOfVE/jSSzKMPyPp6EzJm3o2vK68 9LD6Ksr+s2ZbX2GMGy6yLQwXSWDU3uqJutIHuvElI5FuD1XlF0Ebdb6AcTN5E5aF/6 g4IL+72+IJ3MKpJKGUF4TeWwSR727VCuDDNP99bgDwOMKKoLNyXXV9lsYbMLTxlVhW ucSd4fpHnrS4A== From: Michael Ellerman To: Nathan Chancellor , Segher Boessenkool Cc: David Laight , Nick Desaulniers , LKML , "# 3.4.x" , clang-built-linux , Paul Mackerras , linuxppc-dev Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp In-Reply-To: <20190904231554.GA42450@archlinux-threadripper> References: <878srdv206.fsf@mpe.ellerman.id.au> <20190828175322.GA121833@archlinux-threadripper> <20190828184529.GC127646@archlinux-threadripper> <6801a83ed6d54d95b87a41c57ef6e6b0@AcuMS.aculab.com> <20190903055553.GC60296@archlinux-threadripper> <20190903193128.GC9749@gate.crashing.org> <20190904002401.GA70635@archlinux-threadripper> <1bcd7086f3d24dfa82eec03980f30fbc@AcuMS.aculab.com> <20190904130135.GN9749@gate.crashing.org> <20190904231554.GA42450@archlinux-threadripper> Date: Wed, 11 Sep 2019 04:30:38 +1000 Message-ID: <87mufcypf5.fsf@mpe.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org Nathan Chancellor writes: > On Wed, Sep 04, 2019 at 08:01:35AM -0500, Segher Boessenkool wrote: >> On Wed, Sep 04, 2019 at 08:16:45AM +0000, David Laight wrote: >> > From: Nathan Chancellor [mailto:natechancellor@gmail.com] >> > > Fair enough so I guess we are back to just outright disabling the >> > > warning. >> > >> > Just disabling the warning won't stop the compiler generating code >> > that breaks a 'user' implementation of setjmp(). >> >> Yeah. I have a patch (will send in an hour or so) that enables the >> "returns_twice" attribute for setjmp (in ). In testing >> (with GCC trunk) it showed no difference in code generation, but >> better save than sorry. >> >> It also sets "noreturn" on longjmp, and that *does* help, it saves a >> hundred insns or so (all in xmon, no surprise there). >> >> I don't think this will make LLVM shut up about this though. And >> technically it is right: the C standard does say that in hosted mode >> setjmp is a reserved name and you need to include to access >> it (not ). > > It does not fix the warning, I tested your patch. > >> So why is the kernel compiled as hosted? Does adding -ffreestanding >> hurt anything? Is that actually supported on LLVM, on all relevant >> versions of it? Does it shut up the warning there (if not, that would >> be an LLVM bug)? > > It does fix this warning because -ffreestanding implies -fno-builtin, > which also solves the warning. LLVM has supported -ffreestanding since > at least 3.0.0. There are some parts of the kernel that are compiled > with this and it probably should be used in more places but it sounds > like there might be some good codegen improvements that are disabled > with it: > > https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/ For xmon.c and crash.c I think using -ffreestanding would be fine. They're both crash/debug code, so we don't care about minor optimisation differences. If anything we don't want the compiler being too clever when generating that code. cheers