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=-17.4 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_IN_DEF_DKIM_WL autolearn=ham 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 7A1D8C433DF for ; Mon, 19 Oct 2020 19:30:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EB8C022283 for ; Mon, 19 Oct 2020 19:30:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="fo55Fkmp" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730832AbgJSTao (ORCPT ); Mon, 19 Oct 2020 15:30:44 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60784 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730021AbgJSTao (ORCPT ); Mon, 19 Oct 2020 15:30:44 -0400 Received: from mail-pf1-x444.google.com (mail-pf1-x444.google.com [IPv6:2607:f8b0:4864:20::444]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7887BC0613CE for ; Mon, 19 Oct 2020 12:30:44 -0700 (PDT) Received: by mail-pf1-x444.google.com with SMTP id w21so547112pfc.7 for ; Mon, 19 Oct 2020 12:30:44 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=EU0W4JOrAsDzHZM0eV3e2S82ZfAJTNvMxl+0MvAvfDg=; b=fo55FkmpDj2Wd4crXqqRBUpvhedGsQAZs0ESCJjBuDL3zYKONdan7hYDQFaCu3xvL4 Pd0w+dTDPRCOLS1wEK6lbWHhMJVFXRIzmk0axXsy5lmZmczHwGodHywaotzBd26ID2mh Rk+w3lBoII+3/Al9kOh6oLl/qZORekVRmEokgc9pOeocR9Ts+DUkGSrmpDqcf6FygnRE XTBStEHrM5sbXsD4LKaLGQeocyFnv3mZfFbUOBtMTygMPBIGqotlHBAiqwIIHoX+A6Y3 mu4/ptCbNp57ytRdurmXG2ll2s76//3jR9ZVtLAIgFMNSmMTugXuFlVbQOgIzQzTSxiz wnqg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=EU0W4JOrAsDzHZM0eV3e2S82ZfAJTNvMxl+0MvAvfDg=; b=mKkOA2lR76qqf7vg2YF6M4L4aQn+ZKA8NBCXAdf8smeUSTv3v4T5GnoCIiiPWpN/DN F+wjxIM8xteJ1hseJoHOar/Fi/byIp+gfvT0OmhSqR+Pt3JWrHZoyHPTfGouJcVRG18z 1ye1rem8DmywPY7SceCm7cPXjT9H5VB6SeYj3XcT5bSIzXCj1Xv1sH+84YGJdByMEQYO 2fyG2nNyJUECrqZSRhApU3yn2o+FFj/C1YCVRMq4QMWFZvasAPibCGqvdbsD+16L1OJi QLmh5LW7ipxs4muz/LDdfO0NGXO8qeC166cdlraTmEnCjG5fAQDvHmSNW8FR1K1MB5VJ BDVg== X-Gm-Message-State: AOAM533H8nQcHPkUq8VC2sebOENtZKyRSrIp7VJW2c5LRJVne8WMW8qK 7q9kZ+BWO9pj6F11qHQF6EE3RSnT5tgQ/4vf7xUjNA== X-Google-Smtp-Source: ABdhPJwDXgjoOZFe5sUJ+y5bswoNcDzvWO8SoMgKBYYDcSFtvREyvztwYPGgp8ewpMupFHP/ZGUuP9U3zMq3LtWyj10= X-Received: by 2002:a63:f74a:: with SMTP id f10mr1082418pgk.263.1603135843831; Mon, 19 Oct 2020 12:30:43 -0700 (PDT) MIME-Version: 1.0 References: <20201014110132.2680-1-mark@klomp.org> <20201017120135.4004-1-mark@klomp.org> In-Reply-To: <20201017120135.4004-1-mark@klomp.org> From: Nick Desaulniers Date: Mon, 19 Oct 2020 12:30:32 -0700 Message-ID: Subject: Re: [PATCH V2] Only add -fno-var-tracking-assignments workaround for old GCC versions. To: Mark Wielaard Cc: LKML , Masahiro Yamada , Michal Marek , Linux Kbuild mailing list , Ian Rogers , Andi Kleen , linux-toolchains@vger.kernel.org, Segher Boessenkool , Florian Weimer , Sedat Dilek Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-toolchains@vger.kernel.org On Sat, Oct 17, 2020 at 5:02 AM Mark Wielaard wrote: > > Some old GCC versions between 4.5.0 and 4.9.1 might miscompile code > with -fvar-tracking-assingments (which is enabled by default with -g -O2). > commit 2062afb4f added -fno-var-tracking-assignments unconditionally to > work around this. But newer versions of GCC no longer have this bug, so > only add it for versions of GCC before 5.0. This allows various tools > such as a perf probe or gdb debuggers or systemtap to resolve variable > locations using dwarf locations in more code. > > Changes in V2: > - Update commit message explaining purpose. > - Explicitly mention GCC version in comment. > - Wrap workaround in ifdef CONFIG_CC_IS_GCC > > Signed-off-by: Mark Wielaard > Acked-by: Ian Rogers > Reviewed-by: Andi Kleen > Cc: linux-toolchains@vger.kernel.org > Cc: Nick Desaulniers > Cc: Segher Boessenkool > Cc: Florian Weimer > Cc: Sedat Dilek > --- > Makefile | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/Makefile b/Makefile > index 51540b291738..964754b4cedf 100644 > --- a/Makefile > +++ b/Makefile > @@ -813,7 +813,11 @@ KBUILD_CFLAGS += -ftrivial-auto-var-init=zero > KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang > endif > > -DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments) > +# Workaround for GCC versions < 5.0 > +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801 > +ifdef CONFIG_CC_IS_GCC > +DEBUG_CFLAGS := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments)) Thanks for adding the comment. That will help us find+remove this when the kernel's minimum supported version of GCC advances to gcc-5. The current minimum supported version of GCC according to Documentation/process/changes.rst is gcc-4.9 (so anything older is irrelevant, and we drop support for it). If gcc 4.9 supports `-fno-var-tracking-assignments` (it looks like it does: https://godbolt.org/z/oa53f5), then we should drop the `cc-option` call, which will save us a compiler invocation for each invocation of `make`. > +endif > > ifdef CONFIG_DEBUG_INFO > ifdef CONFIG_DEBUG_INFO_SPLIT > -- > 2.18.4 > -- Thanks, ~Nick Desaulniers