linux-kernel - Re: [PATCH] Only add -fno-var-tracking-assignments workaround for old GCC versions.

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Message-ID: <CAP-5=fUT-1-CR-KMMsrpzgw9b3nBooeY05=YU9XKa5enO9SK+A@mail.gmail.com>
Date:   Sat, 10 Oct 2020 15:21:28 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Mark Wielaard <mark@...mp.org>
Cc:     Andi Kleen <andi@...stfloor.org>, linux-toolchains@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Phillips, Kim" <kim.phillips@....com>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH] Only add -fno-var-tracking-assignments workaround for old
 GCC versions.

On Sat, Oct 10, 2020 at 3:08 PM Mark Wielaard <mark@...mp.org> 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.
>
> Signed-off-by: Mark Wielaard <mark@...mp.org>

Acked-by: Ian Rogers <irogers@...gle.com>

Thanks,
Ian

> ---
>  Makefile | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index f84d7e4ca0be..4f4a9416a87a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -813,7 +813,9 @@ 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 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801
> +# for old versions of GCC.
> +DEBUG_CFLAGS   := $(call cc-ifversion, -lt, 0500, $(call cc-option, -fno-var-tracking-assignments))
>
>  ifdef CONFIG_DEBUG_INFO
>  ifdef CONFIG_DEBUG_INFO_SPLIT
> --
> 2.18.4
>