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

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Date:   Wed, 14 Oct 2020 17:31:39 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Mark Wielaard <mark@...mp.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        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 Mon, Oct 12, 2020 at 9:12 PM Mark Wielaard <mark@...mp.org> wrote:
>
> Hi,
>
> On Mon, 2020-10-12 at 11:59 -0700, Nick Desaulniers wrote:
> > On Sat, Oct 10, 2020 at 3:57 PM Ian Rogers <irogers@...gle.com>
> > wrote:
> > > On Sat, Oct 10, 2020 at 3:08 PM Mark Wielaard <mark@...mp.org>
> > > wrote:
> > > > -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))
> >
> > Should this be wrapped in: `ifdef CONFIG_CC_IS_GCC`/`endif`?
>
> I don't think so. It wasn't before. And call cc-option makes sure to
> only add the flag if the compiler supports it (clang doesn't and it
> also has a much higher version).
>

I am also in favour of `ifdef CONFIG_CC_IS_GCC` to clearly say this is
a GCC bug.

For the comment something like:

# Workaround for GCC version <= 5.0
# GCC Bug: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61801>

Think of people grepping in the Linux source code for supported or
broken compiler (versions)...
As a reference see ClangBuiltLinux issue #427 "audit use of __GNUC__".
[2] says:
"There's also a ton of __GNUC_MINOR__ checks against unsupported GCC versions."

- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues/427
[2] https://github.com/ClangBuiltLinux/linux/issues/427#issuecomment-700935241