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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 19 Oct 2020 12:30:32 -0700
Message-ID: <CAKwvOdnXPs7-0Q8tdDiNGAWDpT3tc6EkjhDhZLdogwokG4JhUg@mail.gmail.com>
Subject: Re: [PATCH V2] Only add -fno-var-tracking-assignments workaround for
 old GCC versions.
To:     Mark Wielaard <mark@klomp.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Ian Rogers <irogers@google.com>,
        Andi Kleen <andi@firstfloor.org>,
        linux-toolchains@vger.kernel.org,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Florian Weimer <fw@deneb.enyo.de>,
        Sedat Dilek <sedat.dilek@gmail.com>
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On Sat, Oct 17, 2020 at 5:02 AM Mark Wielaard <mark@klomp.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. 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 <mark@klomp.org>
> Acked-by: Ian Rogers <irogers@google.com>
> Reviewed-by: Andi Kleen <andi@firstfloor.org>
> Cc: linux-toolchains@vger.kernel.org
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Segher Boessenkool <segher@kernel.crashing.org>
> Cc: Florian Weimer <fw@deneb.enyo.de>
> Cc: Sedat Dilek <sedat.dilek@gmail.com>
> ---
>  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