linux-kernel - Re: [PATCH] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO

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Message-ID: <CAKwvOdnRDC8tYx4H4ueTv44-+rdwevv+PNyEifzUcjqOjQTA8w@mail.gmail.com>
Date:   Wed, 2 Dec 2020 15:12:44 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Fangrui Song <maskray@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: Re: [PATCH] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4

On Mon, Nov 30, 2020 at 7:42 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Tue, Dec 1, 2020 at 12:29 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > The -gdwarf-4 flag is supported by GCC 4.5+, and also by Clang.
> >
> > You can see it at https://godbolt.org/z/6ed1oW

(this link misses -gdwarf-4 for clang, but I think that's ok)

> >
> >   For gcc 4.5.3 pane,    line 37:    .value 0x4
> >   For clang 10.0.1 pane, line 117:   .short 4
> >
> > Given Documentation/process/changes.rst stating GCC 4.9 is the minimal
> > version, this cc-option is unneeded.
> >
> > Note
> > ----
> >
> > CONFIG_DEBUG_INFO_DWARF4 controls the DWARF version only for C files.
> >
> > As you can see in the top Makefile, -gdwarf-4 is only passed to CFLAGS.
> >
> >   ifdef CONFIG_DEBUG_INFO_DWARF4
> >   DEBUG_CFLAGS    += -gdwarf-4
> >   endif
> >
> > This flag is used when compiling *.c files.
> >
> > On the other hand, the assembler is always given -gdwarf-2.
> >
> >   KBUILD_AFLAGS   += -Wa,-gdwarf-2
> >
> > Hence, the debug info that comes from *.S files is always DWARF v2.
> > This is simply because GAS supported only -gdwarf-2 for a long time.
> >
> > Recently, GAS gained the support for --dwarf-[3|4|5] options. [1]
> > And, also we have Clang integrated assembler. So, the debug info
> > for *.S files might be improved if we want.
> >
> > In my understanding, the current code is intentional, not a bug.
> >
> > [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=31bf18645d98b4d3d7357353be840e320649a67d
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> > ---
> >
> >  lib/Kconfig.debug | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index cb8ef4fd0d02..7f80c25497a1 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -258,7 +258,6 @@ config DEBUG_INFO_SPLIT
> >
> >  config DEBUG_INFO_DWARF4
> >         bool "Generate dwarf4 debuginfo"
> > -       depends on $(cc-option,-gdwarf-4)
> >         help
> >           Generate dwarf4 debug info. This requires recent versions
> >           of gcc and gdb. It makes the debug information larger.
> > --
> > 2.27.
>
>
> Maybe, this can be squashed into Nick's patch.

I have no preference.

Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

(I'll reroll this into my DWARFv5 series either way, but when/how you
merge this doesn't matter to me)

-- 
Thanks,
~Nick Desaulniers