linux-kernel - Re: [PATCH v3 5/5] Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO

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Date:   Sat, 10 Sep 2022 05:56:42 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>,
        X86 ML <x86@...nel.org>,
        Dmitrii Bundin <dmitrii.bundin.a@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Alexey Alexandrov <aalexand@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v3 5/5] Makefile.debug: set -g unconditional on CONFIG_DEBUG_INFO_SPLIT

On Wed, Sep 7, 2022 at 1:59 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> Dmitrii, Fangrui, and Mashahiro note:
>
>   Before GCC 11 and Clang 12 -gsplit-dwarf implicitly uses -g2.
>
> Fix CONFIG_DEBUG_INFO_SPLIT for gcc-11+ & clang-12+ which now need -g
> specified in order for -gsplit-dwarf to work at all.
>
> -gsplit-dwarf has been mutually exclusive with -g since support for
> CONFIG_DEBUG_INFO_SPLIT was introduced in
> commit 866ced950bcd ("kbuild: Support split debug info v4")
> I don't think it ever needed to be.
>
> Link: https://lore.kernel.org/lkml/20220815013317.26121-1-dmitrii.bundin.a@gmail.com/
> Link: https://lore.kernel.org/lkml/CAK7LNARPAmsJD5XKAw7m_X2g7Fi-CAAsWDQiP7+ANBjkg7R7ng@mail.gmail.com/
> Link: https://reviews.llvm.org/D80391
> Cc: Andi Kleen <ak@...ux.intel.com>
> Reported-by: Dmitrii Bundin <dmitrii.bundin.a@...il.com>
> Reported-by: Fangrui Song <maskray@...gle.com>
> Reported-by: Masahiro Yamada <masahiroy@...nel.org>
> Suggested-by: Dmitrii Bundin <dmitrii.bundin.a@...il.com>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes v2 -> v3:
> * Pick up Nathan's RB tag.
>
> Changes v1 -> v2:
> * Add reference to 866ced950bcd, cc Andi, in commit message.
>
>  scripts/Makefile.debug | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/scripts/Makefile.debug b/scripts/Makefile.debug
> index 2845145c1220..c20f8f2e76bf 100644
> --- a/scripts/Makefile.debug
> +++ b/scripts/Makefile.debug
> @@ -1,10 +1,4 @@
> -DEBUG_CFLAGS   :=
> -
> -ifdef CONFIG_DEBUG_INFO_SPLIT
> -DEBUG_CFLAGS   += -gsplit-dwarf
> -else
> -debug-cflags-y += -g
> -endif
> +DEBUG_CFLAGS   := -g
>
>  debug-flags-$(CONFIG_DEBUG_INFO_DWARF4)        += -gdwarf-4
>  debug-flags-$(CONFIG_DEBUG_INFO_DWARF5)        += -gdwarf-5
> @@ -15,6 +9,8 @@ ifeq ($(CONFIG_CC_IS_CLANG)$(CONFIG_AS_IS_GNU),yy)
>  KBUILD_AFLAGS  += $(addprefix -Wa$(comma), $(debug-flags-y))
>  endif
>
> +debug-flags-$(CONFIG_DEBUG_INFO_SPLIT) += -gsplit-dwarf
> +


This patch changes the behavior that is not mentioned in the commit log.




Previously, -gsplit-dwarf was passed only when compiling *.c files.
(DEBUG_CFLAGS).

Now it is passed also when compiling *.S files.
(debug-flags-y is appended to KBUILD_AFLAGS)


Please confirm if this makes sense, and if so,
please mention it in the commit log.



As far as I tested, I did not see this change was useful.

When *.S is compiled to *.o, *.dwo is also created,
but it does not contain any debug info.



>  ifdef CONFIG_DEBUG_INFO_REDUCED
>  DEBUG_CFLAGS   += -fno-var-tracking
>  ifdef CONFIG_CC_IS_GCC
> --
> 2.37.2.789.g6183377224-goog
>


-- 
Best Regards
Masahiro Yamada