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

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Date:   Tue, 1 Dec 2020 12:41:50 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...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 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
>
>   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.




-- 
Best Regards
Masahiro Yamada