linux-kernel - Re: [PATCH v4 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO

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Date:   Wed, 13 Jan 2021 10:48:41 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-kbuild@...r.kernel.org, linux-arch@...r.kernel.org,
        Jakub Jelinek <jakub@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>
Subject: Re: [PATCH v4 1/3] Remove $(cc-option,-gdwarf-4) dependency from
 CONFIG_DEBUG_INFO_DWARF4

On Tue, Jan 12, 2021 at 04:32:33PM -0800, Nick Desaulniers wrote:
> From: Masahiro Yamada <masahiroy@...nel.org>
> 
> 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>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
>  lib/Kconfig.debug | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 78361f0abe3a..dd7d8d35b2a5 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.
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>