linux-kernel - Re: [PATCH v5 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO

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Date:   Fri, 15 Jan 2021 13:59:58 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        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>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>, Yonghong Song <yhs@...com>,
        Jiri Olsa <jolsa@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH v5 1/3] Remove $(cc-option,-gdwarf-4) dependency from
 CONFIG_DEBUG_INFO_DWARF4

On 2021-01-15, 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]

The gas commit description has a typo. The supported options are -gdwarf-[345] or --gdwarf-[345].
-gdwarf2 and --gdwarf2 are kept for compatibility.

Looks good otherwise.

>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
>
>Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>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 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
>