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

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Message-ID: <CA+icZUXjiNaHgYpBXB_DO76XznfqOG-rpH5i6NzhBx-gGoN=ZA@mail.gmail.com>
Date:   Fri, 15 Jan 2021 23:03:13 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <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 Fri, Jan 15, 2021 at 11:00 PM Fangrui Song <maskray@...gle.com> wrote:
>
> 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.
>

Good catch Fangrui.
Yesterday, I wondered - after a cut-n-paste from the patch's changelog
- why I did not see anything --dwarfX related in my logs.

- Sedat -

> >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
> >