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From:   Masahiro Yamada <masahiroy@kernel.org>
Date:   Sat, 16 Jan 2021 08:54:08 +0900
Message-ID: <CAK7LNATAAZ_LH_q8x9A7FwBy=kqMd8Z0rVm-wuC1QqxpgsnBQg@mail.gmail.com>
Subject: Re: [PATCH v5 1/3] Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Sedat Dilek <sedat.dilek@gmail.com>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Clang-Built-Linux ML <clang-built-linux@googlegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Jakub Jelinek <jakub@redhat.com>,
        Fangrui Song <maskray@google.com>,
        Caroline Tice <cmtice@google.com>,
        Nick Clifton <nickc@redhat.com>, Yonghong Song <yhs@fb.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
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Precedence: bulk

On Sat, Jan 16, 2021 at 6:51 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Fri, Jan 15, 2021 at 1:48 PM Sedat Dilek <sedat.dilek@gmail.com> wrote:
> >
> > On Fri, Jan 15, 2021 at 10:06 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > From: Masahiro Yamada <masahiroy@kernel.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
> > >
> > > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> > > Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> > > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> >
> > Subject misses a "kbuild:" label like in all other patches.
> > You have:
> > "Remove $(cc-option,-gdwarf-4) dependency from CONFIG_DEBUG_INFO_DWARF4"
>
> Ack, I wonder how that happened? Ah well, will fix in v6; thanks for
> the feedback.


I will apply this in my tree,
adding "kbuild:" and fixing the typo pointed out by Fangrui.

You do not need to resend this one.


> >
> > - Sedat -
> >
> > > ---
> > >  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
> > >
>
>
>
> --
> Thanks,
> ~Nick Desaulniers


--
Best Regards
Masahiro Yamada