linux-kernel - Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

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Message-ID: <CAK7LNAS_hxevOS7hKxepyCBVU-4j87Yf5Y8DB6mFq+4xuaz3AA@mail.gmail.com>
Date:   Tue, 1 Dec 2020 12:38:16 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Fāng-ruì Sòng <maskray@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jakub Jelinek <jakub@...hat.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH v2 3/4] Kbuild: make DWARF version a choice

On Tue, Dec 1, 2020 at 5:27 AM 'Fāng-ruì Sòng' via Clang Built Linux
<clang-built-linux@...glegroups.com> wrote:
>
> On Mon, Nov 30, 2020 at 10:05 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > On Wed, Nov 4, 2020 at 9:53 AM 'Nick Desaulniers' via Clang Built
> > Linux <clang-built-linux@...glegroups.com> wrote:
> > >
> > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
> > > explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
> > > way that's forward compatible with existing configs, and makes adding
> > > future versions more straightforward.
> > >
> > > Suggested-by: Fangrui Song <maskray@...gle.com>
> > > Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > ---
> > >  Makefile          | 14 ++++++++------
> > >  lib/Kconfig.debug | 19 +++++++++++++++----
> > >  2 files changed, 23 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 75b1a3dcbf30..e23786a4c1c7 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -826,12 +826,14 @@ else
> > >  DEBUG_CFLAGS   += -g
> > >  endif
> > >
> > > -ifndef LLVM_IAS
> > > -KBUILD_AFLAGS  += -Wa,-gdwarf-2
> > > -endif
> > > -
> > > -ifdef CONFIG_DEBUG_INFO_DWARF4
> > > -DEBUG_CFLAGS   += -gdwarf-4
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> > > +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> > > +DEBUG_CFLAGS   += -gdwarf-$(dwarf-version-y)
> > > +ifneq ($(dwarf-version-y)$(LLVM_IAS),21)
> > > +# Binutils 2.35+ required for -gdwarf-4+ support.
> > > +dwarf-aflag    := $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> > > +DEBUG_CFLAGS   += $(dwarf-aflag)
> >
> > This changes the behavior.
> >
> > For the Dwarf-2 case,
> >
> > Previously, -gdwarf-2 was passed to $(CC),
> > so the debug info was generated by gcc.
> >
> > Now, -Wa,-gdwarf-2 is passed to $(CC).
> > -gdwarf-2 is handled by GNU as.
> > So, the source info points to /tmp/<hash>.s
> > instead of the original .c file.
> >
> >
> >
> > Handling the Dwarf capability is very complicated.
> >
> > Are you still working for v3?
> >
> >
> >
> > > +KBUILD_AFLAGS  += $(dwarf-aflag)
> > >  endif
> > >
> > >  ifdef CONFIG_DEBUG_INFO_REDUCED
> > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > > index 83a860126897..03c494eefabd 100644
> > > --- a/lib/Kconfig.debug
> > > +++ b/lib/Kconfig.debug
> > > @@ -256,14 +256,25 @@ config DEBUG_INFO_SPLIT
> > >           to know about the .dwo files and include them.
> > >           Incompatible with older versions of ccache.
> > >
> > > +choice
> > > +       prompt "DWARF version"
> > > +       help
> > > +         Which version of DWARF debug info to emit.
> > > +
> > > +config DEBUG_INFO_DWARF2
> > > +       bool "Generate DWARF v2 debuginfo"
> > > +       help
> > > +         Generate DWARF v2 debug info.
> > > +
> > >  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.
> > > -         But it significantly improves the success of resolving
> > > -         variables in gdb on optimized code.
> > > +         Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> > > +         It makes the debug information larger, but it significantly
> > > +         improves the success of resolving variables in gdb on optimized code.
> > > +
> > > +endchoice # "DWARF version"
> > >
> > >  config DEBUG_INFO_BTF
> > >         bool "Generate BTF typeinfo"
> > > --
> > > 2.29.1.341.ge80a0c044ae-goog
>
>
> We can bump -Wa,-gdwarf-2 to -Wa,-gdwarf-3 since GNU actually emits
> DWARF v3 DW_AT_ranges (see
> https://sourceware.org/bugzilla/show_bug.cgi?id=26850 )
> This can avoid the `warning: DWARF2 only supports one section per
> compilation unit` warning for Clang.


I am not a DWARF spec expert.

Please teach me.

In my understanding, "DWARF2 only supports one section ..."
is warned only when building .S files with LLVM_IAS=1

If this is due to the limitation of DWARF v2, why is it OK to
build .c files with LLVM_IAS?



>
> Deleting -Wa,-gdwarf-2 also sounds good to me if people can verify
> their debugging experience is not regressed (I believe it is useless).
>
>
> --
> 宋方睿
>
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-- 
Best Regards
Masahiro Yamada