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

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Date:   Mon, 30 Nov 2020 12:45:15 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     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>,
        Fangrui Song <maskray@...gle.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 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?

Yes, I plan to revisit the series based on all of the feedback thus
far.  Lately I'm focused on enabling LLVM_IAS=1 for Android; but I
would like to see this land so that the Linux kernel may provide
coverage and feedback to the toolchain developers for DWARF v5 (as
well as reduced binary image sizes).  Maybe later this week I'll have
time to revisit.
-- 
Thanks,
~Nick Desaulniers