linux-kernel - Re: [PATCH v2 2/4] Kbuild: do not emit debug info for assembly with LLVM

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Message-ID: <CAKwvOd=9iqLgdtAWe2h-9n=KUWm_rjCCJJYeop8PS6F+AA0VtA@mail.gmail.com>
Date:   Mon, 9 Nov 2020 10:28:21 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        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>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Alistair Delva <adelva@...gle.com>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] Kbuild: do not emit debug info for assembly with LLVM_IAS=1

On Wed, Nov 4, 2020 at 10:58 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Tue, Nov 03, 2020 at 04:53:41PM -0800, Nick Desaulniers wrote:
> > Clang's integrated assembler produces the warning for assembly files:
> >
> > warning: DWARF2 only supports one section per compilation unit
> >
> > If -Wa,-gdwarf-* is unspecified, then debug info is not emitted.  This
>
> Is this something that should be called out somewhere? If I understand
> this correctly, LLVM_IAS=1 + CONFIG_DEBUG_INFO=y won't work? Maybe this
> should be handled in Kconfig?

Specifically, debug info will not be emitted, for assembler source
files. It will still be emitted for C source files (via -gdwarf-*).
-Wa,-gdwarf-* only affects assembler file sources.

>
> > will be re-enabled for new DWARF versions in a follow up patch.
> >
> > Enables defconfig+CONFIG_DEBUG_INFO to build cleanly with
> > LLVM=1 LLVM_IAS=1 for x86_64 and arm64.
> >
> > Cc: <stable@...r.kernel.org>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/716
> > Reported-by: Nathan Chancellor <natechancellor@...il.com>
> > Suggested-by: Dmitry Golovin <dima@...ovin.in>
>
> If you happen to respin, Dmitry deserves a Reported-by tag too :)

Sure.

>
> > Suggested-by: Sedat Dilek <sedat.dilek@...il.com>
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> Regardless of the other two comments, this is fine as is as a fix for
> stable to unblock Android + CrOS since we have been running something
> similar to it in CI:
>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
>
> > ---
> >  Makefile | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/Makefile b/Makefile
> > index f353886dbf44..75b1a3dcbf30 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -826,7 +826,9 @@ else
> >  DEBUG_CFLAGS += -g
> >  endif
> >
> > +ifndef LLVM_IAS
>
> Nit: this should probably match the existing LLVM_IAS check

Sure, will send a v3.  Going to just send this for now, as it's
blocking some downstream work I'm trying to get done in Android.

>
> ifneq ($(LLVM_IAS),1)
>
> >  KBUILD_AFLAGS        += -Wa,-gdwarf-2
> > +endif
> >
> >  ifdef CONFIG_DEBUG_INFO_DWARF4
> >  DEBUG_CFLAGS += -gdwarf-4
> > --
> > 2.29.1.341.ge80a0c044ae-goog
> >



-- 
Thanks,
~Nick Desaulniers