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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 3 Dec 2020 16:17:36 -0800
Message-ID: <CAKwvOd=m9+CxsBuG_DLSx1gaMmOmpm05xeU353As_tPKUij2qA@mail.gmail.com>
Subject: Re: [PATCH] Kbuild: implement support for DWARF5
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Masahiro Yamada <masahiroy@kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        linux-toolchains@vger.kernel.org
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On Thu, Dec 3, 2020 at 2:56 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> On Tue, Nov 3, 2020 at 4:17 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Tue, Nov 03, 2020 at 04:05:36PM -0800, Nick Desaulniers wrote:
> > > On Tue, Nov 3, 2020 at 4:00 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> > > >
> > > > On Wed, Oct 21, 2020 at 06:21:06PM -0700, Nick Desaulniers wrote:
> > > > > Further -gdwarf-X where X is an unsupported value doesn't
> > > > > produce an error in $(CC).
> > > >
> > > > Do you have more details here? On godbolt.org, gcc does report an error
> > > > for unsupported dwarf versions.
> > > >
> > > > https://godbolt.org/z/G35798
> > > >
> > > > gcc does not seem to pass the -gdwarf-* options to the assembler when
> > > > compiling C source. For assembler, gcc will pass an appropriate option
> > > > depending on the version of binutils it was configured with: if the
> > > > assembler doesn't support dwarf-5 it can call it with --gdwarf2 for eg.
> > > >
> > > > If the user is using a properly configured toolchain it doesn't look
> > > > like it should be an issue to just use cc-option?
> > >
> > > I wrote the base patch back in May, and didn't revisit until recently.
> > > I could have sworn the cc-option silently failed for the check
> > > cc-option does, which is /dev/null input.  I need to recheck that, but
> > > it doesn't hurt to simply include it for now, which I've done in a v2
> > > I'm about to send.
> > > --
> > > Thanks,
> > > ~Nick Desaulniers
> >
> > This is giving me deja vu about the -gz=zlib option.
> >
> > Didn't Masahiro fix the cc-option issue with
> >   4d0831e8a029 ("kconfig: unify cc-option and as-option")
> >
> > The existing -Wa,-gdwarf-2 in the Makefile seems bogus, btw. GCC 4.9.0
> > at least appears to pass on --gdwarf2 automatically.
>
> It looks like we don't need -Wa,-gdwarf-2 when -gdwarf-2 is set. So I
> can probably drop
> +DEBUG_CFLAGS   += $(dwarf-aflag)
> from v2.  Will retest though.

That's needed for non LLVM_IAS=1 builds so that clang informs GAS to
assembler using DWARF Version 5; otherwise every translation unit
fails to assemble with an error from GAS.
-- 
Thanks,
~Nick Desaulniers