linux-kernel - Re: [PATCH v2 4/4] Kbuild: implement support for DWARF v5

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Date:   Thu, 3 Dec 2020 15:28:14 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
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>,
        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 4/4] Kbuild: implement support for DWARF v5

On Thu, Dec 3, 2020 at 3:22 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Tue, Nov 24, 2020 at 9:28 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > On Tue, Nov 03, 2020 at 04:53:43PM -0800, Nick Desaulniers wrote:
> > > DWARF v5 is the latest standard of the DWARF debug info format.
> > >
> > > Feature detection of DWARF5 is onerous, especially given that we've
> > > removed $(AS), so we must query $(CC) for DWARF5 assembler directive
> > > support.  GNU `as` only recently gained support for specifying
> > > -gdwarf-5.
> >
> > With gcc, using -gdwarf-5 even without -Wa,--gdwarf-5 results in
> > considerably smaller debug info. gcc does not seem to generate the .file 0
> > directive that causes older GNU as to barf.
> >
> > Should the assembler support check be restricted to CC_IS_CLANG?
>
> No, because if LLVM_IAS=1 then the assembler support need not be checked.

Also, if your version of GCC supports DWARF Version 5, but your
version of GAS does not, then I'm more inclined to not allow
CONFIG_DEBUG_INFO_DWARF5 to be selectable, rather than mix and match
or partially support this for one but not the other.  Either all tools
used support DWARF 5, or you don't get to use DWARF 5.

> > >  config DEBUG_INFO_BTF
> > > diff --git a/scripts/test_dwarf5_support.sh b/scripts/test_dwarf5_support.sh
> > > new file mode 100755
> > > index 000000000000..156ad5ec4274
> > > --- /dev/null
> > > +++ b/scripts/test_dwarf5_support.sh
> > > @@ -0,0 +1,9 @@
> > > +#!/bin/sh
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +
> > > +# Test that assembler accepts -gdwarf-5 and .file 0 directives, which were bugs
> > > +# in binutils < 2.35.
> > > +# https://sourceware.org/bugzilla/show_bug.cgi?id=25612
> > > +# https://sourceware.org/bugzilla/show_bug.cgi?id=25614
> > > +set -e
> > > +echo '.file 0 "filename"' | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
> >
> > This also actually needs --gdwarf-5 to really check the support for the
> > option, but older versions should error on the .file 0 in any case.
>
> Based on Jakub's feedback on the earlier thread
> https://lore.kernel.org/lkml/20201104121934.GT3788@tucnak/
> it sounds like the dwarf version also needs to be dumped since GCC 5 <
> x < 7 accepts --gdwarf-5, but did not produce DWARF Version 5 debug
> info.

Sigh...llvm-readelf doesn't accept --debug-dump=info for checking the
DWARF version; llvm-dwarfdump works with no args...at this point I'm
tempted to just version check GCC.
-- 
Thanks,
~Nick Desaulniers