linux-kernel - Re: [PATCH] Kbuild: implement support for DWARF5

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Date:   Tue, 3 Nov 2020 14:21:22 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Jakub Jelinek <jakub@...hat.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-toolchains@...r.kernel.org,
        Alistair Delva <adelva@...gle.com>,
        Nick Clifton <nickc@...hat.com>
Subject: Re: [PATCH] Kbuild: implement support for DWARF5

On Mon, Nov 2, 2020 at 12:18 AM Jakub Jelinek <jakub@...hat.com> wrote:
>
> On Mon, Nov 02, 2020 at 11:20:41AM +0900, Masahiro Yamada wrote:
> > > --- /dev/null
> > > +++ b/scripts/test_dwarf5_support.sh
> > > @@ -0,0 +1,4 @@
> > > +#!/bin/sh
> > > +# SPDX-License-Identifier: GPL-2.0
> > > +set -eu
> > > +echo ".file 0 \"asdf\"" | $* -Wa,-gdwarf-5 -c -x assembler -o /dev/null -
> >
> >
> >
> > Please tell me how this script detects the dwarf-5 capability.
> >
> >
> > This script fails for GCC 10.
>
> One thing is GCC DWARF-5 support, that is whether the compiler
> will support -gdwarf-5 flag, and that support should be there from
> GCC 7 onwards.

I should improve my Kconfig check; I don't actually have a test for
-gdwarf-5 for the compiler.  In godbolt, it looks like -gdwarf-5
produces an error from GCC up until GCC 5.1.  Does (5.1 < GCC < 7) not
produce DWARF5?  Maybe there's a more specific test you had in mind?

>
> Another separate thing is whether the assembler does support
> the -gdwarf-5 option (i.e. if you can compile assembler files
> with -Wa,-gdwarf-5) for GNU as I think that is binutils 35.1,
> i.e. very new); but only if you want to pass the -Wa,-gdwarf-5
> only when compiling *.s and *.S files.  That option is about whether
> the assembler will emit DWARF5 or DWARF2 .debug_line.
> It is fine to compile C sources with -gdwarf-5 and use DWARF2
> .debug_line for assembler files if as doesn't support it.
>
> Yet another thing is if you can pass -Wa,-gdwarf-5 even when
> compiling C files.  There are several bugs in that category that have been
> fixed only in the last few days on binutils trunk, I'd suggest
> just not to bother, GCC 11 will have proper test for fixed assembler
> and will pass -gdwarf-5 to as when compiling even C sources with -gdwarf-5.

Do you have links?  I would prefer to do feature detection rather than
version detection when possible.  If the bug is so severe that we
think we should scuttle support for old versions, I'm ok with that,
but I want to be able to link to hard proof in a commit message so
that in 6 months when I forget why we made a certain decision, we have
historical record in the commit message/git blame.
-- 
Thanks,
~Nick Desaulniers