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

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Message-ID: <20201102081810.GB3788@tucnak>
Date:   Mon, 2 Nov 2020 09:18:10 +0100
From:   Jakub Jelinek <jakub@...hat.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        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
Subject: Re: [PATCH] Kbuild: implement support for DWARF5

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.

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.
The reason is to get DWARF5 .debug_line (.debug_line is usually produced
by the assembler, not compiler, from .file/.loc directives).

	Jakub