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

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Date:   Wed, 4 Nov 2020 13:19:34 +0100
From:   Jakub Jelinek <jakub@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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 Tue, Nov 03, 2020 at 02:21:22PM -0800, Nick Desaulniers wrote:
> > > 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?

No.  After all, those versions also predate DWARF5.
All 5.1 - 6.x did was start accepting -gdwarf-5 as experimental option
that enabled some small DWARF subset (initially only a few DW_LANG_* codes
newly added to DWARF5 drafts).  Only GCC 7 (released after DWARF 5 has
been finalized) started emitting DWARF5 section headers and got most of the
DWARF5 changes in, e.g. including switching over most of the now
standardized GNU extensions from their DW_*_GNU_* codes to DWARF5 DW_*).
With GCC 5/6, you get:
echo 'int i;' | gcc -c -o /tmp/test.o -xc - -gdwarf-5; readelf -wi /tmp/test.o | grep Version:
   Version:       4
while with 7+
   Version:       5
instead.

>  Maybe there's a more specific test you had in mind?

Guess what you want to test is what version you actually get in .debug_info
if you compile with -gdwarf-5.

> > 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

The
https://gcc.gnu.org/r11-3693
https://gcc.gnu.org/r11-4338
commits contain those tests in gcc/configure.ac

	Jakub