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

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Date:   Fri, 4 Dec 2020 12:06:08 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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 03, 2020 at 03:28:14PM -0800, Nick Desaulniers wrote:
> 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.
> 

Why? Does this actually cause any problems?

It seems like the options for gcc can actually be very straightforward:
you just need a cc-option check, and then add -gdwarf-N to both CFLAGS
and AFLAGS and you're done.  Adding the -Wa flag is superfluous and
carries the risk of interfering with what the compiler driver does. Just
let the gcc driver handle the details.

Clang/IAS is almost as straightforward, with the only additional edge
case being that for assembler files, DWARF 2 doesn't work, so the CFLAGS
is the same -gdwarf-N, but AFLAGS gets -gdwarf-N only if N > 2.

The messy case is only Clang/IAS=0, which needs to check the support
from the external assembler, and needs CFLAGS of -gdwarf-N and AFLAGS of
-Wa,--gdwarf-N, because Clang doesn't pass that option on to an external
assembler. This is why I was asking if the assembler support check can
be restricted to CC_IS_CLANG: nothing but Clang/IAS=0 actually requires
that check.