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

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Date:   Thu, 10 Dec 2020 19:29:10 -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 10, 2020 at 03:18:45PM -0800, Nick Desaulniers wrote:
> On Fri, Dec 4, 2020 at 9:06 AM Arvind Sankar <nivedita@...m.mit.edu> wrote:
> >
> > 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.
> 
> Oh, I see. Yeah, that might be a nicer approach.  What should we do in
> the case of gcc < 7 though, where -gdwarf-5 won't produce DWARF v5?
> Maybe that's ok, but the intent behind the Kconfig check was to
> prevent the option from being selectable if the tools do not support
> it.  Maybe it's more flexible to pass the arguments along, and hope
> for the best?
> 
> As a gcc-5 user, I might be surprised if I chose
> CONFIG_DEBUG_INFO_DWARF5 if what I got was not actually DWARF v5; it
> does violate the principle of least surprise.  Maybe that doesn't
> matter though?

Even the current gcc documentation still says "DWARF Version 5 is only
experimental".  If the user wants to try it out, I think it's fine to
let them get whatever subset their tool chain produces, as long as it's
not completely broken. Your latest help text does say that gcc 7+ is
required, maybe add another sentence saying that gcc 5+ only has partial
support for some draft DWARF 5 features?

Thanks.