llvm dev - May 2016 - [RFC] Fail early on unimplementable append linkage uses

Append linkage is magical. It doesn't correspond to anything available
in real object files (ELF/COFF/MachO). In those files sections are
appended, but symbols are just resolved.

That means that appending linkage can only be used with llvm special
symbols that never hit a real .o.

Long term I think we should delete it an replace with explicit use of section.

But for now I would like to just error early if we try to actually put
one in an object file. That is what http://reviews.llvm.org/D20179
does. Is everyone OK with that with a langref update?

Cheers,
Rafael

On 13 May 2016 at 13:43, Rafael Espíndola <llvm-dev at lists.llvm.org>
wrote:
> That means that appending linkage can only be used with llvm special
> symbols that never hit a real .o.
So they should have been resolved by emission time, or it's a back-end
error.

Do you have an idea of what could be affected if we start emitting an
error, here? Test-Suite, other programs?

cheers,
--renato

On 13 May 2016 at 08:51, Renato Golin <renato.golin at linaro.org>
wrote:
> On 13 May 2016 at 13:43, Rafael Espíndola <llvm-dev at
lists.llvm.org> wrote:
>> That means that appending linkage can only be used with llvm special
>> symbols that never hit a real .o.
>
> So they should have been resolved by emission time, or it's a back-end
error.
Not resolved, translated. For example, llvm.ctors gets expanded to
whatever form the object file uses for processing static constructors.
> Do you have an idea of what could be affected if we start emitting an
> error, here? Test-Suite, other programs?
No. Clang only produces the well known ones. Anything producing a
appending linkage global that actually got written to the output would
hit a link error if it tried combining any two of those files (and can
get the same behaviour with an external linkage).

Cheers,
Rafale

I think appending linkage was intended to abstract over the
platform-specific details of arranging a custom section with start and
end symbols. Nobody ever implemented it though, so until they are
implemented, rejecting them early sounds good. :)

On Fri, May 13, 2016 at 5:43 AM, Rafael Espíndola
<llvm-dev at lists.llvm.org> wrote:
> Append linkage is magical. It doesn't correspond to anything available
> in real object files (ELF/COFF/MachO). In those files sections are
> appended, but symbols are just resolved.
>
> That means that appending linkage can only be used with llvm special
> symbols that never hit a real .o.
>
> Long term I think we should delete it an replace with explicit use of
section.
>
> But for now I would like to just error early if we try to actually put
> one in an object file. That is what http://reviews.llvm.org/D20179
> does. Is everyone OK with that with a langref update?
>
> Cheers,
> Rafael
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On Fri, May 13, 2016 at 9:14 AM Reid Kleckner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I think appending linkage was intended to abstract over the
> platform-specific details of arranging a custom section with start and
> end symbols. Nobody ever implemented it though, so until they are
> implemented, rejecting them early sounds good. :)
>
+1 from me

>
> On Fri, May 13, 2016 at 5:43 AM, Rafael Espíndola
> <llvm-dev at lists.llvm.org> wrote:
> > Append linkage is magical. It doesn't correspond to anything
available
> > in real object files (ELF/COFF/MachO). In those files sections are
> > appended, but symbols are just resolved.
> >
> > That means that appending linkage can only be used with llvm special
> > symbols that never hit a real .o.
> >
> > Long term I think we should delete it an replace with explicit use of
> section.
> >
> > But for now I would like to just error early if we try to actually put
> > one in an object file. That is what http://reviews.llvm.org/D20179
> > does. Is everyone OK with that with a langref update?
> >
> > Cheers,
> > Rafael
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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