[llvm-dev] [llc] Producing ptx assembly for different target architectures - possible bug?

[Artem Belevich via llvm-dev]

On Mon, Oct 22, 2018 at 5:45 AM Lorenz Braun via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> i just found out that i can use llc to also produce ptx assembly for
> GPUs. I noticed that the produced ptx assembly seems to be targeted at
> the gpu architecture sm_20 by default.
>

This is currently the default CPU type for NVPTX back-end.


>
> Is there a way to explicitly demand different or additional target
> architectures like sm_30 for example?
>

It works the same way as for the other back-ends. You specify the CPU
variant with -mcpu=. E.g. for sm_30 you should use -mcpu=sm_30


>
> When i compile a cuda kernel with gpu arch. sm_30 using clang++ the
> .target directive in the ptx assembly will be set to sm_30. However when
> i save the bitcode of the same compilation and hand it to llc the
> .target directive is sm_20. There is an attribute in the bitcode that
> say "target-cpu"="sm_30". The information that sm_30 is required is
> still there.


It's a *function* attribute which, generally speaking, can't be used as the
default for the whole module. It also does not do much in NVPTX back-end.
Eventually it will be used to enforce that -mcpu=XXX is the same or higher
than the all target-cpu attributes in a module. This is one of the areas
where NVPTX can't implement what the attribute was intended to do -- target
different CPU variants within the same module. It's doable on x86 where the
same ISA can represent instructions for different CPU variants, but can't
be done in PTX which requires everything in the module to be for the same
GPU.


> I can imagine that llc might not process this information. Could this be a
> bug?
>

Not really. It's more of a feature in clang/llvm which NVPTX back-end can't
implement.

--Artem



>
> I am currently using llvm 7.0 and i can provide the bitcode if anyone
> wants to reproduce the problem.
>
> Best regards
> Lorenz
>
> --
> Lorenz Braun
> Research Associate
> Institute of Computer Engineering (ZITI)
> INF 368
> Raum 528
> 69120 Heidelberg
>
> Phone: +49 6221 54-16441
> lorenz.braun at ziti.uni-heidelberg.de
>
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>


-- 
--Artem Belevich
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