[llvm-dev] NVPTX Back-end: relocatable device code support for dynamic parallelism

[Justin Lebar via llvm-dev]

Sorry for the long delay in replying, I'm not good at reading the mailing list.

> My guess is, that this feature is not supported. Does anyone know is this is the case?

Clang does not support dynamic parallelism or relocatable CUDA code
today.  Patches -- and documentation fixes to mention this at
https://llvm.org/docs/CompileCudaWithLLVM.html -- are certainly
welcome.

-Justin

On Fri, Jun 9, 2017 at 4:31 AM, Lorenz Braun via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi everyone,
>
> CUDA allows to call some runtime functions also from the device code. On a
> multi-GPU system this allows the GPU to determine its device id on its own
> via cudaGetDevice().
> Unfortunately i cannot get it working when compiling with clang. When
> compiling with nvcc relocatable device code needs to be set to true
> (-rdc=true) and the cudadevrt is needed when linking [0]. I did not found
> such switches to turn rdc for clang. Just compiling does not work as ptxas
> does not find the function cudaGetDevice().
>
> My guess is, that this feature is not supported. Does anyone know is this is
> the case?
>
> I also tried to find out what nvcc is doing when setting rdc to on, but hat
> a few problem trying to understand whats going on. I will attach the verbose
> output of nvcc. I
> have no clue what the binaries cudafe/cudafe++ and cicc are doing so its
> rather hard to guess whats happening.
> There are additional options like -D__CUDACC_RDC__, --device-c and
> --compile-only that are not used when rdc is off. All but --device-c can be
> used with clang and i can compile my program, however i can't get it to run
> properly.  For each runtime call i get an unknown error with code 30.
>
> I have few hope, that someone already has figured out how to use get rdc to
> work with clang, but i will be grateful for any hint. To whom could i write
> to regarding this problem? Maybe the NVPTX developers can help?
>
> [0]
> http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#toolkit-support-for-dynamic-parallelism
>
> --
> Lorenz Braun
> Research Associate
> Institute of Computer Engineering (ZITI)
> B6, 26, Building B, Office B2.20
> 68131 Mannheim
>
> Phone: +49-621-181-2696
> lorenz.braun at ziti.uni-heidelberg.de
>
>
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