[llvm-dev] Executing OpenMP 4.0 code on Nvidia's GPU

[Arpith C Jacob via llvm-dev]

Hi Ahmed,

I am experimenting with LTO, but as you said, it's still *very* hacky.

Here's what I did.  First compile the CUDA GPU OMP runtime with Clang
(rather than nvcc) to bitcode.  When I looked at Clang-CUDA a couple of
weeks ago I could only get device side bitcode by using the temporary files
generated after passing -save-temps to Clang.  The OMP-GPU version of LLVM
that you are using is not up to date with trunk, so I had to do a bit of
massaging on the generated IR.

I then had to manually link the various device side bitcodes, call opt,
llc, ptxas, and finally link it with the host object file.

We don't have support for this in the driver as yet but once we move to
trunk I will look into streamlining this.

Thanks,
Arpith



From:	Ahmed ElTantawy <ahmede at ece.ubc.ca>
To:	Arpith C Jacob/Watson/IBM at IBMUS
Cc:	llvm-dev at lists.llvm.org, "Bataev, Alexey"
            <alexey.bataev at intel.com>
Date:	01/20/2016 08:44 AM
Subject:	Re: Executing OpenMP 4.0 code on Nvidia's GPU
Sent by:	ahmed.mohammed.eltantawy at gmail.com



Hi,

I see now that the linking happens at the binary level. I was wondering
whether it is possible to link to the OpenMP runtime library at the LLVM IR
level (to enable LTO optimizations for the code after library calls has
been replaced).

I have done this before by linking to the bitcode of a file that contains
the compiled CUDA implementation of the OpenMP runtime library. But it was
a bit hacky, and offloading was not supported yet. Is it there a
cleaner/standard way to do this ?

Thanks.

On Wed, Jan 20, 2016 at 5:09 AM, Ahmed ElTantawy <ahmede at ece.ubc.ca> wrote:
  Hi Arpith,

  That is exactly what it is :).

  My bad, I thought I copied over the libraries to where LIBRARY_PATH
  pointing but apparently it was copied to a wrong destination.

  Thanks a lot.

  On Wed, Jan 20, 2016 at 4:51 AM, Arpith C Jacob <acjacob at us.ibm.com>
  wrote:
   Hi Ahmed,

   nvlink is unable to find the GPU OMP runtime library in its path. Does
   LIBRARY_PATH point to the right location? You could try passing the "-v"
   option to clang to get more information.

   Regards,
   Arpith




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