Support Pocl for CPU-only OpenCL implementation

[jserv]

Current GPU-accelerated PoW is implemented via OpenCL, and Travis-CI in general doesn't have GPUs. The good thing about OpenCL is that you can use a CPU implementation of OpenCL, such as Pocl, which uses Clang as an OpenCL C frontend and LLVM for kernel compiler implementation, and as a portability layer.

Pocl packages are available in recent Debian/Ubuntu repositories:

sudo apt install libpocl-dev

Utilities such as clinfo can find the new pocl ICD via

/etc/OpenCL/vendors/pocl.icd

[jserv]

Sample usage on Pocl and Travis: https://github.com/boostorg/compute/blob/master/.travis.yml

[jserv]

PoW linking to Pocl was not functioned as expected:

[dcurl] Implementation GPU (OpenCL) is initialized successfully
test-pow: tests/test-pow.c:111: main: Assertion `pow_ctx' failed.
Aborted (core dumped)

Reported by clinfo:

Number of platforms                               1
  Platform Name                                   Portable Computing Language
  Platform Vendor                                 The pocl project
  Platform Version                                OpenCL 2.0 pocl 0.13, LLVM 3.8.1
  Platform Profile                                FULL_PROFILE
  Platform Extensions                             cl_khr_icd
  Platform Extensions function suffix             POCL

  Platform Name                                   Portable Computing Language

[furuame]

The reason might be that I only configure GPU devices on the dcurl.

dcurl/src/clcontext.c

Line 171 in 5dc6479

clGetDeviceIDs(platform[i], CL_DEVICE_TYPE_GPU, 0, NULL, &num_devices);

[furuame]

@jserv Should we create a now BUILD option in Makefile for CI test?

E.g.

#if defined(BUILD_CI)
    clGetDeviceIDs(platform[i], CL_DEVICE_TYPE_CPU, 0, NULL, &num_devices);
#else
    clGetDeviceIDs(platform[i], CL_DEVICE_TYPE_GPU, 0, NULL, &num_devices);
#endif

[jserv]

@jserv Should we create a now BUILD option in Makefile for CI test?

Never write code dedicated to CI. We can provide some shotcuts or helper functions instead.

[furuame]

Maybe we should reserve the flexibility to let the user can configure their own OpenCL implementation. And we can provide some pre-tested config file like GPU or Pocl.

[jserv]

I agree with the way to improve the flexibility of adopting various OpenCL implementations like Pocl.

[marktwtn]

Since the current CI system is buildkite running on our own machine, we do not worry about the GPU problem.
Shall we close this issue or leave it as a future enhancement?

[jserv]

Close the historical issue