Halide can't seem to find OpenCL

[stewpend0us]

I'm using the 17.0.2 binaries on ubuntu 22.04. The following code always returns false:

host = Halide::get_host_target();
host.set_feature(Halide::Target::OpenCL);
return Halide::host_supports_target_device(host);

I have opencl installed (according to clinfo). I did have to specify set(OpenCL_LIBRARY "/usr/lib/x86_64-linux-gnu/libnvidia-opencl.so.1") in my CMakeLists.txt to get it building.

Is it possible to use OpenCL with the binaries or do I need to build from source?

[stewpend0us]

Figured it out. The following threads were helpful (but a pain to find!):
https://lists.csail.mit.edu/pipermail/halide-dev/2016-July/002453.html
https://lists.csail.mit.edu/pipermail/halide-dev/2018-May/003232.html

My CmakeLists.txt doesn't need to deal with OpenCL at all. Only Halide.

And in my code:

// you can still export HL_JIT_TARGET="host-cuda" if you want but this way it'll always be set to opencl by default.
// surely there's a way to do this without environment variables...but this is fine for now
setenv("HL_JIT_TARGET", "host-opencl-debug", false); // -debug is optional...
Halide::Target host = Halide::get_jit_target_from_environment();

Now a call to host.has_gpu_feature() returns true.

[abadams]

I'm a bit puzzled, because the first piece of code you wrote should have worked. I think there might be a bug here. Is it possible that other JIT code without opencl was already run in the same process?

[stewpend0us]

It's pretty basic I think. Here's what I'm doing:

setenv("HL_JIT_TARGET", "host-opencl-debug", false);
Halide::Target T = Halide::get_jit_target_from_environment();
//T.set_feature(Halide::Target::OpenCL); // Other than the debug stuff this actually does have the same result 
RCLCPP_INFO(get_logger(), "Halide Target: %s has GPU? %s! host supports? %s", T.to_string().c_str(), T.has_gpu_feature() ? "YES" : "NO", Halide::host_supports_target_device(T) ? "YES" : "NO");

pipeline = halide_pipeline(); // actually a Func

#if defined(LOWER)
try
{
    pipeline.compile_to_lowered_stmt("stmt.html", pipeline.infer_arguments(), Halide::StmtOutputFormat::HTML);
    RCLCPP_INFO(get_logger(), "generated stmt.html");
}
catch (...)
{
    RCLCPP_WARN(get_logger(), "failed to generate stmt.html");
}
#endif
pipeline.compile_jit(T);
pipeline.print_loop_nest();

if LOWER is defined I get:

Halide Target: x86-64-linux-avx-avx2-debug-f16c-fma-jit-opencl-sse41 has GPU? YES! host supports? NO
HALIDE ERROR: Error: Schedule for Func gray requires <Default_GPU> but no compatible target feature is enabled in target x86-64-linux-avx-avx2-f16c-fma-sse41

otherwise I get:

Halide Target: x86-64-linux-avx-avx2-debug-f16c-fma-jit-opencl-sse41 has GPU? YES! host supports? NO
produce gray:
  gpu_block y.yo<Default_GPU>:
    gpu_block x.xo<Default_GPU>:
      gpu_thread y.xi in [0, 1]<Default_GPU>:
        gpu_thread x.yi in [0, 1]<Default_GPU>:
          gray(...) = ...
Entering Pipeline gray
Target: x86-64-linux-avx-avx2-debug-f16c-fma-jit-opencl-sse41-user_context
 Input Buffer input: 0x612b3ffdd3b8 -> buffer(0, 0x0, 0x737704612010, 0, uint16, {0, 1920, 1}, {0, 1080, 1920})
 Input (void const *) __user_context: 0x7ffe809a6110
 Output Buffer gray: 0x612b3f73a6a8 -> buffer(0, 0x0, 0x0, 0, float32, {0, 480, 1}, {0, 270, 480})
CL: halide_opencl_initialize_kernels (user_context: 0x7ffe809a6110, state_ptr: 0x737713847000, program: 0x7377138450e0, size: 4721
    load_libopencl (user_context: 0x7ffe809a6110)
HALIDE ERROR: Error: OpenCL API not found: clGetPlatformIDs

So I guess it really is that it can't find OpenCL on my machine after all. Here's the output of clinfo -l:

Platform #0: Intel(R) OpenCL HD Graphics
 `-- Device #0: Intel(R) HD Graphics 530 [0x191b]

[abadams]

I think that means dlsym is not finding that function in whatever libOpenCL.so it managed to open.

[stewpend0us]

Thanks. Sorry to waste your time with this. This fixed it for me:

sudo ln -sv libOpenCL.so.1 /usr/lib/x86_64-linux-gnu/libOpenCL.so

not sure why the so.1 version exists but so does not.

[stewpend0us]

The call to compile_to_lowered_stmt was failing because I wasn't passing the 4th argument (Target)...sigh.