AdaptiveCpp installation instructions for SPIR-V/OpenCL

You will need an OpenCL implementation, and the OpenCL icd loader. The OpenCL library can be specified using cmake -DOpenCL_LIBRARY=/path/to/libOpenCL.so.

In order to generate correct code, AdaptiveCpp needs to use its own fork of the Khronos LLVM-SPIRV translator hosted at https://github.com/AdaptiveCpp/SPIRV-LLVM-Translator. It will not work with the upstream translator. When building, AdaptiveCpp will automatically fetch and build the llvm-spirv translator for the right LLVM version.

The OpenCL backend can be enabled using cmake -DWITH_OPENCL_BACKEND=ON when building AdaptiveCpp. In order to run code successfully on an OpenCL device, it must support SPIR-V ingestion and, ideally, the Intel USM (unified shared memory) extension. If the Intel USM extensions are not supported, but the OpenCL device supports at least OpenCL coarse-grained SVM memory, malloc_device (but not malloc_host and malloc_shared are supported). If the device supports fine-grained system SVM, all USM forms are supported.