Portable Computing Language (PoCL) for Ventus GPGPU

See README-pocl.md for original README.

See https://github.com/THU-DSP-LAB/llvm-project for detailed build guide with Ventus llvm based OpenCL Compiler and icd loader.

Original PoCL build instruction doesn't work for Ventus GPGPU.

TODOs

TODOs are divided into 3 parts, first part contains jobs required to make sure OpenCL tests can be tested with pocl+spike, second part contains jobs required to enable real Ventus GPGPU work flow, third part contains generic TODOs.

TODOs (Part 1)

• Correctly report Ventus GPGPU virtual device after pocl is statically/dynamally linked with spike(Currently ventus device is hardcoded).

• Determine how we implement workitem builtins, if using workitem.S implementation from ventus-llvm, some extra work should to be done in ventus pocl driver, otherwise we need to redesign how workitem builtins are implemented.

  1). Functions such as insertPrologue from ParallelRegion.cc inserts instructions to get local_id_x/y/z which are preset by WorkitemReplication::ProcessFunction, this behavior is weird.

  2). Workitem variables local_size_x/y/z can have dynamic size which to be determined at runtime, this is done in pocl WorkitemHandler.

• Make sure pocl ventus driver is correctly initialized, such as pocl_ventus_init is double checked, basically every items in struct _cl_device_id should be properly initialized.

• Make sure there is no native(host) device related code in ventus pocl driver, such as kernel.so in function llvm_codegen should be renamed to kernel.elf or something else(a static elf file). Also the native kernel execution dlopen(kernel.so) should be replaced by an elf loader(Should already be done by spike fesrv elf loader).

• Kernel metadata buffer and kernel args buffer should be prepared in pocl, then passed to spike by asking spike to store those buffer into the lowerest address of physical memory of Ventus GPGPU, related CSRs should be initialized with the kernel metadata buffer base address.

• Check the behavior of all the OpenCL APIs for Ventus GPGPU are correctly coded(Some API implementations in common.c may not work for Ventus GPGPU), make sure all the examples and tests can be kicked off(may not run through) to spike(via spike elf loader).

• Device side printf support. A buffer with size PRINTF_BUFFER_SIZE on device side should be reserved, and a CSR or event may be needed to notify OCL driver to copy device side printf buffer to host side.

TODOs (Part 2)

• Ventus GPGPU kernel mode driver(kmd) should be provided and bridged with pocl.
• Correctly report Ventus GPGPU device after pocl(umd) can locate ventus kmd and read necessory information from the kmd(such as supported extensions etc).

TODOs (Part 3)

• Fix hardcoded library search path for libworkitem.a in ventus.c.