The [OpenCL 1.2 extension specification][cl-12-ext] specifies the following set of OpenCL C extensions. These extensions have been grouped together into the khr_opencl_c_1_2 extension object.

• cl_khr_global_int32_base_atomics

• cl_khr_global_int32_extended_atomics

• cl_khr_local_int32_base_atomics

• cl_khr_local_int32_extended_atomics

• cl_khr_byte_addressable_store

• cl_khr_fp64

The include/extension/khr_opencl_c_1_2.h header file and source/extension/khr_opencl_c_1_2.cpp source file define how the extensions listed above are reported to the OpenCL application by extending the clGetDeviceInfo entry point. This extension does not provided any additional OpenCL API entry points.

The ICD is provided by Khronos to allow multiple hardware vendor’s drivers to coexist on the same system and avoid an application suffering from linker errors when attempting to link against those drivers.

The OpenCL entry point definitions do not actually contain the implementation, where the work is done, instead they call into the cl namespace. For each OpenCL entry point, such as clGetPlatformIDs. There is a matching function in the cl namespace that is invoked, in this example cl::GetPlatformIDs. This was done to provide a clean boundary between the ICD and the implementation of the OpenCL entry points.

CL_API_ENTRY cl_int CL_API_CALL clGetPlatformIDs(const cl_uint num_entries,
                                                 cl_platform_id *platforms,
                                                 cl_uint *const num_platforms) {
  return cl::GetPlatformIDs(num_entries, platforms, num_platforms);
}

Any object created by the driver, such as cl_command_queue, must contain a pointer to the ICD dispatch table. This is the mechanism used by the ICD to determine which driver an API object works with; it functions in much the same way as a C++ virtual function table but because of this similarity the OpenCL API objects must not themselves contain a virtual function table. The ICD specifies that any object created by the driver must reserve the first sizeof(void*) bytes in its structure for the ICD dispatch table.

The Kernel Debug - cl_codeplay_kernel_debug extension grants developers the ability to specify build options which enable attaching a debugger to a kernel being executed on a device.

• The -g flag, providing emission of debug symbols in the compiled kernel.

• The -S <path> flag, setting the source code location in debug info to the specified path. Our runtime then creates this file if it does not already exist on disk, allowing the debugger to display kernel source code without manual configuration.

The Extra Build Options - cl_codeplay_extra_build_options extension allows the user to specify additional flags handled by the clBuildProgram and clCompileProgram entry-points.

• The -cl-llvm-stats flag allowing llvm to print the statistics from all the passes that have any.

• The -cl-precache-local-sizes=<sizes> build option allows for the pre-caching of kernel compilation for the specified local work group sizes.

The Kernel Exec Info - cl_codeplay_kernel_exec_info extension adds support for allowing additional information other than argument values to be passed to a kernel. This base extension doesn’t provide support for any particular parameter types but is intended to be built upon by future extensions that require this support. For example Intel USM extension USM - cl_intel_unified_shared_memory requires support for clSetKernelExecInfo which is part of the 2.0 API. Instead, USM will be able to use this extension to support 1.2.

cl_int clSetKernelExecInfoCODEPLAY(cl_kernel kernel,
                                   cl_kernel_exec_info param_name,
                                   size_t param_value_size,
                                   const void* param_value)

The Performance Counters - cl_codeplay_performance_counters extension allows the application to enable and get results from a set of supported, likely hardware, performance counters.

The flow of execution is as follows:

1. Query the devices’ supported performance counters using clGetDeviceInfo with a param_name of CL_DEVICE_PERFORMANCE_COUNTERS_CODEPLAY.

2. Create a cl_command_queue using clCreateCommandQueueWithProperties with a properties key of CL_QUEUE_PERFORMANCE_COUNTERS_CODEPLAY and a value of type cl_performance_counter_config_codeplay* containing a list of cl_performance_counter_desc_codeplay structures specifying which performance counters should be enabled using uuids attained in step 1.

3. Enqueuing a workload ensuring to provide a cl_event.

4. Waiting for the workload to complete execution.

5. Querying the performance counter results using clGetEventProfilingInfo with a param_name of CL_PROFILING_COMMAND_PERFORMANCE_COUNTERS_CODEPLAY.

6. Read the performance counter results using the storage member of cl_performance_counter_codeplay to select the correct anonymous union member of each cl_performance_counter_result_codeplay structure.

The Soft Math - cl_codeplay_soft_math extension allows a developer to force the math builtins used to be sourced from Abacus. An additional build option is supported for clCompileProgram and clBuildProgram to specify this "-codeplay-soft-math".

When a customer compiler backend consumes an executable, it is free to replace builtin functions with the optimized equivalents for the target platform. For instance, even though Abacus provides a conformant implementation of the count leading zeros clz builtin function, many mux targets will have a hardware instruction that allows this function to be implemented more efficiently. With the "-codeplay-soft-math" option specified, the mux backend will not use any hardware optimized builtins, and instead rely on the Abacus functionality.

Being able to specify that the mux target cannot use a more efficient implementation of Abacus builtin functionality allows testing and performance metrics to be performed much more easily.

The Whole Function Vectorization - cl_codeplay_wfv extension provides a mechanism to vectorize an OpenCL across the primary work-item dimension, using our whole function vectorization library VECZ.

An additional build option -cl-wfv={always|auto|never} is supported for clCompileProgram and clBuildProgram to enable/disable whole function vectorization. These choices are described in detail in the extension specification.

A new entry point clGetKernelWFVInfoCODEPLAY is added to allow whole function vectorization information to be queried for a specified kernel, given a device and local work size.

clGetKernelWFVInfoCODEPLAY supports two parameter queries:

• CL_KERNEL_WFV_STATUS_CODEPLAY queries the status of whole function vectorization.

• CL_KERNEL_WFV_WIDTHS_CODEPLAY queries the list of widths for each work-item dimension.

Intel’s USM - cl_intel_unified_shared_memory extension adds support for Unified Shared Memory(USM) to OpenCL, where rather than cl_mem handles, memory allocations are represented as pointers in the host application. Unified Shared Memory additionally provides fine-grained control over placement and accessibility of an allocation. This is a device extension rather than a platform extension as every device in the OpenCL platform may not provided the minimum level of support required to implement the extension.

Only host and device allocations are supported, not shared USM allocations which are intended to migrate between the host and one or more devices. This is because our goal is to support SYCL 2020 Explicit USM which does not require this capability.

Extension requires clSetKernelExecInfo from the OpenCL version 2.0 API which we don’t implement as part of our default 1.2 implementation. However, we have backported the entry point to 1.2 in extension cl_codeplay_kernel_exec_info. Therefore to support the USM behaviour provided by clSetKernelExecInfo in a 1.2 build the user can use clSetKernelExecInfoCODEPLAY with cl_codeplay_kernel_exec_info enabled. Alternatively an OpenCL version 3.0 build of the oneAPI Construction Kit can be used without our cl_codeplay_kernel_exec_info extension, where the entry point is available as core to be optionally implemented.

The API and enum definitions for USM are part of CL/cl_ext.h in the external OpenCL-Headers repository.

USM functionality is tested in UnitCL by the tests found in the directory source/cl_intel_unified_shared_memory of UnitCL, which can be run with --gtest_filter=*USM* or build target ninja check-ock-UnitCL-USM.

Support for clHostMemAllocINTEL() is an optional feature of the extension which may be queried for using CL_DEVICE_HOST_MEM_CAPABILITIES_INTEL. Currently we only support host USM allocations if a ComputeMux device can use host allocated memory by reporting mux_allocation_capabilities_cached_host and the ComputeMux device has the same pointer size as host.

To support USM functionality in SYCL, ComputeCpp has found it necessary to generate SPIR-V without address space information in it. To enable this the USM Generic Storage Class - spv_codeplay_usm_generic_storage_class extension was created. With this extension enabled a SPIR-V module can pass the Generic storage class for all of its pointer type declarations to indicate that no address space information is included in the declaration. Our SPIR-V translator interprets all such pointer type declarations as having address space 0. This address space was chosen primarily because this is the address space ComputeCpp uses internally for this, and secondarily because it means that function scope variables can remain as allocas, which is helpful for later optimizations.

Note that this overrides the normal semantics of storage class Generic. We wrote the extension this way rather than adding a new storage class so we could keep using existing SPIR-V tools without needing to fork them to add support.

oneAPI Construction Kit implements version 0.9.1 of the provisional cl_khr_command_buffer extension.

The simultaneous-use optional capability is supported, which allows the same command-buffer to be repeatedly enqueued without blocking in user code.

cl_khr_command_buffer is intended as a base extension that other future extensions will layer upon. These will provide additional features such as recording a command-buffer across multiple command-queues, potentially associated with different devices, as well as the ability to modify a command-buffer recording in between replays.

Our implementation currently has the following limitations which need resolved before the extension can be turned on by default in the oneAPI Construction Kit:

• Event profiling is not implemented (see CA-3322).

oneAPI Construction Kit implements version 0.9.0 of the provisional cl_khr_command_buffer_mutable_dispatch extension.

cl_khr_command_buffer_mutable_dispatch builds upon cl_khr_command_buffer to allow users to modify kernel execution commands between enqueues of a command-buffer.

oneAPI Construction Kit implements the cl_khr_extended_async_copies extension.

This has a default implementation which does simple copies of memory. This can be replaced by a customer implementation where it can be accelerated.

oneAPI Construction Kit implements the cl_intel_required_subgroup_size extension.

This has a default implementation which does not report any available subgroup sizes for any device. The compiler will report an error for any kernel given the intel_reqd_sub_group_size attribute with a size that is not reported by the device. There is currently no handling of the attribute if a target does report a set of sub-group sizes, as no in-tree target does so.

Furthermore, all kernels report the same non-zero value for CL_KERNEL_SPILL_MEM_SIZE_INTEL.