openvino/docs/IE_DG/Extensibility_DG/CPU_Kernel.md

How to Implement Custom CPU Layers

The primary vehicle for the performance of the CPU codepath in the Inference Engine is the Intel® Math Kernel Library for Deep Neural Networks (Intel® MKL-DNN), and new CPU kernels extend the Inference Engine plugin for the Intel MKL-DNN. Implementing the InferenceEngine::ILayerExecImpl defines a general CPU-side extension. There are no Intel MKL-DNN specifics in the way you need to implement a kernel.

Implementation Class

All custom kernels for the CPU plugin should be inherited from the InferenceEngine::ILayerExecImpl interface. Based on that, declaration of a kernel implementation class can look as follows:

@snippet cpu_kernel.hpp cpu_implementation:header

Class Fields

The provided implementation has several fields:

• add of the type int64_t is an attribute of a custom operation
• inShape of the type ngraph::Shape is an input shape
• outShape of the type ngraph::Shape is an output shape
• error of the type std::string is a field to handle errors from a constructor

Constructor of Implementation

An implementation constructor checks parameters of nGraph operation, stores needed attributes, and stores an error message in the case of an error.

@snippet cpu_kernel.cpp cpu_implementation:ctor

getSupportedConfigurations

InferenceEngine::ILayerExecImpl::getSupportedConfigurations method returns all supported configuration formats (input/output tensor layouts) for your implementation. To specify formats of data, use InferenceEngine::TensorDesc. Refer to the Memory Primitives section for instructions on how to do it.

@snippet cpu_kernel.cpp cpu_implementation:getSupportedConfigurations

init

InferenceEngine::ILayerExecImpl::init method gets a runtime-selected configuration from a vector that is populated from the getSupportedConfigurations method and checks the parameters:

@snippet cpu_kernel.cpp cpu_implementation:init

execute

InferenceEngine::ILayerExecImpl::execute method accepts and processes the actual tenors as input/output blobs:

@snippet cpu_kernel.cpp cpu_implementation:execute

Register Implementation in Extension Class

To register custom kernel implementation in the Extension class, implement the following methods:

• getImplTypes
• getImplementation

getImplTypes

InferenceEngine::IExtension::getImplTypes returns a vector of implementation types for an operation.

@snippet extension.cpp extension:getImplTypes

getImplementation

InferenceEngine::IExtension::getImplementation returns the kernel implementation with a specified type for an operation.

@snippet extension.cpp extension:getImplementation

Load Extension with Executable Kernels to Plugin

Use the AddExtension method of the general plugin interface to load your primitives:

InferenceEngine::Core core;
// Load CPU extension as a shared library
auto extension_ptr = make_so_pointer<InferenceEngine::IExtension>("<shared lib path>");
// Add extension to the CPU device
core.AddExtension(extension_ptr, "CPU");