How to write a new operator


Here are the base types needed. For details, please refer to the design docs.

  • class OpProtoAndCheckerMaker: Describes an Operator’s input, output, attributes and description, mainly used to interface with Python API.
  • framework::OperatorBase: Operator (Op)base class.
  • framework::OpKernel: Base class for Op computation kernel.
  • framework::OperatorWithKernel: Inherited from OperatorBase, describing an operator with computation kernels.

Operators can be categorized into two groups: operator with kernel(s) and operator without kernel(s). An operator with kernel(s) inherits from OperatorWithKernel while the one without kernel(s) inherits from OperatorBase. This tutorial focuses on implementing operators with kernels. In short, an operator includes the following information:

Information Where is it defined
OpProtoMake definition `.cc`files, Backward Op does not need an OpProtoMake interface.
Op definition `.cc` files
Kernel implementation The kernel methods shared between CPU and CUDA are defined in `.h` files. CPU-specific kernels live in `.cc` files, while CUDA-specific kernels are implemented in `.cu`files.
Registering the Op Ops are registered in `.cc` files; For Kernel registration, `.cc` files contain the CPU implementation, while `.cu` files contain the CUDA implementation.

New Operator implementations are added to the list paddle/operators, with file names in the format *_op.h (if applicable), *, * (if applicable).** The system will use the naming scheme to automatically build operators and their corresponding Python extensions.**

Let’s take matrix multiplication operator, MulOp, as an example to introduce the writing of an Operator with Kernel.

Implementing C++ Types

Defining ProtoMaker

Matrix Multiplication can be written as $Out = X * Y$, meaning that the operation consists of two inputs and pne output.

First, define ProtoMaker to describe the Operator’s input, output, and additional comments:

class MulOpMaker : public framework::OpProtoAndCheckerMaker {
  MulOpMaker(OpProto *proto, OpAttrChecker *op_checker)
      : OpProtoAndCheckerMaker(proto, op_checker) {
    AddInput("X", "(Tensor), 2D tensor of size (M x K)");
    AddInput("Y", "(Tensor), 2D tensor of size (K x N)");
    AddOutput("Out", "(Tensor), 2D tensor of size (M x N)");
Two Element Mul Operator.
The equation is: Out = X * Y

MulOpMakeris inherited fromframework::OpProtoAndCheckerMaker, consisting of 2 variables in the constructor:

  • framework::OpProto stores Operator input and variable attribute, used for generating Python API interfaces.
  • framework::OpAttrChecker is used to validate variable attributes.

The constructor utilizes AddInput, AddOutput, and AddComment, so that the corresponding information will be added to OpProto.

The code above adds two inputs X and Y to MulOp, an output Out, and their corresponding descriptions, in accordance to Paddle’s naming convention.

An additional example ScaleOp is implemented as follows:

template <typename AttrType>
class ScaleOpMaker : public framework::OpProtoAndCheckerMaker {
  ScaleOpMaker(OpProto *proto, OpAttrChecker *op_checker)
      : OpProtoAndCheckerMaker(proto, op_checker) {
    AddInput("X", "The input tensor of scale operator.").NotInGradient();
    AddOutput("Out", "The output tensor of scale operator.").NotInGradient();
    AddComment(R"DOC(Scale operator
The equation is: Out = scale*X
    AddAttr<AttrType>("scale", "scale of scale operator.").SetDefault(1.0);

Note AddAttr<AttrType>("scale", "...").SetDefault(1.0); adds scaleconstant as an attribute, and sets the default value to 1.0.

Defining Operator

The following code defines the interface for MulOp:

class MulOp : public framework::OperatorWithKernel {
  using framework::OperatorWithKernel::OperatorWithKernel;

  void InferShape(const framework::InferShapeContext &ctx) const override {
    auto dim0 = ctx.Input<Tensor>("X")->dims();
    auto dim1 = ctx.Input<Tensor>("Y")->dims();
    PADDLE_ENFORCE_EQ(dim0.size(), 2,
                      "input X(%s) should be a tensor with 2 dims, a matrix",
    PADDLE_ENFORCE_EQ(dim1.size(), 2,
                      "input Y(%s) should be a tensor with 2 dims, a matrix",
        dim0[1], dim1[0],
        "First matrix's width must be equal with second matrix's height.");
    ctx.Output<Tensor>("Out")->Resize({dim0[0], dim1[1]});

MulOp is inherited from OperatorWithKernel. Its public member

using framework::OperatorWithKernel::OperatorWithKernel;

expresses an operator constructor using base class OperatorWithKernel, alternatively written as

MulOp(const std::string &type, const framework::VariableNameMap &inputs,
      const framework::VariableNameMap &outputs,
      const framework::AttributeMap &attrs)
  : OperatorWithKernel(type, inputs, outputs, attrs) {}

InferShape interface needs to be re-written.InferShape is a constant method and cannot modify Op’s member variables, its constant member const framework::InferShapeContext &ctx can be used to extract input, output, and attributes. It functions to

  • 1). validate and error out early: it checks input data dimensions and types.
  • 2). configures the tensor shape in the output.

Usually OpProtoMaker and Op‘s type definitions are written in .cc files, which also include the registration methods introduced later.

Defining OpKernel

MulKernel inherits framework::OpKernel, which includes the following templates:

  • typename DeviceContext denotes device context type. When different devices, namely the CPUDeviceContext and the CUDADeviceContext, share the same kernel, this template needs to be added. If they don’t share kernels, this must not be added. An example of a non-sharing kernel is OnehotCrossEntropyOpKernel.
  • typename T denotes data type, such as float or double.

MulKernel types need to rewrite the interface for Compute.

  • Compute takes one input parameter: const framework::ExecutionContext& context.
  • Compared with InferShapeContext, ExecutionContext includes device types, and can similarly extract input, output, and attribute variables.
  • Compute implements the computation logics of an OpKernel.

MulKernel‘s implementation of Compute is as follows:

template <typename DeviceContext, typename T>
class MulKernel : public framework::OpKernel {
void Compute(const framework::ExecutionContext& context) const override {
  auto* X = context.Input<Tensor>("X");
  auto* Y = context.Input<Tensor>("Y");
  auto* Z = context.Output<Tensor>("Out");
  auto& device_context = context.template device_context<DeviceContext>();
  math::matmul<DeviceContext, T>(*X, false, *Y, false, 1, Z, 0, device_context);

Note that different devices (CPU, CUDA)share one Op definition; whether or not they share the same OpKernel depends on whether Compute calls functions can support both devices.

MulOp‘s CPU and CUDA share the same Kernel. A non-sharing OpKernel example can be seen in OnehotCrossEntropyOpKernel.

To ease the writing of OpKernel compute, and for reusing code cross-device, Eigen-unsupported Tensor module is used to implement Compute interface. To learn about how the Eigen library is used in PaddlePaddle, please see usage document.

This concludes the forward implementation of an operator. Next its operation and kernel need to be registered in a .cc file.

The definition of its corresponding backward operator, if applicable, is similar to that of an forward operator. Note that a backward operator does not include a ProtoMaker.

Registering Operator and OpKernel

  • In .cc files, register forward and backward operator classes and the CPU kernel.

    namespace ops = paddle::operators;
    REGISTER_OPERATOR(mul, ops::MulOp, ops::MulOpMaker,
    REGISTER_OPERATOR(mul_grad, ops::MulGradOp)
    REGISTER_OP_CPU_KERNEL(mul, ops::MulKernel<paddle::platform::CPUDeviceContext, float>);
                  ops::MulGradKernel<paddle::platform::CPUDeviceContext, float>);

    In that code block,

    • REGISTER_OPERATOR registers the ops::MulOp class, type named mul, its type ProtoMaker is ops::MulOpMaker, registering ops::MulOpGrad as mul_grad.
    • REGISTER_OP_WITHOUT_GRADIENT registers an operator without gradient.
    • REGISTER_OP_CPU_KERNEL registers ops::MulKernel class and specialized template types paddle::platform::CPUPlace and float, which also registers ops::MulGradKernel.
  • Registering CUDA Kernel in .cu files

    • Note that if CUDA Kernel is implemented using the Eigen unsupported module, then on top of .cu, a macro definition #define EIGEN_USE_GPU is needed, such as
    // if use Eigen unsupported module before include head files
    #define EIGEN_USE_GPU
    namespace ops = paddle::operators;
    REGISTER_OP_CUDA_KERNEL(mul, ops::MulKernel<paddle::platform::CUDADeviceContext, float>);
                           ops::MulGradKernel<paddle::platform::CUDADeviceContext, float>);


Run the following commands to compile.

# maybe you need to rerun cmake
make mul_op

Python Binding

The system will automatically bind to Python and link it to a generated library.

Unit Tests

Unit tests for an operator include

  1. comparing a forward operator’s implementations on different devices,
  2. comparing a backward operator’s implementation on different devices, and
  3. a scaling test for the backward operator.

Here, we introduce the unit tests for MulOp.

Testing Forward Operators

A forward operator unit test inherits unittest.TestCase and defines metaclass __metaclass__ = OpTestMeta. More concrete tests are performed in OpTestMeta. Testing a forward operator requires the following:

  1. Defining input, output and relevant attributes in setUp method.
  2. Generating random input data.
  3. Implementing the same computation logic in a Python script.
  4. Call check gradient function to check the backward operator.
import unittest
import numpy as np
from op_test import OpTest

class TestMulOp(OpTest):
    def setUp(self):
        self.op_type = "mul"
        self.inputs = {
            'X': np.random.random((32, 84)).astype("float32"),
            'Y': np.random.random((84, 100)).astype("float32")
        self.outputs = {'Out':['X'], self.inputs['Y'])}

    def test_check_output(self):

    def test_check_grad_normal(self):
        self.check_grad(['X', 'Y'], 'Out', max_relative_error=0.5)

    def test_check_grad_ingore_x(self):
            ['Y'], 'Out', max_relative_error=0.5, no_grad_set=set("X"))

    def test_check_grad_ingore_y(self):
            ['X'], 'Out', max_relative_error=0.5, no_grad_set=set('Y'))

Get its output, and compare it with the forward operator’s own output.

The code above first loads required packages. In addition, we have

  • self.op_type = "mul" defines the type that is identical to what the operator’s registered type.
  • self.inputs defines input, with type numpy.array and initializes it.
  • self.outputs defines output and completes the same operator computation in the Python script, and returns its result from the Python script.

Testing Backward Operators

Some key points in checking gradient above include:

  • test_normal calls check_grad to validate scaling tests’ correctness and stability through numeric methods.
    • The first variable ["X", "Y"] appoints X and Y to be scale tested.
    • The second variable "Out" points to the network’s final output target Out.
    • The third variable max_relative_error points to the maximum relative tolerance error during scaling tests.
  • test_check_grad_ingore_x and test_check_grad_ingore_ybranches test the cases where there is only one scaling input.

Compiling and Running

Any new unit testing file of the format test_*.py added to the director python/paddle/fluid/tests/unittests/ is automatically added to the project to compile.

Note that unlike the compile test for Ops, running unit tests requires compiling the entire project and requires compiling with flag WITH_TESTING on i.e. cmake paddle_dir -DWITH_TESTING=ON.

After successfully compiling the project, run the following command to run unit tests:

make test ARGS="-R test_mul_op -V"


ctest -R test_mul_op


  • The type with which an operator is registered needs to be identical to the Op’s name. Registering REGISTER_OPERATOR(B, ...) in will cause unit testing failures.
  • If the operator does not implement a CUDA kernel, please refrain from creating an empty * file, or else unit tests will fail.
  • If multiple operators rely on some shared methods, a file NOT named *_op.* can be created to store them, such as gather.h.