masahi commented on a change in pull request #4602: [Docs] Bring Your Own Codegen Guide -- Part 1 URL: https://github.com/apache/incubator-tvm/pull/4602#discussion_r363205221
########## File path: docs/dev/relay_bring_your_own_codegen.rst ########## @@ -0,0 +1,514 @@ +.. Licensed to the Apache Software Foundation (ASF) under one + or more contributor license agreements. See the NOTICE file + distributed with this work for additional information + regarding copyright ownership. The ASF licenses this file + to you under the Apache License, Version 2.0 (the + "License"); you may not use this file except in compliance + with the License. You may obtain a copy of the License at + +.. http://www.apache.org/licenses/LICENSE-2.0 + +.. Unless required by applicable law or agreed to in writing, + software distributed under the License is distributed on an + "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY + KIND, either express or implied. See the License for the + specific language governing permissions and limitations + under the License. + +============================= +Bring Your Own Codegen To TVM +============================= +**Author**: `Zhi Chen <https://github.com/zhiics>`_, `Cody Hao Yu <https:://github.com/comaniac>`_ + +As the number of hardware devices targeted by deep learning workloads keeps increasing, the required knowledge for users to achieve high performance on various devices keeps increasing as well. To free data scientists from worrying about the performance when developing a new model, hardware vendors either provide libraries such as MKLDNN or cuDNN with many commonly used deep learning operators, or provide frameworks such as TensorRT to let users describe their models in a certain way to achieve high performance. However, users have to learn a new programming interface when they attempt to work on a new library or device. As a result, the demand for a unified programming interface becomes more and more important to 1) let all users and hardware vendors stand on the same page, and 2) provide a feasible solution to allow specialized hardware or library to only support widely used operators with extremely high performance, but fallback unsupported operators to general devices like CPU/GPU. + +In this develop guide, we demonstrate how a hardware vendor can easily implement your own codegen and register it as a Relay backend compiler to support your hardware device/library. This guide covers two types of codegen based on different graph representations you need: + +**1. You want to generate C code.** + +If your hardware already has a well-optimized C/C++ library, such as Intel CBLAS/MKL to CPU and NVIDIA CUBLAS to GPU, then this is what you are looking for. Fortunately, C source code module is fully compatible with TVM runtime module, which means the generated code could be compiled by any C/C++ compiler with proper compilation flags, so the only task you have is to implement a codegen that generates C code for subgraphs and a C source module to integrate into TVM runtime module. We will demonstrate how to implement a C code generator for your hardware in the following section. + +**2. You want to generate any other graph representations.** + +Your hardware may require other forms of graph representation, such as JSON. In this case, you need to implement not only a codegen but a customized TVM runtime module to let TVM runtime know how this graph representation should be executed. If you already have a complete graph execution engine for your hardware, such as TensorRT for GPU, then this is a solution you can consider. + +After you finished the codegen and runtime, you can then let your customers annotate their models with your customized tag to make use of them. The tutorial for end-users to annotate and launch a specific codegen is **here (TBA)**. + +********************* +Implement a C Codegen +********************* + +In this part, we demonstrate how to implement a codegen that generates C code with pre-implemented operator functions. To simplify, our example codegen does not depend on third-party libraries. Instead, we manually implement two function macros in C: + +.. code-block:: c++ + + #define CSOURCE_BINARY_OP_1D(p_ID_, p_OP_, p_DIM1_) \ + extern "C" void p_ID_(float* a, float* b, float* out) { \ + for (int64_t i = 0; i < p_DIM1_; ++i) { \ + out[i] = a[i] p_OP_ b[i]; \ + } \ + } + + #define CSOURCE_BINARY_OP_2D(p_ID_, p_OP_, p_DIM1_, p_DIM2_) \ + extern "C" void p_ID_(float* a, float* b, float* out) { \ + for (int64_t i = 0; i < p_DIM1_; ++i) { \ + for (int64_t j = 0; j < p_DIM2_; ++j) { \ + int64_t k = i * p_DIM2_ + j; \ + out[k] = a[k] p_OP_ b[k]; \ + } \ + } \ + } + +With the two macros, we can generate binary operators for 1-D and 2-D tensors. For example, given a subgraph as follows. Assuming all inputs are 2-D tensors with shape (10, 10). + +:: + + gcc_input0 + | + add <-- gcc_input1 + | + subtract <-- gcc_input2 + | + multiply <-- gcc_input3 + | + out + +Our goal is to generate the following compilable code to execute the subgraph: + +.. code-block:: c++ + + #include <tvm/runtime/c_runtime_api.h> + #include <dlpack/dlpack.h> + #include <cstdint> + #include <cstring> + #include <iostream> + + #define GCC_BINARY_OP_1D(p_ID_, p_OP_, p_DIM1_) \ + extern "C" void p_ID_(float* a, float* b, float* out) { \ + for (int64_t i = 0; i < p_DIM1_; ++i) { \ + out[i] = a[i] p_OP_ b[i]; \ + } \ + } + + #define GCC_BINARY_OP_2D(p_ID_, p_OP_, p_DIM1_, p_DIM2_) \ + extern "C" void p_ID_(float* a, float* b, float* out) { \ + for (int64_t i = 0; i < p_DIM1_; ++i) { \ + for (int64_t j = 0; j < p_DIM2_; ++j) { \ + int64_t k = i * p_DIM2_ + j; \ + out[k] = a[k] p_OP_ b[k]; \ + } \ + } \ + } + + // Note 1 + GCC_BINARY_OP_2D(gcc_0_0, *, 10, 10); + GCC_BINARY_OP_2D(gcc_0_1, -, 10, 10); + GCC_BINARY_OP_2D(gcc_0_2, +, 10, 10); + + // Note 2 + extern "C" void gcc_0_(float* gcc_input0, float* gcc_input1, + float* gcc_input2, float* gcc_input3, float* out) { + float* buf_0 = (float*)malloc(4 * 100); + float* buf_1 = (float*)malloc(4 * 100); + gcc_0_2(gcc_input0, gcc_input1, buf_0); + gcc_0_1(buf_0, gcc_input2, buf_1); + gcc_0_0(buf_1, gcc_input3, out); + free(buf_0); + free(buf_1); + } + + // Note 3 + extern "C" int gcc_0(TVMValue* value, int* type_code, int nargs) { + if (nargs != 5) { + printf("Expect 5 args, but get %d", nargs); + return 1; + } + DLTensor* arg0 = static_cast<DLTensor*>(value[0].v_handle); + DLTensor* arg1 = static_cast<DLTensor*>(value[1].v_handle); + DLTensor* arg2 = static_cast<DLTensor*>(value[2].v_handle); + DLTensor* arg3 = static_cast<DLTensor*>(value[3].v_handle); + DLTensor* out = static_cast<DLTensor*>(value[4].v_handle); + gcc_0_(static_cast<float*>(arg0->data), static_cast<float*>(arg1->data), + static_cast<float*>(arg2->data), static_cast<float*>(arg3->data), + static_cast<float*>(out->data)); + return 0; + } + +Here we highlight the notes marked in the above code: + +* **Note 1** is the function implementation for the three nodes in the subgraph. + +* **Note 2** is a function to execute the subgraph by allocating intermediate buffers and invoking corresponding functions. + +* **Note 3** is a TVM runtime compatible wrapper function with unified arguments. It accepts and unpacks the packed data ``TVMValue``, and invokes the corresponding function to execute the subgraph. Due to the unified function arguments, the TVM runtime can directly invoke ``gcc_0`` to execute the subgraph without additional efforts. With the above code generated, TVM is able to compile it along with the rest parts of the graph and export a single library for deployment. + +In the rest of this section, we will implement a codegen step-by-step to generate the above code. Your own codegen has to be located at ``src/relay/backend/contrib/<your-codegen-name>/``. In our example, we name our codegen "codegen_c" and put it under ``src/relay/backend/contrib/codegen_c/codegen.cc``. Feel free to check this file for a complete implementation. + +Specifically, we are going to implement two classes in this file and here is their relationship: + +:: + + subgraph subgraph + TVM backend -----------------------------> CSourceCodegen -------------> CodegenC + ^ | ^ | + | | | | + ---------------------------------------- ------------------------ + generated C source runtime module generated C code + +When TVM backend finds a function (subgraph) in a Relay graph is annotated with the registered compiler tag (``ccompiler`` in this example), TVM backend invokes ``CSourceCodegen`` and passes the subgraph. ``CSourceCodegen``'s member function ``CreateCSourceModule`` will 1) generate C code for the subgraph, and 2) wrap the generated C code to a C source runtime module for TVM backend to compile and deploy. In particular, the C code generation is transparent to ``CodegenC`` class because it provides many useful utilities to ease the code generation implementation. The following sections will implement these two classes in bottom-up order. + +Implement CodegenC +================== + +In ``src/relay/backend/contrib/codegen_c/codegen.cc``, we first create a codegen class skeleton under the namespace of ``tvm.relay.contrib``: + +.. code-block:: c++ + + #include <tvm/relay/expr_functor.h> + #include <tvm/relay/transform.h> + #include <tvm/relay/type.h> + #include <tvm/runtime/module.h> + #include <tvm/runtime/object.h> + + #include <fstream> + #include <sstream> + + #include "codegen_c.h" + + namespace tvm { + namespace relay { + namespace contrib { + + class CodegenC : public ExprVisitor, public CodegenCBase { + public: + explicit CodegenC(const std::string& id) { this->ext_func_id_ = id; } + + void VisitExpr_(const VarNode* node) { ; } + void VisitExpr_(const CallNode* call) final { ; } + std::string JIT() { ; } + + private: + /*! \brief The function id that represents a C source function. */ + std::string ext_func_id_ = ""; + /*! \brief The index of a wrapped C function. */ + int func_idx = 0; + /*! \brief The index of allocated buffers. */ + int buf_idx_ = 0; + /*! \brief The arguments of a C compiler compatible function. */ + std::vector<std::string> ext_func_args_; + /*! \brief The statements of a C compiler compatible function. */ + std::vector<std::string> ext_func_body; + /*! \brief The declaration statements of a C compiler compatible function. */ + std::vector<std::string> func_decl_; + /*! \brief The declaration statements of buffers. */ + std::vector<std::string> buf_decl_; + /*! \brief The name and index pairs for output. */ + std::vector<std::pair<std::string, int>> out_; + } + +The ``CodegenC`` class inherits two classes: ``ExprVisitor`` provides abilities to traverse subgraphs and collects the required information and generate subgraph functions such as ``gcc_0_``; ``CodegenCBase`` provides abilities and utilities to generate wrapper functions such as ``gcc_0`` in the above example. As can be seen, we only need to implement three functions in this codegen class to make it work. + +Code Generation for Operators +----------------------------- + +We first implement ``VisitExpr_(const CallNode* call)``. This function visits all call nodes when traversing the subgraph. Each call node contains an operator that we want to offload to your hardware. As a result, we need to generate the corresponding C code with correct operators in topological order. We implement this function step-by-step as follows. + +**1. Generate the function declaration** + +Example Result: ``GCC_BINARY_OP_2D(gcc_0_0, *, 10, 10);`` + +To generate the function declaration, as shown above, we need 1) a function name (e.g., ``gcc_0_0``), 2) the type of operator (e.g., ``*``), and 3) the input tensor shape (e.g., ``(10, 10)``). Fortunately, this information can be obtained easily from ``CallNode``: + +.. code-block:: c++ + + std::ostringstream macro_stream; + std::ostringstream decl_stream; + std::ostringstream buf_stream; + + // Generate a unique function name you like. + std::string func_name = ext_func_id_ + "_" + std::to_string(func_idx++); + + // Make function declaration string. + macro_stream << "CSOURCE_BINARY_OP_" << call->args.size() << "D(" << func_name << ", "; + + // Check the operator type. + if (IsOp(call, "add")) { + macro_stream << "+"; + } else if (IsOp(call, "subtract")) { + macro_stream << "-"; + } else if (IsOp(call, "multiply")) { + macro_stream << "*"; + } else { + LOG(FATAL) << "Unrecognized op"; + } + + // Extract the input tensor shape. + auto in_shape = GetShape(call->args[0]->checked_type()); + for (size_t i = 0; i < in_shape.size(); ++i) { + macro_stream << ", " << in_shape[i]; + } + macro_stream << ");"; + func_decl_.push_back(macro_stream.str()); + +As can be seen, we push the generated code to class member variables ``func_decl_``. It means after we finish traversing the entire subgraph, we have collected all required function declarations and the only thing we need to do is writing them out to be compiled. The rest implementations of ``VisitExpr_(const CallNode* call)`` also follow this concept. + +**2. Generate the function call** + +Example Result: ``gcc_0_0(buf_1, gcc_input3, out);`` + +After generating the function declaration, we need to generate a function call with proper inputs and outputs. To know which inputs or buffers we should put when calling this function, we have to visit its arguments: + +.. code-block:: c++ + + bool first = true; + decl_stream << func_name << "("; + for (size_t i = 0; i < call->args.size(); ++i) { + VisitExpr(call->args[i]); // Note 1 + for (auto out : out_) { + if (!first) { + decl_stream << ", "; + } + first = false; + decl_stream << out.first; + } + } + // Note 2 + +Again, we want to highlight the notes in the above code: + +**Note 1**: ``VisitExpr(call->args[i])`` is a recursive call to visit arguments of the current function. An argument could be an output of another node or an input tensor. In our example implementation, we make sure every node updates a class variable ``out_`` before leaving the visitor. Here is an illustration: + +:: + + arg_node arg_node <- Visit arg (Note 1) arg_node + | | | + curr_node <- Process curr_node curr_node <- Put "buf_0" as an input buffer + + (a) out_ = {} (b) out_ = {} (c) out_ = {("buf_0", 20)} + + +We can see in the above figure, class variable ``out_`` is empty before visiting the argument node, and it was filled with the output buffer name and size of ``arg_node``. AS a result, when we finished visiting the argument node, we know the proper input buffer we should put by looking at ``out_``. You will find out how do we update ``out_`` at the end of this section as well as the next section. + +**Note 2**: You may notice that we did not close the function call string in this step. The current function call string looks like: ``gcc_0_0(buf_1, gcc_input3``. This is because we have not put the last argument (i.e., the output) to this call. The output of a function call could be either an allocated temporary buffer or the subgraph output tensor. To simplify, in this example, we allocate an output buffer for every call node (next step) and copy the result in the very last buffer to the output tensor. + +**3. Generate the output buffer** + +Example Result: ``float* buf_0 = (float*)malloc(4 * 100);`` + +As mentioned in the previous step, in addition to the subgraph input and output tensors, we may also need buffers to keep the intermediate results. To generate the buffer, we extract the shape information to determine the buffer type and size: + +.. code-block:: c++ + + // This example only supports single output. + auto type_node = call->checked_type().as<TensorTypeNode>(); + CHECK(type_node != nullptr && runtime::TypeMatch(type_node->dtype, kDLFloat, 32)) + << "Only support single output tensor with float type"; + + // Generate a unique buffer name. + std::string out = "buf_" + std::to_string(buf_idx_++); + + // Extract the shape to be the buffer size. + auto out_shape = GetShape(call->checked_type()); + int out_size = 1; + for (size_t i = 0; i < out_shape.size(); ++i) { + out_size *= out_shape[i]; + } + + // Make the buffer allocation and push to the buffer declarations. + buf_stream << "float* " << out << " = (float*)std::malloc(4 * " << out_size << ");"; + buf_decl_.push_back(buf_stream.str()); + +After we have allocated the output buffer, we can now close the function call string and push the generated function call to a class variable ``ext_func_body``. + +.. code-block:: c++ + + decl_stream << ", " << out << ");"; + ext_func_body.push_back(decl_stream.str()); + +**4. Update output buffer** + +To let the next node, which accepts the output of the current call node as its input, know which buffer should it take, we need to update the class variable ``out_`` before leaving this visit function: Review comment: which buffer it should take ---------------------------------------------------------------- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. For queries about this service, please contact Infrastructure at: [email protected] With regards, Apache Git Services
