JIT Kernel

JIT(Just In Time) Kernel contains actually generated code and some other implemenations with the same logic. Each implementation has its own condition to use, defined in CanBeUsed. They are combined together to get the best performance of one single independent function. They could be some very simple functions like vector multiply, or some complicated functions like LSTM. And they can be composed with some other exited jit kernels to build up a complex function. Currently it's only supported on CPU yet.

Contents

PaddlePaddle/Paddle/paddle/fluid/
├── ...
└── lite/
    ├── .../
    └── jit/
        ├── ...
        ├── gen/
        │   └── ...
        |── more/
        │   ├── ...
        │   ├── mkl/
        │   │   └── ...
        │   ├── mkldnn/
        │   │   └── ...
        │   ├── mix/
        │   │   └── ...
        │   ├── intrinsic/
        │   │   └── ...
        │   └── openblas/
        │       └── ...
        └── refer/
            └── ...

All basical definations of jit kernels are addressed in lite/backends/x86/jit including these three key folders refer, gen, more. There is only one unique name for each kernel while may have seraval implementations with same functionality.

• refer: Each kernel must have one reference implementation on CPU, and it should only focus on the correctness and should not depends on any third-party libraries.
• gen: The code generated should be kept here. They should be designed focusing on the best performance, which depends on Xbyak.
• more: All other implementations should be kept in this folder with one directory corresponding to one library kind or method kind, such as mkl, mkldnn, openblas or intrinsic code. Each implementation should have it advantage.

How to use

We present these methods to get the functions:

• GetAllCandidateFuncs. It can return all the implementations supported. All of the implementations can get the same result. You can do some runtime benchmark to choose which should actually be used.
• GetDefaultBestFunc. It only return one default function pointer, which is tuning offline with some genenal configures and attributes. This should cover most situations.
• KernelFuncs::Cache(). It can get the default functions and save it for next time with the same attribute.
• GetReferFunc. It can only get the reference code in CPU, and all the others implementations have same logic with this reference code.

And here are some examples:

Get from cache:

    using T = float;
    jit::seq_pool_attr_t attr(width, jit::SeqPoolType::kSum);
    auto seqpool_func = jit::KernelFuncs<jit::SeqPoolTuple<T>, platform::CPUPlace>::Cache().At(attr);
    seqpool_func(src_data, dst_data, &attr);

Get all implementations and run once:

    using T = float;
    jit::seq_pool_attr_t attr(width, jit::SeqPoolType::kSum);
    auto funcs = jit::GetAllCandidateFuncsWithTypes<jit::SeqPoolTuple<T>, platform::CPUPlace>(attr);
    for (auto f : funcs) {
        LOG(INFO) << "Kernel implementation type: " << f.first;
        f.second(src_data, dst_data, &attr);
    }

All kernels are inlcuded in lite/backends/x86/jit/kernels.h, which is automatically generated in compile time, you can only include this one header to get all the registered kernels.

Solid Test

• Unit Test All functions should be compared with the corresponding reference functions, including data tyep float and double.
• Benchmark All functions should be tested, and make sure the jit::GetDefaultBestFunc function obtain the best performance with all attributes.

How to add new kernel

Required

1. Add your_key at KernelType.
2. Add your new KernelTuple which must include your_key. It should be a combination of the data type, attribute type and function type. You can refer SeqPoolTuple.
3. Add reference function of your_key. Note:
   • this should be run on CPU and do not depend on any third-party.
   • Add USE_JITKERNEL_REFER_LITE(your_key) in refer/CmakeLists.txt to make sure this code can be used.
4. Add unit test in test.cc, and verfiy at least float and double. Test more data type for some special functions if necessary, for example int8.
5. Add functions in benchmark.cc to test all function of same KernelType. Make sure GetDefaultBestFunc always get the best one.

Optional

Add more implementations of your_kery for performance enhancement.

1. Add functions based on generated code in gen. It should be derived from JitCode and should have correpsonding creator from JitCodeCreator which will be registered on the your_key.
2. If new attribute type is added, you should specialize JitCodeKey of this type.
3. Add more functions in more，you can use any third party you wish, like mkl, mkldnn or intrinsic code to reach the best performance.