优化C ++模板执行 [英] Optimize C++ template executions

问题描述

我正在研究性能至关重要的项目.该应用程序正在处理大量数据.代码是用C ++编写的，我需要进行一些更改.

I am working on project where the performance is critical. The application is processing a huge amount of data. Code is written in C++ and I need to do some changes.

给出了以下代码(这不是我的代码，我将其简化到最低限度):

There is given following code (It is NOT my code and I simplified it to minimum):

void process<int PARAM1, int PARAM2>() {
    // processing the data
}

void processTheData (int param1, int param2) { // wrapper

    if (param1 == 1 && param2 == 1) { // Ugly looking block of if's
        process<1, 1>();
    else if(param1 == 1 && param2 == 2) {
        process<1, 2>();
    else if(param1 == 1 && param2 == 3) {
        process<1, 3>();
    else if(param1 == 1 && param2 == 4) {
        process<1, 4>();
    else if(param1 == 2 && param2 == 1) {
        process<2, 1>();
    else if(param1 == 2 && param2 == 2) {
        process<2, 2>();
    else if(param1 == 2 && param2 == 3) {
        process<2, 3>();
    else if(param1 == 2 && param2 == 4) {
        process<2, 4>();
    }   // and so on....

}

主要功能:

int main(int argc, char *argv[]) {

    factor1 = atoi(argv[1]);
    factor2 = atoi(argv[2]);

    // choose some optimal param1 and param2
    param1 = choseTheOptimal(factor1, factor2);
    param2 = choseTheOptimal(factor1, factor2);

    processTheData(param1, param2); //start processing

    return 0;
}

希望代码看起来很清楚.

Hopefully the code looks clear.

功能:

• 过程是处理数据的核心功能，
• processTheData 是流程函数的包装.

• process is the core function that is processing the data,
• processTheData is a wrapper of the process function.

参数( param1 和 param2 )接受的值数量有限(假设大约10 x 10).

There is a limited number of values that the params (param1 and param2) takes (Let's say about 10 x 10).

在执行之前不知道 param1 和 param2 的值.

The values of param1 and param2 are NOT known before execution.

如果我简单地重写 process 函数，以便它使用函数参数而不是模板常量(意味着 process(int PARAM1，int PARAM2))，则该处理约为慢十倍.

If I simply rewrite the process function so it uses the function parameters instead of template constants (means process(int PARAM1, int PARAM2)) then the processing is about 10 times slower.

由于上述 PARAM1 和 PARAM2 必须是 process 函数的常数.

Because of the above the PARAM1 and PARAM2 must be the constant of process function.

在 processTheData 函数中是否有任何聪明的方法来消除if的这个丑陋块?

Is there any smart way to get rid of this ugly block of if's located in processTheData function?

推荐答案

类似.

#include <array>
#include <utility>

template<int PARAM1, int PARAM2>
void process() {
    // processing the data
}

// make a jump table to call process<X, Y> where X is known and Y varies    
template<std::size_t P1, std::size_t...P2s>
constexpr auto make_table_over_p2(std::index_sequence<P2s...>)
{
    return std::array<void (*)(), sizeof...(P2s)>
    {
        &process<int(P1), int(P2s)>...
    };
}

// make a table of jump tables to call process<X, Y> where X and Y both vary    
template<std::size_t...P1s, std::size_t...P2s>
constexpr auto make_table_over_p1_p2(std::index_sequence<P1s...>, std::index_sequence<P2s...> p2s)
{
    using element_type = decltype(make_table_over_p2<0>(p2s));
    return std::array<element_type, sizeof...(P1s)>
    {
        make_table_over_p2<P1s>(p2s)...
    };
}


void processTheData (int param1, int param2) { // wrapper

    // make a 10x10 jump table
    static const auto table = make_table_over_p1_p2(
        std::make_index_sequence<10>(), 
        std::make_index_sequence<10>()
    ) ;

    // todo - put some limit checks here

    // dispatch
    table[param1][param2]();
}

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