C/C++ 中的自展开宏循环 [英] Self-unrolling macro loop in C/C++

问题描述

我目前正在处理一个项目，其中每个周期都很重要.在分析我的应用程序时，我发现一些内部循环的开销非常高，因为它们只包含一些机器指令.此外，这些循环中的迭代次数在编译时是已知的.

所以我想不是用 copy & 手动展开循环.我可以使用宏在编译时展开循环，以便以后可以轻松修改.

我的印象是这样的:

#define LOOP_N_TIMES(N, CODE) <在此处插入魔法>

这样我就可以替换 for (int i = 0; i < N, ++i) { do_stuff();} 与:

#define INNER_LOOP_COUNT 4LOOP_N_TIMES(INNER_LOOP_COUNT, do_stuff();)

它自己展开:

do_stuff();做东西();做东西();做东西();

由于大多数时候 C 预处理器对我来说仍然是个谜，我不知道如何实现这一点，但我知道这一定是可能的，因为 Boost 似乎有一个 BOOST_PP_REPEAT 宏.不幸的是，我不能在这个项目中使用 Boost.

解决方案

您可以使用模板展开.请参阅示例的反汇编 .

<小时>

生活在 Coliru

模板<无符号N>struct faux_unroll {模板 静态无效调用(F const& f){F();faux_unroll<N-1>::call(f);}};模板 <>struct faux_unroll<0u>{模板 静态无效调用(F const&){}};#include #include int main() {srand(时间(0))；双 r = 0;faux_unroll<10>::call([&] { r += 1.0/rand(); });std::cout <<r;}

I am currently working on a project, where every cycle counts. While profiling my application I discovered that the overhead of some inner loop is quite high, because they consist of just a few machine instruction. Additionally the number of iterations in these loops is known at compile time.

So I thought instead of manually unrolling the loop with copy & paste I could use macros to unroll the loop at compile time so that it can be easily modified later.

What I image is something like this:

#define LOOP_N_TIMES(N, CODE) <insert magic here>

So that I can replace for (int i = 0; i < N, ++i) { do_stuff(); } with:

#define INNER_LOOP_COUNT 4
LOOP_N_TIMES(INNER_LOOP_COUNT, do_stuff();)

And it unrolls itself to:

do_stuff(); do_stuff(); do_stuff(); do_stuff();

Since the C preprocessor is still a mystery to me most of the time, I have no idea how to accomplish this, but I know it must be possible because Boost seems to have a BOOST_PP_REPEAT macros. Unfortunately I can't use Boost for this project.

解决方案

You can use templates to unroll. See the disassembly for the sample Live on Godbolt

But -funroll-loops has the same effect for this sample.

---

Live On Coliru

template <unsigned N> struct faux_unroll {
    template <typename F> static void call(F const& f) {
        f();
        faux_unroll<N-1>::call(f);
    }
};

template <> struct faux_unroll<0u> {
    template <typename F> static void call(F const&) {}
};

#include <iostream>
#include <cstdlib>

int main() {
    srand(time(0));

    double r = 0;
    faux_unroll<10>::call([&] { r += 1.0/rand(); });

    std::cout << r;
}

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