什么是constexpr的等效函数参数？ [英] What is the function parameter equivalent of constexpr?

问题描述

我们正试图加速Clang和Visual C ++下的一些代码（GCC和ICC是OK）。我们认为我们可以使用 constexpr 来告诉Clang一个值是一个编译时常数，但它导致编译错误：

We are trying to speedup some code under Clang and Visual C++ (GCC and ICC is OK). We thought we could use constexpr to tell Clang a value is a compile time constant but its causing a compile error:

$ clang++ -g2 -O3 -std=c++11 test.cxx -o test.exe
test.cxx:11:46: error: function parameter cannot be constexpr
unsigned int RightRotate(unsigned int value, constexpr unsigned int rotate)
                                             ^
1 error generated.

以下是减少的情况：

$ cat test.cxx
#include <iostream>

unsigned int RightRotate(unsigned int value, constexpr unsigned int rotate);

int main(int argc, char* argv[])
{
  std::cout << "Rotated: " << RightRotate(argc, 2) << std::endl;
  return 0;
}

unsigned int RightRotate(unsigned int value, constexpr unsigned int rotate)
{
  // x = value; y = rotate
  __asm__ ("rorl %1, %0" : "+mq" (value) : "I" ((unsigned char)rotate));
  return value;
}

GCC和ICC会做正确的事。他们识别表达式 RightRotate（argc，2）中的 2 的值不能根据物理宇宙的规律改变因为我们知道它们，它会将 2 作为编译时常数，并将其传播到汇编代码中。

GCC and ICC will do the right thing. They recognize a value like 2 in the expression RightRotate(argc, 2) cannot change under the laws of the physical universe as we know them, and it will treat 2 as a compile time constant and propagate it into the assembly code.

如果我们删除 constexpr ，则Clang和VC ++将函数集合到 rotate REGISTER 中， 旋转IMMEDIATE 。

If we remove the constexpr, then Clang and VC++ aseembles the function into a rotate REGISTER, which is up to 3x slower than a rotate IMMEDIATE.

我们如何告诉Clang函数参数 rotate 是一个编译时常数，它应该被组装成 rotate IMMEDIATE ，而不是 rotate REGISTER

How do we tell Clang the function parameter rotate is a compile time constant, and it should be assembled into a rotate IMMEDIATE rather than a rotate REGISTER?

推荐答案

您可以使用非类型模板参数：

You could use non-type template arguments for this:

template <unsigned int rotate> RightRotate(unsigned int value) {
     ...
}

d然后调用它

RightRotate<137>(argument); // rotate is 137 here

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