What is the function parameter equivalent of const

问题:

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.

Here is the reduced case:

$ 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 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.

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.

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?

回答1:

You could use non-type template arguments for this:

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

You'd then invoke it as

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