参数化和 "function template partial specialization is not allowed"

Question

这是 What is the function parameter equivalent of constexpr? 原始问题的延续，我们正在尝试加速一些在 Clang 和 VC++ 下执行移位和旋转的代码。 Clang 和 VC++ 没有很好地优化代码，因为它将 shift/rotate 数量视为变量（即，不是 constexpr）。

当我尝试参数化移位量和字长时，结果为：

$ g++ -std=c++11 -march=native test.cxx -o test.exe
test.cxx:13:10: error: function template partial specialization is not allowed
uint32_t LeftRotate<uint32_t, unsigned int>(uint32_t v)
         ^         ~~~~~~~~~~~~~~~~~~~~~~~~
test.cxx:21:10: error: function template partial specialization is not allowed
uint64_t LeftRotate<uint64_t, unsigned int>(uint64_t v)
         ^         ~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

这是测试程序。它比需要的要大一点，所以人们可以看到我们需要同时处理 uint32_t 和 uint64_t（更不用说 uint8_t、uint16_t 和其他类型）。

$ cat test.cxx
#include <iostream>
#include <stdint.h>

template<typename T, unsigned int R>
inline T LeftRotate(unsigned int v)
{
  static const unsigned int THIS_SIZE = sizeof(T)*8;
  static const unsigned int MASK = THIS_SIZE-1;
  return T((v<<R)|(v>>(-R&MASK)));
};

template<uint32_t, unsigned int R>
uint32_t LeftRotate<uint32_t, unsigned int>(uint32_t v)
{
  __asm__ ("roll %1, %0" : "+mq" (v) : "I" ((unsigned char)R));
  return v;
}

#if __x86_64__
template<uint64_t, unsigned int R>
uint64_t LeftRotate<uint64_t, unsigned int>(uint64_t v)
{
  __asm__ ("rolq %1, %0" : "+mq" (v) : "J" ((unsigned char)R));
  return v;
}
#endif

int main(int argc, char* argv[])
{
  std::cout << "Rotated: " << LeftRotate<uint32_t, 2>((uint32_t)argc) << std::endl;
  return 0;
}

根据我尝试实施旋转的方式，我已经经历了多次错误消息迭代。其他错误消息包括 no function template matches function template specialization...。使用 template <> 似乎产生了最难以理解的结果。

我如何参数化偏移量以希望 Clang 和 VC++ 将按预期优化函数调用？

Answer 1

使用模板 class，而不是模板函数：

#include <iostream>
#include <stdint.h>


template<typename T, unsigned int R>
struct LeftRotate {
    static inline T compute(T v)
    {
        static const unsigned int THIS_SIZE = sizeof(T)*8;
        static const unsigned int MASK = THIS_SIZE-1;
        return T((v<<R)|(v>>(-R&MASK)));
    }
};


template<unsigned int R>
struct LeftRotate<uint32_t, R> {
    static inline uint32_t compute(uint32_t v)
    {
        __asm__ ("roll %1, %0" : "+mq" (v) : "I" ((unsigned char)R));
        return v;
    }
};

#if __x86_64__
template<unsigned int R>
struct LeftRotate<uint64_t, R> {
    static inline uint64_t compute(uint64_t v)
    {
        __asm__ ("rolq %1, %0" : "+mq" (v) : "J" ((unsigned char)R));
        return v;
    }
};
#endif

int main(int argc, char* argv[])
{
  std::cout << "Rotated: " << LeftRotate<uint32_t, 2>::compute((uint32_t)argc) << std::endl;
  return 0;
}

Answer 2

另一种方法是将模板常量转换为编译器可以优化掉的常量参数。

第一步：定义a的概念rotate_distance:

template<unsigned int R> using rotate_distance = std::integral_constant<unsigned int, R>;

第 2 步：根据采用此类参数的函数的重载定义旋转函数：

template<unsigned int R>
uint32_t LeftRotate(uint32_t v, rotate_distance<R>)

现在，如果我们愿意，我们可以简单地调用 LeftRotate(x, rotate_distance<y>())，这似乎很好地表达了意图，

或者我们现在可以根据这种形式重新定义 2 参数模板形式：

template<unsigned int Dist, class T>
T LeftRotate(T t)
{
  return LeftRotate(t, rotate_distance<Dist>());
}

完整演示：

#include <iostream>
#include <stdint.h>
#include <utility>

template<unsigned int R> using rotate_distance = std::integral_constant<unsigned int, R>;

template<typename T, unsigned int R>
inline T LeftRotate(unsigned int v, rotate_distance<R>)
{
  static const unsigned int THIS_SIZE = sizeof(T)*8;
  static const unsigned int MASK = THIS_SIZE-1;
  return T((v<<R)|(v>>(-R&MASK)));
}

template<unsigned int R>
uint32_t LeftRotate(uint32_t v, rotate_distance<R>)
{
  __asm__ ("roll %1, %0" : "+mq" (v) : "I" ((unsigned char)R));
  return v;
}

#if __x86_64__
template<unsigned int R>
uint64_t LeftRotate(uint64_t v, rotate_distance<R>)
{
  __asm__ ("rolq %1, %0" : "+mq" (v) : "J" ((unsigned char)R));
  return v;
}
#endif


template<unsigned int Dist, class T>
T LeftRotate(T t)
{
  return LeftRotate(t, rotate_distance<Dist>());
}

int main(int argc, char* argv[])
{
  std::cout << "Rotated: " << LeftRotate((uint32_t)argc, rotate_distance<2>()) << std::endl;
  std::cout << "Rotated: " << LeftRotate((uint64_t)argc, rotate_distance<2>()) << std::endl;
  std::cout << "Rotated: " << LeftRotate<2>((uint64_t)argc) << std::endl;
  return 0;
}

C++11 之前的编译器

在 c++11 之前我们没有 std::integral_constant，所以我们必须制作自己的版本。

为了我们的目的，这就足够了：

template<unsigned int R> struct rotate_distance {};

完整证明 - 注意优化的效果：

https://godbolt.org/g/p4tsQ5

参数化和 "function template partial specialization is not allowed"

Parameterization and "function template partial specialization is not allowed"

c++

function-templates

C++11 之前的编译器