Question

我试图将SSE float [4]加法与标准float [4]加法进行比较。作为演示，我计算了有和没有SSE的总和组件的总和：

#include <iostream>
#include <vector>

struct Point4
{
  Point4()
  {
    data[0] = 0;
    data[1] = 0;
    data[2] = 0;
    data[3] = 0;
  }

  float data[4];
};

void Standard()
{
  Point4 a;
  a.data[0] = 1.0f;
  a.data[1] = 2.0f;
  a.data[2] = 3.0f;
  a.data[3] = 4.0f;

  Point4 b;
  b.data[0] = 1.0f;
  b.data[1] = 6.0f;
  b.data[2] = 3.0f;
  b.data[3] = 5.0f;

  float total = 0.0f;
  for(unsigned int i = 0; i < 1e9; ++i)
  {
    for(unsigned int component = 0; component < 4; ++component)
    {
      total += a.data[component] + b.data[component];
    }
  }

  std::cout << "total: " << total << std::endl;
}

void Vectorized()
{
  typedef float v4sf __attribute__ (( vector_size(4*sizeof(float)) ));

  v4sf a;
  float* aPointer = (float*)&a;
  aPointer[0] = 1.0f; aPointer[1] = 2.0f; aPointer[2] = 3.0f; aPointer[3] = 4.0f;

  v4sf b;
  float* bPointer = (float*)&b;
  bPointer[0] = 1.0f; bPointer[1] = 6.0f; bPointer[2] = 3.0f; bPointer[3] = 5.0f;

  v4sf result;
  float* resultPointer = (float*)&result;
  resultPointer[0] = 0.0f;
  resultPointer[1] = 0.0f;
  resultPointer[2] = 0.0f;
  resultPointer[3] = 0.0f;

  for(unsigned int i = 0; i < 1e9; ++i)
  {
    result += a + b; // Vectorized operation
  }

  // Sum the components of the result (this is done with the "total += " in the Standard() loop
  float total = 0.0f;
  for(unsigned int component = 0; component < 4; ++component)
  {
    total += resultPointer[component];
  }
  std::cout << "total: " << total << std::endl;
}

int main()
{

//  Standard();

  Vectorized();

  return 0;
}

然而，使用标准方法的代码似乎比使用矢量化（〜.4秒）方法更快（〜.2秒）。是因为for循环对v4sf值求和了吗？是否有更好的操作可以用来计算这两种技术之间的差异，并仍然比较输出以确保两者之间没有差异？

Answer 1

然后理由你的版本较慢，因为SSE是你必须每次迭代从SSE寄存器解包到标量寄存器4次，这比你从矢量化加法获得的开销更多。看看反汇编，你应该得到一个更清晰的画面。

我认为你想要做的是以下（SSE更快）：

for(unsigned int i = 0; i < 1e6; ++i)
{
    result += a + b; // Vectorized operation
}

// Sum the components of the result (this is done with the "total += " in the Standard() loop
for(unsigned int component = 0; component < 4; ++component)
{
    total += resultPointer[component];
}

以下内容可能更快：

for(unsigned int i = 0; i < 1e6/4; ++i)
{
    result0 += a + b; // Vectorized operation
    result1 += a + b; // Vectorized operation
    result2 += a + b; // Vectorized operation
    result3 += a + b; // Vectorized operation
}

// Sum the components of the result (this is done with the "total += " in the Standard() loop
for(unsigned int component = 0; component < 4; ++component)
{
    total += resultPointer0[component];
    total += resultPointer1[component];
    total += resultPointer2[component];
    total += resultPointer3[component];
}

简单的SSE循环比非SSE版本慢

1 个答案: