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我正在尝试将 SSE 浮点 [4] 添加与标准浮点 [4] 添加进行比较。作为一个演示，我计算了有和没有 SSE 的总和分量的总和：

#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;
}

但是，使用标准方法的代码似乎比使用矢量化方法（约 0.4 秒）的代码更快（~.2 秒）。是因为 for 循环对 v4sf 值求和吗？有没有更好的操作我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？

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1 回答 1

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然后，您的版本较慢的原因是 SSE 是您必须每次迭代从 SSE 寄存器解压缩到标量寄存器 4 次，这比从矢量化加法中获得的开销更大。看看拆解，你应该得到一个更清晰的画面。

我认为您想要做的是以下（使用 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];
}

此外，以下可能会更快：

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];
}
于 2012-08-29T21:07:30.280 回答