我有C代码处理来自一个1800元素阵列的三个连续值.由ICC 14.0编译的代码比MSVC生成的代码慢约68％(1600对2700 cpu周期).我不明白为什么.有人可以帮忙吗？即使我设置英特尔编译器-O3开关,它也不会改变时序. cpu是Ivy Bridge.
- #include <iostream>
- int main(){
- int data[1200];
- //Dummy-populate data
- for(int y=0; y<1200; y++){
- data[y] = y/2 + 7;
- }
- int counter = 0;
- //Just to repeat the test
- while(counter < 10000){
- int Accum = 0;
- long long start = 0;
- long long end = 0;
- int p = 0;
- start = __rdtsc();
- while(p < 1200){
- unsigned int level1 = data[p];
- unsigned int factor = data[p + 1];
- Accum += (level1 * factor);
- p = p + 2;
- }
- end = __rdtsc();
- std::cout << (end - start) << " " << Accum << std::endl;
- counter++;
- }
- }
解决方法
ICC在这里很糟糕,因为它正在计算每个数据的地址[n]访问ala mov edi,dword ptr [rsp rax * 4 44h] …所有运行时乘法都很昂贵.您应该能够通过重新编码来避免它,因此索引是常量(也可以使用* p_data三次,但这会引入可能对性能产生负面影响的排序问题).
- for (unsigned* p_data = &data[0],*p_end = data + 1800; p_data < p_end; p_data += 3)
- {
- unsigned level1 = p_data[0];
- unsigned level2 = p_data[1];
- unsigned factor = p_data[2];
- Accum1 += level1 * factor;
- Accum2 += level2 * factor;
- }
