在程序优化期间,尝试优化迭代遍历向量的循环,我发现以下事实::: std :: vector :: at()比operator []快得多!
运算符[]比at()快5到10倍,在发布和发布时都是如此. debug builds(VS2008 x86).
在网上读了一下让我意识到at()有边界检查.好的,但是,将操作放慢了10倍?!
有什么理由吗?我的意思是,边界检查是一个简单的数字比较,还是我错过了什么?
问题是这种性能受损的真正原因是什么?
更进一步,有没有办法让它更快?
我肯定会在其他代码部分(我已经有自定义边界检查!)中将所有at()调用与[]交换.
概念证明:
#define _WIN32_WINNT 0x0400
#define WIN32_LEAN_AND_MEAN
#include
#include
#include
#define ELEMENTS_IN_VECTOR 1000000
int main()
{
__int64 freq, start, end, diff_Result;
if(!::QueryPerformanceFrequency((LARGE_INTEGER*)&freq))
throw "Not supported!";
freq /= 1000000; // microseconds!
::std::vector vec;
vec.reserve(ELEMENTS_IN_VECTOR);
for(int i = 0; i < ELEMENTS_IN_VECTOR; i++)
vec.push_back(i);
int xyz = 0;
printf("Press any key to start!");
_getch();
printf(" Running speed test../n");
{ // at()
::QueryPerformanceCounter((LARGE_INTEGER*)&start);
for(int i = 0; i < ELEMENTS_IN_VECTOR; i++)
xyz += vec.at(i);
::QueryPerformanceCounter((LARGE_INTEGER*)&end);
diff_Result = (end - start) / freq;
}
printf("Result/t/t: %u/n/n", diff_Result);
printf("Press any key to start!");
_getch();
printf(" Running speed test../n");
{ // operator []
::QueryPerformanceCounter((LARGE_INTEGER*)&start);
for(int i = 0; i < ELEMENTS_IN_VECTOR; i++)
xyz -= vec[i];
::QueryPerformanceCounter((LARGE_INTEGER*)&end);
diff_Result = (end - start) / freq;
}
printf("Result/t/t: %u/n", diff_Result);
_getch();
return xyz;
}
编辑:
现在该值被赋予“xyz”,因此编译器不会“擦除”它.