感性理解编译优化

Posted 2020-12-18 sweepy

很多人说O2很快，真的吗？

话不多说，先贴图：
环境：

WSL Ubuntu 18.04LTS

sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ g++ file.cpp -o file.out
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ vi ./file.cpp
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ time ./file.out

real    0m0.063s
user    0m0.047s
sys     0m0.016s
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ g++ file.cpp -o file.out -O2
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ time ./file.out

real    0m0.013s
user    0m0.000s
sys     0m0.000s
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ g++ file.cpp -o file.out -O3
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ time ./file.out

real    0m0.016s
user    0m0.016s
sys     0m0.000s
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ g++ file.cpp -o file.out -Ofast
sweepy@DESKTOP-M79ULPC:/mnt/c/Users/zljzl/desktop$ time ./file.out

real    0m0.012s
user    0m0.000s
sys     0m0.016s

源代码是这样的（计算1~10000000的平方和）

#include<iostream>
using namespace std;
int main(){
    int ans=0;
 for(int i=0;i<10000000;i++){
     ans+=i*i;
 }
 return 0;
}

总而言之，(O(fast))真的很快