源代码

```c++
#include <iostream>
#include <vector>
using namespace std;

int main(){
vector<int> a{1,2,3};
for(int i = 0; i < a.size(); i++){
cout << a[i] << ' ';
}
}
```

g++版本

```shell
g++ --version
g++.exe (tdm64-1) 10.3.0
```

分别用不同优化等级编译，然后反编译，循环部分如下

-O0

```c
for( i = 0; ; ++i ){
v3 = i;
if ( v3 >= std::vector<int>::size(v8) )break;
v4 = (unsigned int *)std::vector<int>::operator[](v8, i);
v5 = std::ostream::operator<<(refptr__ZSt4cout, *v4);
std::operator<<<std::char_traits<char>>(v5, 32i64);
}
```

-O1

```c
do{
v5 = ((__int64 (__fastcall *)(void *, _QWORD))std::ostream::operator<<)(
refptr__ZSt4cout,
*(unsigned int *)(v3 + 4 * v4));
v7 = 32;
std::__ostream_insert<char,std::char_traits<char>>(v5, &v7, 1i64);
v3 = v8;
++v4;
}while ( (v9 - v8) >> 2 > v4 );
```

-O2

```c
do{
v5 = (std::ostream *)std::ostream::operator<<(refptr__ZSt4cout, *(unsigned int *)(v3 + 4 * v4));
std::__ostream_insert<char,std::char_traits<char>>(v5);
v3 = v7;
++v4;
}while ( (v8 - v7) >> 2 > v4 );
```

-O3

```c
do{
v5 = (std::ostream *)std::ostream::operator<<(refptr__ZSt4cout, *v4);
std::__ostream_insert<char,std::char_traits<char>>(v5);
++v4;
}while ( v4 != v3 + 3 );
```