关于循环赋值、memset与fill的效率测试

最近一直在做图论的题目。对于初始化的效率要求比较高。正巧我也对这三个函数不是很清楚。

就写了个测试程序来测试效率

测试程序：

#include <bits/stdc++.h>
//#pragma GCC optimize(2)
using namespace std;

#define max 100000000+5
int a[max];
int main()
{
    time_t beg,end;

    cout<<"---------------------------for
";
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 1 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 2 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 3 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 4 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 5 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 6 cost: "<<end-beg<<"MS"<<endl;

    cout<<"----------------register---for
";
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 1 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 2 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 3 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 4 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 5 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    for(register int i=0;i<max;++i) a[i]=0;
    end=clock();
    cout<<"test 6 cost: "<<end-beg<<"MS"<<endl;

    cout<<"---------------------------fill
";
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 1 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 2 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 3 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 4 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 5 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    fill(a,a+max,0);
    end=clock();
    cout<<"test 6 cost: "<<end-beg<<"MS"<<endl;

    cout<<"---------------------------memset
";
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 1 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 2 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 3 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 4 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 5 cost: "<<end-beg<<"MS"<<endl;
    beg=clock();
    memset(a,0,sizeof(a));
    end=clock();
    cout<<"test 6 cost: "<<end-beg<<"MS"<<endl;
    return 0;
}

 开启o2优化之后

综上，当需要数组初始化为0,-1,inf时,memset最快，

其他时候，循环赋值 + O2优化最快