不 要 像 这 样 写 循 环 展 开

你的循环展开是真的吗？

正确写法：

#include<bits/stdc++.h>
using namespace std;
int a[150005],n=120000;
int solve(){
    long long sum1=rand()%n+1,sum2=rand()%n+1,sum3=rand()%n+1,sum4=rand()%n+1,sum5=rand()%n+1,sum6=rand()%n+1,sum7=rand()%n+1,sum8=rand()%n+1;
    for(int i=0;i+8<=n;i+=8){
        sum1+=a[i+1];sum2+=a[i+2];sum3+=a[i+3];sum4+=a[i+4];
        sum5+=a[i+5];sum6+=a[i+6];sum7+=a[i+7];sum8+=a[i+8];
    }
    switch(n&7){
        case 7:sum7+=a[n-6];
        case 6:sum6+=a[n-5];
        case 5:sum5+=a[n-4];
        case 4:sum4+=a[n-3];
        case 3:sum3+=a[n-2];
        case 2:sum2+=a[n-1];
        case 1:sum1+=a[n]; 
    }
    return sum1+sum2+sum3+sum4+sum5+sum6+sum7+sum8;
}
int main(){
    srand(102624);
    for(int i=1;i<=n;++i)a[i]=i;
    long long ans=0;
    for(int i=1;i<=100000;++i)ans^=solve();
    cerr<<ans<<' '<<1.0*clock()/CLOCKS_PER_SEC<<'s'<<'\n';
    return 0;
}

测试结果：

正常 5.822s

开O2：1.647s

循环用了++i：

#include<bits/stdc++.h>
using namespace std;
int a[150005],n=120000;
int solve(){
    long long sum1=rand()%n+1,sum2=rand()%n+1,sum3=rand()%n+1,sum4=rand()%n+1,sum5=rand()%n+1,sum6=rand()%n+1,sum7=rand()%n+1,sum8=rand()%n+1;
    for(int i=0;i+8<=n;){
        sum1+=a[++i];sum2+=a[++i];sum3+=a[++i];sum4+=a[++i];
        sum5+=a[++i];sum6+=a[++i];sum7+=a[++i];sum8+=a[++i];
    }
    switch(n&7){
        case 7:sum7+=a[n-6];
        case 6:sum6+=a[n-5];
        case 5:sum5+=a[n-4];
        case 4:sum4+=a[n-3];
        case 3:sum3+=a[n-2];
        case 2:sum2+=a[n-1];
        case 1:sum1+=a[n]; 
    }
    return sum1+sum2+sum3+sum4+sum5+sum6+sum7+sum8;
}
int main(){
    srand(102624);
    for(int i=1;i<=n;++i)a[i]=i;
    long long ans=0;
    for(int i=1;i<=100000;++i)ans^=solve();
    cerr<<ans<<' '<<1.0*clock()/CLOCKS_PER_SEC<<'s'<<'\n';
    return 0;
}

测试结果：

正常 7.343s

开O2：1.647s

求和变量用了同一个：

#include<bits/stdc++.h>
using namespace std;
int a[150005],n=120000;
int solve(){
    long long sum=rand()%n+1+rand()%n+1+rand()%n+1+rand()%n+1+rand()%n+1+rand()%n+1+rand()%n+1+rand()%n+1;
    for(int i=0;i+8<=n;i+=8){
        sum+=a[i+1];sum+=a[i+2];sum+=a[i+3];sum+=a[i+4];
        sum+=a[i+5];sum+=a[i+6];sum+=a[i+7];sum+=a[i+8];
    }
    switch(n&7){
        case 7:sum+=a[n-6];
        case 6:sum+=a[n-5];
        case 5:sum+=a[n-4];
        case 4:sum+=a[n-3];
        case 3:sum+=a[n-2];
        case 2:sum+=a[n-1];
        case 1:sum+=a[n]; 
    }
    return sum;
}
int main(){
    srand(102624);
    for(int i=1;i<=n;++i)a[i]=i;
    long long ans=0;
    for(int i=1;i<=100000;++i)ans^=solve();
    cerr<<ans<<' '<<1.0*clock()/CLOCKS_PER_SEC<<'s'<<'\n';
    return 0;
}

测试结果：

正常 24.492s

开O2：2.961s