Optimize this code (like what compiler would reconstruct an equivalent source code as):

int fn (int a, int b)
{
int sum = 0;
for(int i=4*a;i0;i--)
{
sum+=b*i*i;
}
return sum;
}

Question

Optimize this code (like what compiler would reconstruct an equivalent source code as): 

int fn (int a, int b)
{
    int sum = 0;
    for(int i=4*a;i>0;i--)
    {
        sum+=b*i*i;
    }
    return sum;
}

Sigiloso · Accepted Answer

One of them is replace 4*b with a<<2. But there are more ways I didn't find.

El · Answer

the loop can be replaced by one line (basic series formula) :

sum = b* 4*a *(4*a +1) *(8*a+1)/6

http://www.tug.org/texshowcase/cheat.pdf

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