Is there any benefit of not using double on a 64bi

Question

I always use double to do calculations but double offers far better accuracy than I need (or makes sense, considering that most of the calculations I do are approximations to begin with).

But since the processor is already 64bit, I do not expect that using a type with less bits will be of any benefit.

Am I right/wrong, how would I optimize for speed (I understand that smaller types would be more memory efficient)

here is the test

#include <cmath>
#include <ctime>
#include <cstdio>

template<typename T>
void creatematrix(int m,int n, T **&M){
    M = new T*[m];
    T *M_data = new T[m*n];

    for(int i=0; i< m; ++i) 
    {
        M[i] = M_data + i * n;
    }
}

void main(){
    clock_t start,end;
    double diffs;
    const int N = 4096;
    const int rep =8;

    float **m1,**m2;
    creatematrix(N,N,m1);creatematrix(N,N,m2);

    start=clock();
    for(int k = 0;k<rep;k++){
        for(int i = 0;i<N;i++){
            for(int j =0;j<N;j++)
                m1[i][j]=sqrt(m1[i][j]*m2[i][j]+0.1586);
        }
    }
    end = clock();
    diffs = (end - start)/(double)CLOCKS_PER_SEC;
    printf("time = %lf\n",diffs);


    delete[] m1[0];
    delete[] m1;

    delete[] m2[0];
    delete[] m2;

    getchar();
}

there was no time difference between double and float, however when square root is not used, float is twice as fast.

Answer 1

There are a couple of ways they can be faster:

• Faster I/O: you have only half the bits to move between disk/memory/cache/registers
• Typically the only operations that are slower are square-root and division. As an example, on a Haswell a DIVSS (float division) takes 7 clock cycles, whereas a DIVSD (double division) takes 8-14 (source: Agner Fog's tables).
• If you can take advantage of SIMD instructions, then you can handle twice as many per instruction (i.e. in a 128-bit SSE register, you can operate on 4 floats, but only 2 doubles).
• Special functions (log, sin) can use lower-degree polynomials: e.g. the openlibm implementation of log uses a degree 7 polynomial, whereas logf only needs degree 4.
• If you need higher intermediate precision, you can simply promote float to double, whereas for a double you need either software double-double, or slower long double.

Note that these points also hold for 32-bit architectures as well: unlike integers, there's nothing particularly special about having the size of the format match your architecture, i.e. on most machines doubles are just as "native" as floats.