Computational Physics Lectures: How to optimize codes, from vectorization to parallelization

 

 

 

Number of elements that can acted upon, examples

If we vectorize the code, we can perform, with a 128-bit register four simultaneous operations, that is we have 

for (i = 0; i < n; i+=4){
    a[i] = b[i] + c[i];
    a[i+1] = b[i+1] + c[i+1];
    a[i+2] = b[i+2] + c[i+2];
    a[i+3] = b[i+3] + c[i+3];
}

displayed here as

0 1 2 3
a[0]= a[1]= a[2]= a[3]=
b[0]+ b[1]+ b[2]+ b[3]+
c[0] c[1] c[2] c[3]
Four additions are now done in a single step.