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

Contents

Automatic vectorization and vectorization inhibitors, straight-line code

SIMD instructions perform the same type of operations multiple times. A switch statement leads thus to a non-vectorizable loop since different statemens cannot branch. The following code can however be vectorized since the if statement is implemented as a masked assignment.

  for (int j = 0; j < n; j++) {
    double x  = cos(j*1.0);
    if (x > 0 ) {
       a[j] =  x*sin(j*2.0); 
    }
    else {
       a[j] = 0.0;
    }
  }

These operations can be performed for all data elements but only those elements which the mask evaluates as true are stored. In general, one should avoid branches such as switch, go to, or return statements or if constructs that cannot be treated as masked assignments.