7.5 Loop Rerolling

Many codes have loops that were unrolled manually over several iterations to amortize the cost of the test and branch at each iteration of the for loop. Before KAP can examine them, they must be rerolled to a simpler form, as shown in the following code examples:

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

KAP recognizes the following code as an unrolled loop and rerolls it before applying loop-based optimizations:

for ( i = 0; i<=(n + 1) / 2 * 2 - 1; i++ ) {
      a[i] = b[i] + c[i];
     }

Unrolled summations are also recognized, as follows:

sum = 0.0;
   for ( i=0; i<n; i+=4 ) {
   sum += b[i];
   sum += b[i+1];
   sum += b[i+2];
   sum += b[i+3];
 }

Becomes:

sum = (float )(0.0);
    for ( i = 0; i<=(n + 3) / 4 * 4 - 1; i++ ) {
sum +=  b[i];
  }