The IVDEP Directive

• With indexed addressing IVDEP is the only way to specify no data dependencies to the compiler:

• • here ivdep means that the integer values stored in indx array are all different, I.e. indx is a permutation array
  • assuming no data dependencies will produce faster processor code, because compiler has less constraints on ordering the load-store instructions

• The IVDEP directive to the compiler is not part of the language and its interpretation is not standardized.

void update(int n, float *a, float *b,

int *indx, float s)

{ int i;

#pragma ivdep

for(i=0; i<n; i++) a[indx[i]] += s*b[i];

}

Previous slide

Next slide

Back to first slide

View graphic version