Data Dependence in Parallel Loops

• Two statements or loops have data access conflict when they refer to the same location
• It is resolved by completing the first access before initiating the second one

Forall Loop

• Each statement computes rhs for all index values before store
  forall i = 2 to 10 do
  x[i] = x[i-1] + x[i+1]
  endall
• If it were a sequential loop
  flow dependence from x[i] to x[i-1]
  anti dependence from x[i+1] to x[i]
• Semantics of forall loop
  • Fetch all old values before writing therefore, no flow dependence
  • Two anti dependence from S to S with distances (-1, 1)

Dopar Loop

• Each iteration starts with copies of variables with the values available before the loop
• Value computed in one iteration can not be fetched in another iteration
• If there is conflict between two stores, language model does not resolve. It is likely to be a programmer error