cs394p p. 75

Loop Transformations

Sometimes loops can be transformed to different forms that are faster.


for i := 1 to 1000 do
  for j := 1 to 1000 do
    x[i,j] := y[i,j];

This might be transformed to a single, linear loop:


for i := 1 to 1000000 do  x[i] := y[i];

Then it might be generated as a block-move instruction.

Code motion is moving code to a more favorable location, e.g., moving invariant code out of loops:


for i := 1 to 1000 do
  x[i] := y[i] * sqrt(a);

The code sqrt(a) does not change within the loop, so it could be moved above the loop and its value reused.