Moving Invariant Computations Out of Loops

If a computation inside a loop computes the same value for each iteration, improve performance by moving the computation outside the loop. Compute the value once before the loop begins and use the computed value in the loop. For example, in the following code block, compute(x) is evaluated during each iteration of the DO loop:

do i = 1 to dim(a);
  if (compute(x) eq a[i]) then ...;
end;

If compute(x) is invariant (meaning that it always computes the same value for each iteration of the loop), then the code block can be modified to perform the computation once outside the loop:

computed_x = compute(x);
do i = 1 to dim(a);
  if (computed_x eq a[i]) then ...;
end;