Loop Code Motion

Loop Code Motion

Loop code motion is moving loop-invariant code from inside the loop body to the pre-initialization code for the loop. Any code that has the same value should not be performed inside the loop body. Instead, it should be pre-calculated before the loop, and stored in a temporary variable. This is sometimes called “hoisting” the code out of the loop.

Example: Loop Code Motion: One common example of unnecessary recalculation of loop-invariant values is in the loop test. The code in the Boolean test for the loop is actually part of the loop body.

An example of code that re-calculates the loop limit:

   for (i = 0; i < vec.num_elements(); i++) {
      // ...
   }

The “num_elements” call is probably loop-invariant, assuming the vector doesn't change size during processing. Maybe the “num_elements” function is declared “inline” and the C++ compiler will fix it anyway. Nevertheless, this is a candidate for loop code motion, using a temporary variable instead:

   int n = vec.num_elements();  // Loop-invariant value
   for (i = 0; i < n; i++) {
      // ...
   }

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