Update stale comment before analyzeLoopUnrollCost. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239565 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoopUnrollPass.cpp

@@ -455,13 +455,15 @@ struct EstimatedUnrollCost {
 ///
 /// Complete loop unrolling can make some loads constant, and we need to know
 /// if that would expose any further optimization opportunities.  This routine
-/// estimates this optimization.  It assigns computed number of instructions,
-/// that potentially might be optimized away, to
-/// NumberOfOptimizedInstructions, and total number of instructions to
-/// UnrolledLoopSize (not counting blocks that won't be reached, if we were
-/// able to compute the condition).
-/// \returns false if we can't analyze the loop, or if we discovered that
-/// unrolling won't give anything. Otherwise, returns true.
+/// estimates this optimization.  It computes cost of unrolled loop
+/// (UnrolledCost) and dynamic cost of the original loop (RolledDynamicCost). By
+/// dynamic cost we mean that we won't count costs of blocks that are known not
+/// to be executed (i.e. if we have a branch in the loop and we know that at the
+/// given iteration its condition would be resolved to true, we won't add up the
+/// cost of the 'false'-block).
+/// \returns Optional value, holding the RolledDynamicCost and UnrolledCost. If
+/// the analysis failed (no benefits expected from the unrolling, or the loop is
+/// too big to analyze), the returned value is None.
 Optional<EstimatedUnrollCost>
 analyzeLoopUnrollCost(const Loop *L, unsigned TripCount, ScalarEvolution &SE,
                       const TargetTransformInfo &TTI,