LU, LV: elaborate on comment for overflows

Author: artagnon

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -319,7 +319,7 @@ void addStringMetadataToLoop(Loop *TheLoop, const char *MDString,
 /// In addition if \p EstimatedLoopInvocationWeight is not null it is
 /// initialized with weight of loop's latch leading to the exit.
 /// Returns a valid positive trip count, or std::nullopt when a meaningful
-/// estimate cannot be made.
+/// estimate cannot be made (including when the trip count would overflow).
 std::optional<unsigned>
 getLoopEstimatedTripCount(Loop *L,
                           unsigned *EstimatedLoopInvocationWeight = nullptr);

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -422,8 +422,9 @@ static bool hasIrregularType(Type *Ty, const DataLayout &DL) {
 }
 
 /// Returns "best known" trip count, which is either a valid positive trip count
-/// or std::nullopt, for the specified loop \p L as defined by the following
-/// procedure:
+/// or std::nullopt when an estimate cannot be made (including when the trip
+/// count would overflow), for the specified loop \p L as defined by the
+/// following procedure:
 ///   1) Returns exact trip count if it is known.
 ///   2) Returns expected trip count according to profile data if any.
 ///   3) Returns upper bound estimate if known, and if \p CanUseConstantMax.