[SCEV] Further clarify comments regarding UB and zero stride

Follow on to D109029. I realized we had no mention of mustprogrress in the comment (as it prexisted mustprogress in the codebase). In the process of adding it, I tweaked the preconditions into something I think is more clear. Note that mustprogress is checked in the code. Differential Revision: https://reviews.llvm.org/D109091
llvm · Sep 7, 2021 · 9659069 · 9659069
1 parent ee903a2
commit 9659069
Showing 1 changed file with 3 additions and 3 deletions.
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11631,9 +11631,9 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
     //
     // a) IV is either nuw or nsw depending upon signedness (indicated by the
     //    NoWrap flag).
-    // b) loop is single exit with no side effects.
-    // c) loop has no abnormal exits
-    //
+    // b) the loop is guaranteed to be finite (e.g. is mustprogress and has
+    //    no side effects within the loop)
+    // c) loop has a single static exit (with no abnormal exits)
     //
     // Precondition a) implies that if the stride is negative, this is a single
     // trip loop. The backedge taken count formula reduces to zero in this case.