Comment and clarifying assert.

llvm-svn: 139036

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -319,8 +319,9 @@ namespace llvm {
       const SCEV *getExact(ScalarEvolution *SE) const;
 
       /// getExact - Return the number of times this loop exit may fall through
-      /// to the back edge. The loop is guaranteed not to exit via this block
-      /// before this number of iterations, but may exit via another block.
+      /// to the back edge, or SCEVCouldNotCompute. The loop is guaranteed not
+      /// to exit via this block before this number of iterations, but may exit
+      /// via another block.
       const SCEV *getExact(BasicBlock *ExitingBlock, ScalarEvolution *SE) const;
 
       /// getMax - Get the max backedge taken count for the loop.

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -4090,6 +4090,7 @@ ScalarEvolution::BackedgeTakenInfo::getExact(ScalarEvolution *SE) const {
     else
       BECount = SE->getUMinFromMismatchedTypes(BECount, ENT->ExactNotTaken);
   }
+  assert(BECount && "Invalid not taken count for loop exit");
   return BECount;
 }