Clarified the SCEV getSmallConstantTripCount interface with in-your-face comments.

This interface is misleading and dangerous, but it is actually what we need for unrolling.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147926 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Analysis/ScalarEvolution.h

@@ -730,16 +730,21 @@ namespace llvm {
                                      const SCEV *LHS, const SCEV *RHS);
 
     /// getSmallConstantTripCount - Returns the maximum trip count of this loop
-    /// as a normal unsigned value, if possible. Returns 0 if the trip count is
-    /// unknown or not constant.
-    unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitBlock);
+    /// as a normal unsigned value. Returns 0 if the trip count is unknown or
+    /// not constant. This "trip count" assumes that control exits via
+    /// ExitingBlock. More precisely, it is the number of times that control may
+    /// reach ExitingBlock before taking the branch. For loops with multiple
+    /// exits, it may not be the number times that the loop header executes if
+    /// the loop exits prematurely via another branch.
+    unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitingBlock);
 
     /// getSmallConstantTripMultiple - Returns the largest constant divisor of
     /// the trip count of this loop as a normal unsigned value, if
     /// possible. This means that the actual trip count is always a multiple of
     /// the returned value (don't forget the trip count could very well be zero
-    /// as well!).
-    unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitBlock);
+    /// as well!). As explained in the comments for getSmallConstantTripCount,
+    /// this assumes that control exits the loop via ExitingBlock.
+    unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitingBlock);
 
     // getExitCount - Get the expression for the number of loop iterations for
     // which this loop is guaranteed not to exit via ExitingBlock. Otherwise