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
+    /// as a normal unsigned value. Returns 0 if the trip count is unknown or
-    /// unknown or not constant.
+    /// not constant. This "trip count" assumes that control exits via
-    unsigned getSmallConstantTripCount(Loop *L, BasicBlock *ExitBlock);
+    /// 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!).
+    /// as well!). As explained in the comments for getSmallConstantTripCount,
-    unsigned getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitBlock);
+    /// 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

lib/Analysis/ScalarEvolution.cpp

@@ -3939,13 +3939,19 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
 //
 
 /// getSmallConstantTripCount - Returns the maximum trip count of this loop as a
-/// normal unsigned value, if possible. Returns 0 if the trip count is unknown
+/// normal unsigned value. Returns 0 if the trip count is unknown or not
-/// or not constant. Will also return 0 if the maximum trip count is very large
+/// constant. Will also return 0 if the maximum trip count is very large (>=
-/// (>= 2^32)
+/// 2^32).
-unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
+///
-                                                    BasicBlock *ExitBlock) {
+/// 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 because the loop may exit
+/// prematurely via another branch.
+unsigned ScalarEvolution::
+getSmallConstantTripCount(Loop *L, BasicBlock *ExitingBlock) {
   const SCEVConstant *ExitCount =
-    dyn_cast<SCEVConstant>(getExitCount(L, ExitBlock));
+    dyn_cast<SCEVConstant>(getExitCount(L, ExitingBlock));
   if (!ExitCount)
     return 0;
 
@@ -3968,9 +3974,12 @@ unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
 /// multiple of a constant (which is also the case if the trip count is simply
 /// constant, use getSmallConstantTripCount for that case), Will also return 1
 /// if the trip count is very large (>= 2^32).
-unsigned ScalarEvolution::getSmallConstantTripMultiple(Loop *L,
+///
-                                                       BasicBlock *ExitBlock) {
+/// As explained in the comments for getSmallConstantTripCount, this assumes
-  const SCEV *ExitCount = getExitCount(L, ExitBlock);
+/// that control exits the loop via ExitingBlock.
+unsigned ScalarEvolution::
+getSmallConstantTripMultiple(Loop *L, BasicBlock *ExitingBlock) {
+  const SCEV *ExitCount = getExitCount(L, ExitingBlock);
   if (ExitCount == getCouldNotCompute())
     return 1;