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

1 files changed, 18 insertions, 9 deletions

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 1fe72fd2b9..008f259573 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/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
-/// or not constant. Will also return 0 if the maximum trip count is very large
-/// (>= 2^32)
-unsigned ScalarEvolution::getSmallConstantTripCount(Loop *L,
-                                                    BasicBlock *ExitBlock) {
+/// normal unsigned value. Returns 0 if the trip count is unknown or not
+/// constant. Will also return 0 if the maximum trip count is very large (>=
+/// 2^32).
+///
+/// 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) {
-  const SCEV *ExitCount = getExitCount(L, ExitBlock);
+///
+/// As explained in the comments for getSmallConstantTripCount, this assumes
+/// 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;