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
diff --git a/include/llvm/Analysis/ScalarEvolution.h b/include/llvm/Analysis/ScalarEvolution.h
index dd3c808..727bf1b 100644
--- a/include/llvm/Analysis/ScalarEvolution.h
+++ b/include/llvm/Analysis/ScalarEvolution.h
@@ -730,16 +730,21 @@
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
diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index 1fe72fd..008f259 100644
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -3939,13 +3939,19 @@
//
/// 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 @@
/// 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;