[SCEV] Expose and use maximum constant exit counts for individual loop exits

We were already going to all of the trouble of computing maximum constant exit counts for each loop exit, we might as well expose them through the API.  The change in IndVars is mostly to demonstrate that the wired up code works, but it als very slightly strengthens the transform.  The strengthened case is rather narrow though: it requires one exactly analyzeable exit, one imprecisely analyzeable exit (with the upper bound less than the precise one), and one unanalyzeable exit.  I coudn't construct a reasonably stable test case.

This does increase the memory usage of the BackedgeTakenCount by a factor of 2 in the worst case.

I also noticed the loop in IndVars is O(#Exits ^ 2).  This doesn't change with this patch.  A future patch will cache this result inside of SCEV to avoid requering.

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1244,13 +1244,15 @@ private:
   struct ExitNotTakenInfo {
     PoisoningVH<BasicBlock> ExitingBlock;
     const SCEV *ExactNotTaken;
+    const SCEV *MaxNotTaken;
     std::unique_ptr<SCEVUnionPredicate> Predicate;
 
     explicit ExitNotTakenInfo(PoisoningVH<BasicBlock> ExitingBlock,
                               const SCEV *ExactNotTaken,
+                              const SCEV *MaxNotTaken,
                               std::unique_ptr<SCEVUnionPredicate> Predicate)
-        : ExitingBlock(ExitingBlock), ExactNotTaken(ExactNotTaken),
-          Predicate(std::move(Predicate)) {}
+      : ExitingBlock(ExitingBlock), ExactNotTaken(ExactNotTaken),
+        MaxNotTaken(ExactNotTaken), Predicate(std::move(Predicate)) {}
 
     bool hasAlwaysTruePredicate() const {
       return !Predicate || Predicate->isAlwaysTrue();
@@ -1333,6 +1335,9 @@ private:
     /// Get the max backedge taken count for the loop.
     const SCEV *getMax(ScalarEvolution *SE) const;
 
+    /// Get the max backedge taken count for the particular loop exit.
+    const SCEV *getMax(BasicBlock *ExitingBlock, ScalarEvolution *SE) const;
+
     /// Return true if the number of times this backedge is taken is either the
     /// value returned by getMax or zero.
     bool isMaxOrZero(ScalarEvolution *SE) const;

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -6606,7 +6606,7 @@ const SCEV *ScalarEvolution::getExitCount(const Loop *L,
   case Exact: 
     return getBackedgeTakenInfo(L).getExact(ExitingBlock, this);
   case ConstantMaximum:
-    return getCouldNotCompute();
+    return getBackedgeTakenInfo(L).getMax(ExitingBlock, this);
   };
   llvm_unreachable("Invalid ExitCountKind!");
 }
@@ -6934,6 +6934,16 @@ ScalarEvolution::BackedgeTakenInfo::getExact(BasicBlock *ExitingBlock,
   return SE->getCouldNotCompute();
 }
 
+const SCEV *
+ScalarEvolution::BackedgeTakenInfo::getMax(BasicBlock *ExitingBlock,
+                                           ScalarEvolution *SE) const {
+  for (auto &ENT : ExitNotTaken)
+    if (ENT.ExitingBlock == ExitingBlock && ENT.hasAlwaysTruePredicate())
+      return ENT.MaxNotTaken;
+
+  return SE->getCouldNotCompute();
+}
+
 /// getMax - Get the max backedge taken count for the loop.
 const SCEV *
 ScalarEvolution::BackedgeTakenInfo::getMax(ScalarEvolution *SE) const {
@@ -7025,13 +7035,15 @@ ScalarEvolution::BackedgeTakenInfo::BackedgeTakenInfo(
         BasicBlock *ExitBB = EEI.first;
         const ExitLimit &EL = EEI.second;
         if (EL.Predicates.empty())
-          return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken, nullptr);
+          return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken, EL.MaxNotTaken,
+                                  nullptr);
 
         std::unique_ptr<SCEVUnionPredicate> Predicate(new SCEVUnionPredicate);
         for (auto *Pred : EL.Predicates)
           Predicate->add(Pred);
 
-        return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken, std::move(Predicate));
+        return ExitNotTakenInfo(ExitBB, EL.ExactNotTaken, EL.MaxNotTaken,
+                                std::move(Predicate));
       });
   assert((isa<SCEVCouldNotCompute>(MaxCount) || isa<SCEVConstant>(MaxCount)) &&
          "No point in having a non-constant max backedge taken count!");

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -2662,11 +2662,11 @@ static const SCEV* getMaxBackedgeTakenCount(ScalarEvolution &SE,
   // merge the max and exact information to approximate a version of
   // getConstantMaxBackedgeTakenCount which isn't restricted to just constants.
   SmallVector<const SCEV*, 4> ExitCounts;
-  const SCEV *MaxConstEC = SE.getConstantMaxBackedgeTakenCount(L);
-  if (!isa<SCEVCouldNotCompute>(MaxConstEC))
-    ExitCounts.push_back(MaxConstEC);
   for (BasicBlock *ExitingBB : ExitingBlocks) {
     const SCEV *ExitCount = SE.getExitCount(L, ExitingBB);
+    if (isa<SCEVCouldNotCompute>(ExitCount))
+      ExitCount = SE.getExitCount(L, ExitingBB,
+                                  ScalarEvolution::ConstantMaximum);
     if (!isa<SCEVCouldNotCompute>(ExitCount)) {
       assert(DT.dominates(ExitingBB, L->getLoopLatch()) &&
              "We should only have known counts for exiting blocks that "