[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.

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index fc22680f537..5cbc4066610 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/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;