[SimpleLoopUnswitch] Invalidate the topmost loop with ExitBB as exiting.

SCEV caches the exiting blocks when computing exit counts. In
SimpleLoopUnswitch, we split the exit block of the loop to unswitch.

Currently we only invalidate the loop containing that exit block, but if
that block is the exiting block for a parent loop, we have stale cache
entries. We have to invalidate the top-most loop that contains the exit
block as exiting block. We might also be able to skip invalidating the
loop containing the exit block, if the exit block is not an exiting
block of that loop.

There are also 2 more places in SimpleLoopUnswitch, that use a similar
problematic approach to get the loop to invalidate. If the patch makes
sense, I will also update those places to a similar approach (they deal
with multiple exit blocks, so we cannot directly re-use
getTopMostExitingLoop).

Fixes PR43972.

Reviewers: skatkov, reames, asbirlea, chandlerc

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D70786

1 files changed, 17 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index 13e44765985..d441c6bbf12 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -331,6 +331,20 @@ static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
   }
 }
 
+// Return the top-most loop containing ExitBB and having ExitBB as exiting block
+// or the loop containing ExitBB, if there is no parent loop containing ExitBB
+// as exiting block.
+static Loop *getTopMostExitingLoop(BasicBlock *ExitBB, LoopInfo &LI) {
+  Loop *TopMost = LI.getLoopFor(ExitBB);
+  Loop *Current = TopMost;
+  while (Current) {
+    if (Current->isLoopExiting(ExitBB))
+      TopMost = Current;
+    Current = Current->getParentLoop();
+  }
+  return TopMost;
+}
+
 /// Unswitch a trivial branch if the condition is loop invariant.
 ///
 /// This routine should only be called when loop code leading to the branch has
@@ -415,9 +429,10 @@ static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
   });
 
   // If we have scalar evolutions, we need to invalidate them including this
-  // loop and the loop containing the exit block.
+  // loop, the loop containing the exit block and the topmost parent loop
+  // exiting via LoopExitBB.
   if (SE) {
-    if (Loop *ExitL = LI.getLoopFor(LoopExitBB))
+    if (Loop *ExitL = getTopMostExitingLoop(LoopExitBB, LI))
       SE->forgetLoop(ExitL);
     else
       // Forget the entire nest as this exits the entire nest.