[LV] Don't allow outside uses of IVs if the SCEV is predicated on loop conditions.

This fixes PR33706.
Differential Revision: https://reviews.llvm.org/D35227

llvm-svn: 307837

1 files changed, 7 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 193cc4d1378..eb82ee283d4 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5315,8 +5315,13 @@ void LoopVectorizationLegality::addInductionPhi(
 
   // Both the PHI node itself, and the "post-increment" value feeding
   // back into the PHI node may have external users.
-  AllowedExit.insert(Phi);
-  AllowedExit.insert(Phi->getIncomingValueForBlock(TheLoop->getLoopLatch()));
+  // We can allow those uses, except if the SCEVs we have for them rely
+  // on predicates that only hold within the loop, since allowing the exit
+  // currently means re-using this SCEV outside the loop.
+  if (PSE.getUnionPredicate().isAlwaysTrue()) {
+    AllowedExit.insert(Phi);
+    AllowedExit.insert(Phi->getIncomingValueForBlock(TheLoop->getLoopLatch()));
+  }
 
   DEBUG(dbgs() << "LV: Found an induction variable.\n");
   return;