[LoopRotate] Forget SCEV values in RewriteUsesOfClonedInstructions

This patch fixes problems reported in PR51981.

When rotating a loop it isn't enough to just forget SCEV for that
loop nest. When rotating we might clone some instructions from the
old header into the preheader, and insert new PHI nodes to merge
values together. There could be users of the original value that are
updated to use the PHI result. And those users were not necessarily
depending on a PHI node earlier, so they weren't cleaned up when just
forgetting all SCEV:s for the loop nest. So we need to explicitly
forget those values to avoid invalid cached SCEV expressions.

Reviewed By: fhahn, mkazantsev

Differential Revision: https://reviews.llvm.org/D110813

1 files changed, 6 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
index c5b9d2c..692e60a 100644
--- a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -103,6 +103,7 @@ static void InsertNewValueIntoMap(ValueToValueMapTy &VM, Value *K, Value *V) {
 static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
                                             BasicBlock *OrigPreheader,
                                             ValueToValueMapTy &ValueMap,
+                                            ScalarEvolution *SE,
                                 SmallVectorImpl<PHINode*> *InsertedPHIs) {
   // Remove PHI node entries that are no longer live.
   BasicBlock::iterator I, E = OrigHeader->end();
@@ -125,6 +126,10 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
     // The value now exits in two versions: the initial value in the preheader
     // and the loop "next" value in the original header.
     SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
+    // Force re-computation of OrigHeaderVal, as some users now need to use the
+    // new PHI node.
+    if (SE)
+      SE->forgetValue(OrigHeaderVal);
     SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
     SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal);
 
@@ -563,7 +568,7 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
     SmallVector<PHINode*, 2> InsertedPHIs;
     // If there were any uses of instructions in the duplicated block outside the
     // loop, update them, inserting PHI nodes as required
-    RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap,
+    RewriteUsesOfClonedInstructions(OrigHeader, OrigPreheader, ValueMap, SE,
                                     &InsertedPHIs);
 
     // Attach dbg.value intrinsics to the new phis if that phi uses a value that