Index: llvm/lib/Transforms/Scalar/LoopInterchange.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -1022,6 +1022,39 @@
   return true;
 }
 
+// When the inner loop is a nested loop itself, if there is a def inside the
+// inner loop, i.e., between the inner header and the inner latch, and there
+// is a use in the original inner latch, there will be an lcssa phi node in
+// the original inner latch, and we should check whether it is supported.
+// Note that the original inner latch will become the new outer latch after
+// interchange. We check whether the incoming values of those lcssa phis
+// are available in the new outer latch after interchange.
+static bool areInnerLoopLatchPHIsSupported(Loop *OuterLoop, Loop *InnerLoop) {
+  if (InnerLoop->getSubLoops().empty())
+    return true;
+  // If the original outer latch has only one predecessor, then values defined
+  // inside the inner loop, i.e., in the innermost loop, will be available at
+  // the new outer latch after interchange.
+  if (OuterLoop->getLoopLatch()->getUniquePredecessor() != nullptr)
+    return true;
+
+  // The outer latch has more than one predecessors, i.e., the inner
+  // exit and the inner header.
+  // PHI nodes in the inner latch are lcssa phis where the incoming values
+  // are defined in the innermost loop. Check if those phis are used in the
+  // original inner latch. If that is the case then bail out since those
+  // incoming values may not be available at the new outer latch.
+  BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
+  for (PHINode &PHI : InnerLoopLatch->phis()) {
+    for (auto *U : PHI.users()) {
+      Instruction *UI = cast<Instruction>(U);
+      if (InnerLoopLatch == UI->getParent())
+        return false;
+    }
+  }
+  return true;
+}
+
 bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
                                                   unsigned OuterLoopId,
                                                   CharMatrix &DepMatrix) {
@@ -1057,6 +1090,18 @@
         return false;
       }
 
+  if (!areInnerLoopLatchPHIsSupported(OuterLoop, InnerLoop)) {
+    LLVM_DEBUG(dbgs() << "Found unsupported PHI nodes in inner loop latch.\n");
+    ORE->emit([&]() {
+      return OptimizationRemarkMissed(DEBUG_TYPE, "UnsupportedInnerLatchPHI",
+                                      InnerLoop->getStartLoc(),
+                                      InnerLoop->getHeader())
+             << "Cannot interchange loops because unsupported PHI nodes found "
+                "in inner loop latch.";
+    });
+    return false;
+  }
+
   // TODO: The loops could not be interchanged due to current limitations in the
   // transform module.
   if (currentLimitations()) {
Index: llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll
@@ -0,0 +1,58 @@
+; REQUIRES: asserts
+; RUN: opt < %s -basic-aa -loop-interchange -verify-dom-info -verify-loop-info \
+; RUN:     -S -debug 2>&1 | FileCheck %s
+
+@a = common global i32 0, align 4
+@d = common dso_local local_unnamed_addr global [1 x [6 x i32]] zeroinitializer, align 4
+
+;; After interchanging the innermost and the middle loop, we should not continue
+;; doing interchange for the (new) middle loop and the outermost loop, because of
+;; values defined in the new innermost loop not available in the exiting block of
+;; the entire loop nest.
+; CHECK: Loops are legal to interchange
+; CHECK: Loops interchanged.
+; CHECK: Found unsupported PHI nodes in inner loop latch.
+; CHECK: Not interchanging loops. Cannot prove legality.
+define void @innermost_latch_uses_values_in_middle_header() {
+entry:
+  %0 = load i32, i32* @a, align 4
+  %b = add i32 80, 1
+  br label %outermost.header
+
+outermost.header:                      ; preds = %outermost.latch, %entry
+  %indvar.outermost = phi i32 [ 10, %entry ], [ %indvar.outermost.next, %outermost.latch ]
+  %tobool71.i = icmp eq i32 %0, 0
+  br i1 %tobool71.i, label %middle.header, label %outermost.latch
+
+middle.header:                            ; preds = %middle.latch, %outermost.header
+  %indvar.middle = phi i64 [ 4, %outermost.header ], [ %indvar.middle.next, %middle.latch ]
+  %indvar.middle.wide = zext i32 %b to i64 ; a def in the middle header
+  br label %innermost.header
+
+innermost.header:                                         ; preds = %middle.header, %innermost.latch
+  %indvar.innermost = phi i64 [ %indvar.innermost.next, %innermost.latch ], [ 4, %middle.header ]
+  br label %innermost.body
+
+innermost.body:                                      ; preds = %innermost.header
+  %arrayidx9.i = getelementptr inbounds [1 x [6 x i32]], [1 x [6 x i32]]* @d, i64 0, i64 %indvar.innermost, i64 %indvar.middle
+  store i32 0, i32* %arrayidx9.i, align 4
+  br label %innermost.latch
+
+innermost.latch:                             ; preds = %innermost.body
+  %indvar.innermost.next = add nsw i64 %indvar.innermost, 1
+  %tobool5.i = icmp eq i64 %indvar.innermost.next, %indvar.middle.wide ; corresponding use in the innermost latch
+  br i1 %tobool5.i, label %middle.latch, label %innermost.header
+
+middle.latch:                                      ; preds = %innermost.latch
+  %indvar.middle.next = add nsw i64 %indvar.middle, -1
+  %tobool2.i = icmp eq i64 %indvar.middle.next, 0
+  br i1 %tobool2.i, label %outermost.latch, label %middle.header
+
+outermost.latch:                                      ; preds = %middle.latch, %outermost.header
+  %indvar.outermost.next = add nsw i32 %indvar.outermost, -5
+  %tobool.i = icmp eq i32 %indvar.outermost.next, 0
+  br i1 %tobool.i, label %outermost.exit, label %outermost.header
+
+outermost.exit:                                           ; preds = %outermost.latch
+  ret void
+}