diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp
@@ -1296,11 +1296,20 @@
 }
 
 bool LoopVectorizationLegality::prepareToFoldTailByMasking() {
-
   LLVM_DEBUG(dbgs() << "LV: checking if tail can be folded by masking.\n");
 
-  SmallPtrSet<const Value *, 8> ReductionLiveOuts;
+  for (auto &Induction : getInductionVars())
+    if (any_of(Induction.first->users(), [&](User *U) {
+          return !TheLoop->contains(cast<Instruction>(U));
+        })) {
+      LLVM_DEBUG(
+          dbgs()
+          << "LV: Cannot fold tail by masking, loop has an outside user for "
+          << *Induction.first << "\n");
+      return false;
+    }
 
+  SmallPtrSet<const Value *, 8> ReductionLiveOuts;
   for (auto &Reduction : getReductionVars())
     ReductionLiveOuts.insert(Reduction.second.getLoopExitInstr());
 
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -3321,10 +3321,12 @@
   Value *PostInc = OrigPhi->getIncomingValueForBlock(OrigLoop->getLoopLatch());
   for (User *U : PostInc->users()) {
     Instruction *UI = cast<Instruction>(U);
-    if (!OrigLoop->contains(UI)) {
-      assert(isa<PHINode>(UI) && "Expected LCSSA form");
-      MissingVals[UI] = EndValue;
-    }
+    if (OrigLoop->contains(UI))
+      continue;
+
+    assert(isa<PHINode>(UI) && "Expected LCSSA form");
+    assert(!Cost->foldTailByMasking() && "Unexpected external use of IV");
+    MissingVals[UI] = EndValue;
   }
 
   // An external user of the penultimate value need to see EndValue - Step.
@@ -3334,6 +3336,7 @@
     auto *UI = cast<Instruction>(U);
     if (!OrigLoop->contains(UI)) {
       assert(isa<PHINode>(UI) && "Expected LCSSA form");
+      assert(!Cost->foldTailByMasking() && "Unexpected external use of IV");
 
       IRBuilder<> B(MiddleBlock->getTerminator());
 
diff --git a/llvm/test/Transforms/LoopVectorize/pr52335.ll b/llvm/test/Transforms/LoopVectorize/pr52335.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/LoopVectorize/pr52335.ll
@@ -0,0 +1,45 @@
+; RUN: opt < %s -loop-vectorize -force-vector-width=4 -force-vector-interleave=1 -S | FileCheck %s
+
+define dso_local i32 @test(i32* nocapture %arr, i64 %n) {
+; The vectorizer should refuse to fold the tail by masking because
+; %conv is used outside of the loop. Test for this by checking that
+; %n.vec, the vector trip count, is rounded down to the next multiple of
+; 4. If folding the tail, it would have been rounded up instead.
+;
+; CHECK-LABEL: vector.ph:
+; CHECK-NEXT:    %n.mod.vf = urem i64 [[TC:%.*]], 4
+; CHECK-NEXT:    %n.vec = sub i64 [[TC]], %n.mod.vf
+entry:
+  %cmp1 = icmp ugt i64 %n, 1
+  br i1 %cmp1, label %preheader, label %done
+
+preheader:
+  br label %loop
+
+loop:
+  %conv = phi i64 [ %conv2, %loop ], [ 1, %preheader ]
+  %i = phi i8 [ %inc, %loop ], [ 1, %preheader ]
+  %sub = add nsw i64 %conv, -1
+  %ptr = getelementptr inbounds i32, i32* %arr, i64 %sub
+  store i32 65, i32* %ptr, align 4
+  %inc = add i8 %i, 1
+  %conv2 = zext i8 %inc to i64
+  %cmp2 = icmp ult i64 %conv2, %n
+  br i1 %cmp2, label %loop, label %load_val, !llvm.loop !0
+
+load_val:
+  %final = phi i64 [ %conv, %loop ]
+  %ptr2 = getelementptr inbounds i32, i32* %arr, i64 %final
+  %val = load i32, i32* %ptr2, align 4
+  br label %done
+
+done:
+  %value = phi i32 [ %val, %load_val ], [ 0, %entry ]
+  ret i32 %value
+
+}
+
+!0 = distinct !{!0, !1, !2, !3}
+!1 = !{!"llvm.loop.unroll.disable"}
+!2 = !{!"llvm.loop.vectorize.predicate.enable", i1 true}
+!3 = !{!"llvm.loop.vectorize.enable", i1 true}