Index: lib/Transforms/Utils/LoopVersioning.cpp
===================================================================
--- lib/Transforms/Utils/LoopVersioning.cpp
+++ lib/Transforms/Utils/LoopVersioning.cpp
@@ -112,9 +112,11 @@
                      VersionedLoop->getLoopPreheader(), RuntimeCheck, OrigTerm);
   OrigTerm->eraseFromParent();
 
-  // The loops merge in the original exit block.  This is now dominated by the
-  // memchecking block.
-  DT->changeImmediateDominator(VersionedLoop->getExitBlock(), RuntimeCheckBB);
+  // The loops merge in the original exit block. The exit block is now
+  // dominated by the memchecking block, if the exit block did not dominate
+  // the memchecking block initially.
+  if (!DT->dominates(VersionedLoop->getExitBlock(), RuntimeCheckBB))
+    DT->changeImmediateDominator(VersionedLoop->getExitBlock(), RuntimeCheckBB);
 
   // Adds the necessary PHI nodes for the versioned loops based on the
   // loop-defined values used outside of the loop.
Index: test/Transforms/LoopVersioning/exit-block-dominates-rt-check-block.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopVersioning/exit-block-dominates-rt-check-block.ll
@@ -0,0 +1,33 @@
+; This test ensures loop versioning does not produce an invalid dominator tree
+; if the exit block of the loop (bb0) dominates the runtime check block
+; (bb1 will become the runtime check block).
+
+; RUN: opt -loop-versioning -verify-dom-info -S -o - %s > %t
+; RUN: FileCheck -input-file %t %s
+
+@c1 = external global i16
+
+define void @f(i16 %a) {
+  br label %bb0
+
+bb0:
+  br label %bb1
+
+bb1:
+  %tmp1 = load i16, i16* @c1
+  br label %bb2
+
+bb2:
+  %tmp2 = phi i16 [ %tmp1, %bb1 ], [ %tmp3, %bb2 ]
+  %tmp4 = getelementptr inbounds [1 x i32], [1 x i32]* undef, i32 0, i32 4
+  store i32 1, i32* %tmp4
+  %tmp5 = getelementptr inbounds [1 x i32], [1 x i32]* undef, i32 0, i32 9
+  store i32 0, i32* %tmp5
+  %tmp3 = add i16 %tmp2, 1
+  store i16 %tmp2, i16* @c1
+  %tmp6 = icmp sle i16 %tmp3, 0
+  br i1 %tmp6, label %bb2, label %bb0
+}
+
+; Simple check to make sure loop versioning happened.
+; CHECK-LABEL: bb2.lver.check