Index: lib/Transforms/Utils/LoopVersioning.cpp
===================================================================
--- lib/Transforms/Utils/LoopVersioning.cpp
+++ lib/Transforms/Utils/LoopVersioning.cpp
@@ -61,6 +61,8 @@
 
   // Add the memcheck in the original preheader (this is empty initially).
   BasicBlock *RuntimeCheckBB = VersionedLoop->getLoopPreheader();
+  bool CheckDominatesExit = DT->dominates(RuntimeCheckBB,
+                                          VersionedLoop->getExitBlock());
   std::tie(FirstCheckInst, MemRuntimeCheck) =
       LAI.addRuntimeChecks(RuntimeCheckBB->getTerminator(), AliasChecks);
 
@@ -112,9 +114,13 @@
                      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.  This should now be dominated
+  // by the memchecking block, but several loops may share the same exit block,
+  // so it may already be dominated by one of memcheck dominators.  Only if
+  // the exit block is dominated by memcheck block in original loop, update its
+  // idom.
+  if (CheckDominatesExit)
+    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/noalias-version-twice.ll
===================================================================
--- test/Transforms/LoopVersioning/noalias-version-twice.ll
+++ test/Transforms/LoopVersioning/noalias-version-twice.ll
@@ -1,4 +1,4 @@
-; RUN: opt -basicaa -loop-distribute -scoped-noalias -loop-versioning -S < %s | FileCheck %s
+; RUN: opt -basicaa -loop-distribute -scoped-noalias -loop-versioning -verify-dom-info -S < %s | FileCheck %s
 
 ; Test the metadata generated when versioning an already versioned loop.  Here
 ; we invoke loop distribution to perform the first round of versioning.  It