Index: lib/Analysis/TypeBasedAliasAnalysis.cpp
===================================================================
--- lib/Analysis/TypeBasedAliasAnalysis.cpp
+++ lib/Analysis/TypeBasedAliasAnalysis.cpp
@@ -448,6 +448,18 @@
   if (!EnableTBAA)
     return AliasAnalysis::alias(LocA, LocB);
 
+  // Be conservative when comparing elements of StructType as members of union
+  // MayAlias each other.
+  const Value* VA = LocA.Ptr;
+  const Value* VB = LocB.Ptr;
+  if (VA->getType()->getTypeID() == Type::PointerTyID &&
+      VB->getType()->getTypeID() == Type::PointerTyID) {
+    Type* TyA = cast<PointerType>(VA->getType())->getElementType();
+    Type* TyB = cast<PointerType>(VB->getType())->getElementType();
+    if(TyA->getTypeID() == Type::StructTyID &&
+       TyB->getTypeID() == Type::StructTyID)
+      return AliasAnalysis::alias(LocA, LocB);
+  }
   // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
   // be conservative.
   const MDNode *AM = LocA.TBAATag;
Index: test/Analysis/TypeBasedAliasAnalysis/placement-tbaa.ll
===================================================================
--- test/Analysis/TypeBasedAliasAnalysis/placement-tbaa.ll
+++ test/Analysis/TypeBasedAliasAnalysis/placement-tbaa.ll
@@ -16,9 +16,9 @@
 ;   return f->i;
 ; }
 
-; Basic AA says MayAlias, TBAA says NoAlias
+; Basic AA says MayAlias, TBAA also conservatively says MayAlias
 ; CHECK: MayAlias: i64* %i5, i8** %p
-; CHECK: NoAlias: store i64 %conv, i64* %i5, align 8, !tbaa !6 <->   store i8* null, i8** %p, align 8, !tbaa !9
+; CHECK: MayAlias: store i64 %conv, i64* %i5, align 8, !tbaa !6 <->   store i8* null, i8** %p, align 8, !tbaa !9
 
 %struct.Foo = type { i64 }
 %struct.Bar = type { i8* }