Index: include/llvm/Analysis/AliasSetTracker.h
===================================================================
--- include/llvm/Analysis/AliasSetTracker.h
+++ include/llvm/Analysis/AliasSetTracker.h
@@ -212,7 +212,8 @@
   bool isForwardingAliasSet() const { return Forward; }
 
   /// Merge the specified alias set into this alias set.
-  void mergeSetIn(AliasSet &AS, AliasSetTracker &AST);
+  void mergeSetIn(AliasSet &AS, AliasSetTracker &AST, AliasResult AR,
+                  bool EntryUpdated = false);
 
   // Alias Set iteration - Allow access to all of the pointers which are part of
   // this alias set.
@@ -438,7 +439,8 @@
 
   AliasSet &addPointer(MemoryLocation Loc, AliasSet::AccessLattice E);
   AliasSet *mergeAliasSetsForPointer(const Value *Ptr, LocationSize Size,
-                                     const AAMDNodes &AAInfo);
+                                     const AAMDNodes &AAInfo,
+                                     bool EntryUpdated = false);
 
   /// Merge all alias sets into a single set that is considered to alias any
   /// pointer.
Index: lib/Analysis/AliasSetTracker.cpp
===================================================================
--- lib/Analysis/AliasSetTracker.cpp
+++ lib/Analysis/AliasSetTracker.cpp
@@ -47,7 +47,8 @@
 
 /// mergeSetIn - Merge the specified alias set into this alias set.
 ///
-void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST) {
+void AliasSet::mergeSetIn(AliasSet &AS, AliasSetTracker &AST, AliasResult AR,
+                          bool EntryUpdated) {
   assert(!AS.Forward && "Alias set is already forwarding!");
   assert(!Forward && "This set is a forwarding set!!");
 
@@ -57,17 +58,20 @@
   Alias  |= AS.Alias;
 
   if (Alias == SetMustAlias) {
-    // Check that these two merged sets really are must aliases.  Since both
-    // used to be must-alias sets, we can just check any pointer from each set
-    // for aliasing.
-    AliasAnalysis &AA = AST.getAliasAnalysis();
-    PointerRec *L = getSomePointer();
-    PointerRec *R = AS.getSomePointer();
-
-    // If the pointers are not a must-alias pair, this set becomes a may alias.
-    if (AA.alias(MemoryLocation(L->getValue(), L->getSize(), L->getAAInfo()),
-                 MemoryLocation(R->getValue(), R->getSize(), R->getAAInfo())) !=
-        MustAlias)
+    if (EntryUpdated) {
+      // Check that these two merged sets really are must aliases.  Since both
+      // used to be must-alias sets, we can just check any pointer from each set
+      // for aliasing.
+      AliasAnalysis &AA = AST.getAliasAnalysis();
+      PointerRec *L = getSomePointer();
+      PointerRec *R = AS.getSomePointer();
+
+      // If the pointers are not a must-alias pair, this set becomes a may alias.
+      if (AA.alias(MemoryLocation(L->getValue(), L->getSize(), L->getAAInfo()),
+                   MemoryLocation(R->getValue(), R->getSize(), R->getAAInfo())) !=
+          MustAlias)
+        Alias = SetMayAlias;
+    } else if (AR != MustAlias)
       Alias = SetMayAlias;
   }
 
@@ -295,18 +299,24 @@
 /// the pointer was found.
 AliasSet *AliasSetTracker::mergeAliasSetsForPointer(const Value *Ptr,
                                                     LocationSize Size,
-                                                    const AAMDNodes &AAInfo) {
+                                                    const AAMDNodes &AAInfo,
+                                                    bool EntryUpdated) {
   AliasSet *FoundSet = nullptr;
   for (iterator I = begin(), E = end(); I != E;) {
     iterator Cur = I++;
-    if (Cur->Forward || !Cur->aliasesPointer(Ptr, Size, AAInfo, AA)) continue;
+    if (Cur->Forward)
+      continue;
+
+    AliasResult AR = Cur->aliasesPointer(Ptr, Size, AAInfo, AA);
+    if (AR == NoAlias)
+      continue;
 
     if (!FoundSet) {
       // If this is the first alias set ptr can go into, remember it.
       FoundSet = &*Cur;
     } else {
       // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*Cur, *this);
+      FoundSet->mergeSetIn(*Cur, *this, AR, EntryUpdated);
     }
   }
 
@@ -324,7 +334,7 @@
       FoundSet = &*Cur;
     } else {
       // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*Cur, *this);
+      FoundSet->mergeSetIn(*Cur, *this, MayAlias);
     }
   }
   return FoundSet;
@@ -362,7 +372,7 @@
     // is NoAlias, mergeAliasSetsForPointer(undef, ...) will not find the
     // the right set for undef, even if it exists.
     if (Entry.updateSizeAndAAInfo(Size, AAInfo))
-      mergeAliasSetsForPointer(Pointer, Size, AAInfo);
+      mergeAliasSetsForPointer(Pointer, Size, AAInfo, true);
     // Return the set!
     return *Entry.getAliasSet(*this)->getForwardedTarget(*this);
   }
@@ -605,7 +615,7 @@
     }
 
     // Otherwise, perform the actual merge.
-    AliasAnyAS->mergeSetIn(*Cur, *this);
+    AliasAnyAS->mergeSetIn(*Cur, *this, MayAlias);
   }
 
   return *AliasAnyAS;