Index: include/llvm/Analysis/AliasSetTracker.h
===================================================================
--- include/llvm/Analysis/AliasSetTracker.h
+++ include/llvm/Analysis/AliasSetTracker.h
@@ -212,7 +212,7 @@
   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);
 
   // Alias Set iteration - Allow access to all of the pointers which are part of
   // this alias set.
Index: lib/Analysis/AliasSetTracker.cpp
===================================================================
--- lib/Analysis/AliasSetTracker.cpp
+++ lib/Analysis/AliasSetTracker.cpp
@@ -47,7 +47,7 @@
 
 /// 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) {
   assert(!AS.Forward && "Alias set is already forwarding!");
   assert(!Forward && "This set is a forwarding set!!");
 
@@ -56,20 +56,8 @@
   Access |= AS.Access;
   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)
-      Alias = SetMayAlias;
-  }
+  if (Alias == SetMustAlias && AR != MustAlias)
+    Alias = SetMayAlias;
 
   if (Alias == SetMayAlias) {
     if (WasMustAlias)
@@ -299,12 +287,17 @@
   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.
       FoundSet = &*Cur;   // Remember it.
     } else {              // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
+      FoundSet->mergeSetIn(*Cur, *this, AR); // Merge in contents.
     }
   }
 
@@ -320,7 +313,7 @@
     if (!FoundSet)            // If this is the first alias set ptr can go into.
       FoundSet = &*Cur;       // Remember it.
     else   // Otherwise, we must merge the sets.
-      FoundSet->mergeSetIn(*Cur, *this);     // Merge in contents.
+      FoundSet->mergeSetIn(*Cur, *this, MayAlias); // Merge in contents.
   }
   return FoundSet;
 }
@@ -600,7 +593,7 @@
     }
 
     // Otherwise, perform the actual merge.
-    AliasAnyAS->mergeSetIn(*Cur, *this);
+    AliasAnyAS->mergeSetIn(*Cur, *this, MayAlias);
   }
 
   return *AliasAnyAS;