diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2168,13 +2168,21 @@
     if (!CodeGenOpts.NullPointerIsValid &&
         getContext().getTargetAddressSpace(FI.arg_begin()->type) == 0) {
       Attrs.addAttribute(llvm::Attribute::NonNull);
+      Attrs.addDereferenceableAttr(
+          getMinimumObjectSize(
+              FI.arg_begin()->type.castAs<PointerType>()->getPointeeType())
+              .getQuantity());
+    } else {
+      // FIXME dereferenceable should be correct here, regardless of
+      // NullPointerIsValid. However, dereferenceable currently does not always
+      // respect NullPointerIsValid and may imply nonnull and break the program.
+      // See https://reviews.llvm.org/D66618 for discussions.
+      Attrs.addDereferenceableOrNullAttr(
+          getMinimumObjectSize(
+              FI.arg_begin()->type.castAs<PointerType>()->getPointeeType())
+              .getQuantity());
     }
 
-    Attrs.addDereferenceableAttr(
-        getMinimumObjectSize(
-            FI.arg_begin()->type.castAs<PointerType>()->getPointeeType())
-            .getQuantity());
-
     ArgAttrs[IRArgs.first] = llvm::AttributeSet::get(getLLVMContext(), Attrs);
   }
 
diff --git a/clang/test/CodeGenCXX/this-nonnull.cpp b/clang/test/CodeGenCXX/this-nonnull.cpp
--- a/clang/test/CodeGenCXX/this-nonnull.cpp
+++ b/clang/test/CodeGenCXX/this-nonnull.cpp
@@ -12,8 +12,9 @@
   s.ReturnsVoid();
 
   // CHECK-YES: call void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* nonnull dereferenceable(12) %0)
-  // CHECK-NO: call void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* dereferenceable(12) %0)
+  /// FIXME Use dereferenceable after dereferenceable respects NullPointerIsValid.
+  // CHECK-NO: call void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* dereferenceable_or_null(12) %0)
 }
 
 // CHECK-YES: declare void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* nonnull dereferenceable(12))
-// CHECK-NO: declare void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* dereferenceable(12))
+// CHECK-NO: declare void @_ZN6Struct11ReturnsVoidEv(%struct.Struct* dereferenceable_or_null(12))