--- old/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp	2018-05-16 09:39:25.859409098 +0200
+++ new/src/hotspot/cpu/x86/c1_LIRAssembler_x86.cpp	2018-05-16 09:39:19.667427069 +0200
@@ -1678,9 +1678,9 @@
     // Object is null; update MDO and exit
     Register mdo  = klass_RInfo;
     __ mov_metadata(mdo, md->constant_encoding());
-    Address data_addr(mdo, md->byte_offset_of_slot(data, DataLayout::header_offset()));
-    int header_bits = DataLayout::flag_mask_to_header_mask(BitData::null_seen_byte_constant());
-    __ orl(data_addr, header_bits);
+    Address data_addr(mdo, md->byte_offset_of_slot(data, DataLayout::flags_offset()));
+    int header_bits = BitData::null_seen_byte_constant();
+    __ orb(data_addr, header_bits);
     __ jmp(*obj_is_null);
     __ bind(not_null);
   } else {
@@ -1824,9 +1824,9 @@
       // Object is null; update MDO and exit
       Register mdo  = klass_RInfo;
       __ mov_metadata(mdo, md->constant_encoding());
-      Address data_addr(mdo, md->byte_offset_of_slot(data, DataLayout::header_offset()));
-      int header_bits = DataLayout::flag_mask_to_header_mask(BitData::null_seen_byte_constant());
-      __ orl(data_addr, header_bits);
+      Address data_addr(mdo, md->byte_offset_of_slot(data, DataLayout::flags_offset()));
+      int header_bits = BitData::null_seen_byte_constant();
+      __ orb(data_addr, header_bits);
       __ jmp(done);
       __ bind(not_null);
     } else {