VM: Reland Inline instance object hash code into object header on 64bit.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 341a2e6efdfab62e6491a4eefc65a473ae437c12..0a3f6dcfa91a6fcb76c0bdeb4b34bac9327fffad 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -3433,10 +3433,12 @@ void Assembler::TryAllocate(const Class& cls,
     NOT_IN_PRODUCT(UpdateAllocationStats(cls.id(), space));
     ASSERT(instance_size >= kHeapObjectTag);
     AddImmediate(instance_reg, Immediate(kHeapObjectTag - instance_size));
-    uword tags = 0;
+    uint32_t tags = 0;
     tags = RawObject::SizeTag::update(instance_size, tags);
     ASSERT(cls.id() != kIllegalCid);
     tags = RawObject::ClassIdTag::update(cls.id(), tags);
+    // Extends the 32 bit tags with zeros, which is the uninitialized
+    // hash code.
     MoveImmediate(FieldAddress(instance_reg, Object::tags_offset()),
                   Immediate(tags));
   } else {
@@ -3480,9 +3482,11 @@ void Assembler::TryAllocateArray(intptr_t cid,
 
     // Initialize the tags.
     // instance: new object start as a tagged pointer.
-    uword tags = 0;
+    uint32_t tags = 0;
     tags = RawObject::ClassIdTag::update(cid, tags);
     tags = RawObject::SizeTag::update(instance_size, tags);
+    // Extends the 32 bit tags with zeros, which is the uninitialized
+    // hash code.
     movq(FieldAddress(instance, Array::tags_offset()), Immediate(tags));
   } else {
     jmp(failure);