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

runtime/vm/stub_code_x64.cc

Index: runtime/vm/stub_code_x64.cc
diff --git a/runtime/vm/stub_code_x64.cc b/runtime/vm/stub_code_x64.cc
index 58520becc4e98a7cd200a1bd7678a2281f7c22d3..67a1832ad08cfa12456f29e12328b1232dfad6a9 100644
--- a/runtime/vm/stub_code_x64.cc
+++ b/runtime/vm/stub_code_x64.cc
@@ -1034,21 +1034,23 @@ void StubCode::GenerateUpdateStoreBufferStub(Assembler* assembler) {
   // RDX: Address being stored
   Label reload;
   __ Bind(&reload);
-  __ movq(RAX, FieldAddress(RDX, Object::tags_offset()));
-  __ testq(RAX, Immediate(1 << RawObject::kRememberedBit));
+  __ movl(RAX, FieldAddress(RDX, Object::tags_offset()));
+  __ testl(RAX, Immediate(1 << RawObject::kRememberedBit));
   __ j(EQUAL, &add_to_buffer, Assembler::kNearJump);
   __ popq(RCX);
   __ popq(RAX);
   __ ret();
 
   // Update the tags that this object has been remembered.
+  // Note that we use 32 bit operations here to match the size of the
+  // background sweeper which is also manipulating this 32 bit word.
   // RDX: Address being stored
   // RAX: Current tag value
   __ Bind(&add_to_buffer);
-  __ movq(RCX, RAX);
-  __ orq(RCX, Immediate(1 << RawObject::kRememberedBit));
+  __ movl(RCX, RAX);
+  __ orl(RCX, Immediate(1 << RawObject::kRememberedBit));
   // Compare the tag word with RAX, update to RCX if unchanged.
-  __ LockCmpxchgq(FieldAddress(RDX, Object::tags_offset()), RCX);
+  __ LockCmpxchgl(FieldAddress(RDX, Object::tags_offset()), RCX);
   __ j(NOT_EQUAL, &reload);
 
   // Load the StoreBuffer block out of the thread. Then load top_ out of the
@@ -1132,10 +1134,11 @@ void StubCode::GenerateAllocationStubForClass(Assembler* assembler,
     // RBX: next object start.
     // RDX: new object type arguments (if is_cls_parameterized).
     // Set the tags.
-    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);
+    // 64 bit store also zeros the identity hash field.
     __ movq(Address(RAX, Instance::tags_offset()), Immediate(tags));
     __ addq(RAX, Immediate(kHeapObjectTag));