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

runtime/vm/stub_code_x64.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/assembler.h"
 #include "vm/compiler.h"
 #include "vm/dart_entry.h"
@@ skipping 1016 matching lines @@
   __ pushq(RAX);
   __ pushq(RCX);
 
   Label add_to_buffer;
   // Check whether this object has already been remembered. Skip adding to the
   // store buffer if the object is in the store buffer already.
   // Spilled: RAX, RCX
   // 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
   // StoreBufferBlock and add the address to the pointers_.
   // RDX: Address being stored
   __ movq(RAX, Address(THR, Thread::store_buffer_block_offset()));
   __ movl(RCX, Address(RAX, StoreBufferBlock::top_offset()));
   __ movq(Address(RAX, RCX, TIMES_8, StoreBufferBlock::pointers_offset()), RDX);
 
   // Increment top_ and check for overflow.
@@ skipping 63 matching lines @@
     } else {
       __ j(ABOVE_EQUAL, &slow_case);
     }
     __ movq(Address(RCX, Heap::TopOffset(space)), RBX);
     NOT_IN_PRODUCT(__ UpdateAllocationStats(cls.id(), space));
 
     // RAX: new object start (untagged).
     // 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));
 
     // Initialize the remaining words of the object.
     // RAX: new object (tagged).
     // RBX: next object start.
     // RDX: new object type arguments (if is_cls_parameterized).
     // R9: raw null.
     // First try inlining the initialization without a loop.
     if (instance_size < (kInlineInstanceSize * kWordSize)) {
@@ skipping 1171 matching lines @@
 }
 
 
 void StubCode::GenerateAsynchronousGapMarkerStub(Assembler* assembler) {
   __ int3();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64