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

runtime/vm/assembler_arm64.cc

 // Copyright (c) 2014, 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"  // NOLINT
 #if defined(TARGET_ARCH_ARM64)
 
 #include "vm/assembler.h"
 #include "vm/cpu.h"
 #include "vm/longjump.h"
@@ skipping 1345 matching lines @@
     b(failure, LS);
 
     // Successfully allocated the object, now update top to point to
     // next object start and store the class in the class field of object.
     str(instance_reg, Address(temp_reg, Heap::TopOffset(space)));
 
     ASSERT(instance_size >= kHeapObjectTag);
     AddImmediate(instance_reg, -instance_size + kHeapObjectTag);
     NOT_IN_PRODUCT(UpdateAllocationStats(cls.id(), space));
 
-    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.
     LoadImmediate(TMP, tags);
     StoreFieldToOffset(TMP, instance_reg, Object::tags_offset());
   } else {
     b(failure);
   }
 }
 
 
 void Assembler::TryAllocateArray(intptr_t cid,
                                  intptr_t instance_size,
@@ skipping 23 matching lines @@
 
     // Successfully allocated the object(s), now update top to point to
     // next object start and initialize the object.
     str(end_address, Address(temp1, Heap::TopOffset(space)));
     add(instance, instance, Operand(kHeapObjectTag));
     LoadImmediate(temp2, instance_size);
     NOT_IN_PRODUCT(UpdateAllocationStatsWithSize(cid, temp2, space));
 
     // 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.
     LoadImmediate(temp2, tags);
     str(temp2, FieldAddress(instance, Array::tags_offset()));  // Store tags.
   } else {
     b(failure);
   }
 }
 
 
 Address Assembler::ElementAddressForIntIndex(bool is_external,
                                              intptr_t cid,
@@ skipping 145 matching lines @@
   str(tmp, Address(addr, 7), kUnsignedByte);
   if (sz == kDoubleWord) {
     return;
   }
   UNIMPLEMENTED();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_ARM64