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

runtime/vm/intermediate_language_dbc.cc

Index: runtime/vm/intermediate_language_dbc.cc
diff --git a/runtime/vm/intermediate_language_dbc.cc b/runtime/vm/intermediate_language_dbc.cc
index 4e675296f48f63250bde9a11d461470d701a4e60..e9c4c770e3c239189c12c83ba79964755ed6a1ca 100644
--- a/runtime/vm/intermediate_language_dbc.cc
+++ b/runtime/vm/intermediate_language_dbc.cc
@@ -1011,7 +1011,7 @@ EMIT_NATIVE_CODE(AllocateObject,
       Isolate* isolate = Isolate::Current();
       if (Heap::IsAllocatableInNewSpace(instance_size) &&
           !cls().TraceAllocation(isolate)) {
-        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);
@@ -1045,6 +1045,7 @@ EMIT_NATIVE_CODE(AllocateObject,
       tags = RawObject::SizeTag::update(instance_size, tags);
       ASSERT(cls().id() != kIllegalCid);
       tags = RawObject::ClassIdTag::update(cls().id(), tags);
+      // tags also has the initial zero hash code on 64 bit.
       if (Smi::IsValid(tags)) {
         const intptr_t tags_kidx = __ AddConstant(Smi::Handle(Smi::New(tags)));
         __ AllocateOpt(locs()->out(0).reg(), tags_kidx);
@@ -1656,6 +1657,7 @@ EMIT_NATIVE_CODE(Box, 1, Location::RequiresRegister(), LocationSummary::kCall) {
     uword tags = 0;
     tags = RawObject::SizeTag::update(instance_size, tags);
     tags = RawObject::ClassIdTag::update(compiler->double_class().id(), tags);
+    // tags also has the initial zero hash code on 64 bit.
     if (Smi::IsValid(tags)) {
       const intptr_t tags_kidx = __ AddConstant(Smi::Handle(Smi::New(tags)));
       __ AllocateOpt(out, tags_kidx);