Allow property indexes to refer to slots inside the object header.

src/x64/stub-cache-x64.cc

Index: src/x64/stub-cache-x64.cc
diff --git a/src/x64/stub-cache-x64.cc b/src/x64/stub-cache-x64.cc
index b120efb376b6c5def906887d3dbf8f29ae53ecb0..866a14232a217d30a334c85eda6ae5255e6316b2 100644
--- a/src/x64/stub-cache-x64.cc
+++ b/src/x64/stub-cache-x64.cc
@@ -350,18 +350,23 @@ void StubCompiler::GenerateFastPropertyLoad(MacroAssembler* masm,
                                             Register dst,
                                             Register src,
                                             Handle<JSObject> holder,
-                                            int index) {
-  // Adjust for the number of properties stored in the holder.
-  index -= holder->map()->inobject_properties();
-  if (index < 0) {
-    // Get the property straight out of the holder.
-    int offset = holder->map()->instance_size() + (index * kPointerSize);
+                                            PropertyIndex index) {
+  if (index.IsHeaderIndex()) {
+    int offset = index.HeaderIndex() * kPointerSize;
     __ movq(dst, FieldOperand(src, offset));
   } else {
-    // Calculate the offset into the properties array.
-    int offset = index * kPointerSize + FixedArray::kHeaderSize;
-    __ movq(dst, FieldOperand(src, JSObject::kPropertiesOffset));
-    __ movq(dst, FieldOperand(dst, offset));
+    // Adjust for the number of properties stored in the holder.
+    int slot = index.FieldIndex() - holder->map()->inobject_properties();
+    if (slot < 0) {
+      // Get the property straight out of the holder.
+      int offset = holder->map()->instance_size() + (slot * kPointerSize);
+      __ movq(dst, FieldOperand(src, offset));
+    } else {
+      // Calculate the offset into the properties array.
+      int offset = slot * kPointerSize + FixedArray::kHeaderSize;
+      __ movq(dst, FieldOperand(src, JSObject::kPropertiesOffset));
+      __ movq(dst, FieldOperand(dst, offset));
+    }
   }
 }