Reduce number of JSObject direct slots to 5

neildhar · facebook-github-bot · commit 1b24ca22b69f · 2021-05-19T19:42:22.000-07:00
Summary:
The previous changes shave 4 bytes off the size of objects. However,
due to heap alignment, we need to shrink `JSObject` by 8 bytes in
order to decrease its actual size after alignment. We can do that by
removing a direct property slot.

Reviewed By: dulinriley

Differential Revision: D28526281

fbshipit-source-id: 408d80cfebbf63c3112345099940e01cda450ae1
diff --git a/include/hermes/VM/JSObject.h b/include/hermes/VM/JSObject.h
@@ -390,7 +390,7 @@ class JSObject : public GCCell {
   static const PropStorage::size_type NAMED_PROPERTY_SLOTS = 0;
 
   /// Number of property slots allocated directly inside the object.
-  static const PropStorage::size_type DIRECT_PROPERTY_SLOTS = 6;
+  static const PropStorage::size_type DIRECT_PROPERTY_SLOTS = 5;
 
   static bool classof(const GCCell *cell) {
     return kindInRange(
@@ -1558,6 +1558,10 @@ constexpr size_t JSObject::cellSizeJSObject() {
           directPropsOffset() +
               sizeof(GCSmallHermesValue) * DIRECT_PROPERTY_SLOTS,
       "unexpected padding");
+  static_assert(
+      heapAlignSize(sizeof(JSObjectAndDirectProps)) ==
+          sizeof(JSObjectAndDirectProps),
+      "Wasted direct slot due to alignment");
   return sizeof(JSObjectAndDirectProps);
 }
 
diff --git a/lib/VM/JSObject.cpp b/lib/VM/JSObject.cpp
@@ -62,8 +62,7 @@ void ObjectBuildMeta(const GCCell *cell, Metadata::Builder &mb) {
       "directProp1",
       "directProp2",
       "directProp3",
-      "directProp4",
-      "directProp5"};
+      "directProp4"};
   for (unsigned i = mb.getJSObjectOverlapSlots();
        i < JSObject::DIRECT_PROPERTY_SLOTS;
        ++i) {
