Move cellkind number calculation to CellKind.h

Summary:
Move the calculation of the number of CellKinds to `CellKind.h`
so that we can use it in other headers.

Given that it is now a constexpr value, we can simplify the
`MetadataStorage` construction to be a `std::array` and initialiser
list.

Reviewed By: dulinriley

Differential Revision: D27991149

fbshipit-source-id: e6d5bb818bca9ee8b332891356c278f5b4aeadd7

include/hermes/VM/CellKind.h

@@ -8,6 +8,7 @@
 #ifndef HERMES_VM_CELLKIND_H
 #define HERMES_VM_CELLKIND_H
 
+#include <cstddef>
 #include <type_traits>
 
 namespace hermes {
@@ -43,6 +44,11 @@ cellKindsContiguousAscending(CellKind v1, CellKind v2, T... rest) {
 
 const char *cellKindStr(CellKind kind);
 
+static constexpr size_t kNumCellKinds = 0
+#define CELL_KIND(name) +1
+#include "hermes/VM/CellKinds.def"
+    ;
+
 } // namespace vm
 } // namespace hermes
 

lib/VM/BuildMetadata.cpp

@@ -17,33 +17,6 @@
 namespace hermes {
 namespace vm {
 
-static size_t getNumCellKinds() {
-  // This embeds the same value as the CellKind enum, but adds one more at the
-  // end to know how many values it contains.
-  enum CellKinds {
-#define CELL_KIND(name) name,
-#include "hermes/VM/CellKinds.def"
-#undef CELL_KIND
-    numKinds,
-  };
-  return numKinds;
-}
-
-/// Creates and populates the storage for the metadata of the classes.
-/// The caller takes ownership of the memory.
-static const Metadata *buildStorage() {
-  // For each class of object, initialize its metadata
-  // Only run this once per class, not once per Runtime instantiation.
-  Metadata *storage = new Metadata[getNumCellKinds()];
-  size_t i = 0;
-#define CELL_KIND(name) \
-  storage[i++] = buildMetadata(CellKind::name##Kind, name##BuildMeta);
-#include "hermes/VM/CellKinds.def"
-#undef CELL_KIND
-  assert(i == getNumCellKinds() && "Incorrect number of metadata populated");
-  return storage;
-}
-
 Metadata buildMetadata(CellKind kind, BuildMetadataCallback *builder) {
   const GCCell *base;
 #ifdef HERMES_UBSAN
@@ -66,10 +39,16 @@ Metadata buildMetadata(CellKind kind, BuildMetadataCallback *builder) {
 }
 
 MetadataTable getMetadataTable() {
+  // For each class of object, initialize its metadata
+  // Only run this once per class, not once per Runtime instantiation.
   // We intentionally leak memory here in order to avoid any static destructors
   // running at exit time.
-  static const Metadata *storage = buildStorage();
-  return MetadataTable(storage, getNumCellKinds());
+  static const auto *storage = new std::array<Metadata, kNumCellKinds>{
+#define CELL_KIND(name) buildMetadata(CellKind::name##Kind, name##BuildMeta),
+#include "hermes/VM/CellKinds.def"
+#undef CELL_KIND
+  };
+  return *storage;
 }
 
 } // namespace vm