8336256: memcpy short value to int local is incorrect in VtableStubs::unsafe_hash

Reviewed-by: stuefe, shade, kvn

Author: reinrich

src/hotspot/share/code/vtableStubs.cpp

@@ -262,7 +262,8 @@ inline uint VtableStubs::unsafe_hash(address entry_point) {
   address vtable_type_addr = vtable_stub_addr + offset_of(VtableStub, _type);
   address vtable_index_addr = vtable_stub_addr + offset_of(VtableStub, _index);
   bool is_vtable_stub = *vtable_type_addr == static_cast<uint8_t>(VtableStub::Type::vtable_stub);
-  int vtable_index;
+  short vtable_index;
+  static_assert(sizeof(VtableStub::_index) == sizeof(vtable_index), "precondition");
   memcpy(&vtable_index, vtable_index_addr, sizeof(vtable_index));
   return hash(is_vtable_stub, vtable_index);
 }