use F14NodeMap in folly::dynamic

Summary:
F14NodeMap is a safe drop-in replacement for std::unordered_map. It is smaller
than std::unordered_map (32 bytes instead of 56 bytes) and has been shown
to be faster in many production use cases. This diff replaces the hash table
inside folly::dynamic. The effect is that sizeof(folly::dynamic) will drop from 64
bytes to 40 bytes, and use cases that use folly::dynamic as an object may also
get a CPU win.

Reviewed By: nbronson

Differential Revision: D9518110

fbshipit-source-id: 72daba13577c560bbb88c4d68a457afedf1af38e

folly/dynamic-inl.h

@@ -141,13 +141,13 @@ dynamic numericOp(dynamic const& a, dynamic const& b) {
 
 /*
  * We're doing this instead of a simple member typedef to avoid the
- * undefined behavior of parameterizing std::unordered_map<> with an
+ * undefined behavior of parameterizing F14NodeMap<> with an
  * incomplete type.
  *
  * Note: Later we may add separate order tracking here (a multi-index
  * type of thing.)
  */
-struct dynamic::ObjectImpl : std::unordered_map<dynamic, dynamic> {};
+struct dynamic::ObjectImpl : F14NodeMap<dynamic, dynamic> {};
 
 //////////////////////////////////////////////////////////////////////
 
@@ -855,8 +855,9 @@ struct dynamic::GetAddrImpl<std::string> {
   }
 };
 template <> struct dynamic::GetAddrImpl<dynamic::ObjectImpl> {
-  static_assert(sizeof(ObjectImpl) <= sizeof(Data::objectBuffer),
-    "In your implementation, std::unordered_map<> apparently takes different"
+  static_assert(
+      sizeof(ObjectImpl) <= sizeof(Data::objectBuffer),
+      "In your implementation, F14NodeMap<> apparently takes different"
       " amount of space depending on its template parameters.  This is "
       "weird.  Make objectBuffer bigger if you want to compile dynamic.");
 

folly/dynamic.h

@@ -57,7 +57,6 @@
 #include <ostream>
 #include <string>
 #include <type_traits>
-#include <unordered_map>
 #include <utility>
 #include <vector>
 
@@ -65,6 +64,7 @@
 
 #include <folly/Range.h>
 #include <folly/Traits.h>
+#include <folly/container/F14Map.h>
 #include <folly/json_pointer.h>
 
 namespace folly {
@@ -608,15 +608,14 @@ struct dynamic : private boost::operators<dynamic> {
 
     /*
      * Objects are placement new'd here.  We have to use a char buffer
-     * because we don't know the type here (std::unordered_map<> with
+     * because we don't know the type here (F14NodeMap<> with
      * dynamic would be parameterizing a std:: template with an
      * incomplete type right now).  (Note that in contrast we know it
      * is ok to do this with fbvector because we own it.)
      */
     std::aligned_storage<
-      sizeof(std::unordered_map<int,int>),
-      alignof(std::unordered_map<int,int>)
-    >::type objectBuffer;
+        sizeof(F14NodeMap<int, int>),
+        alignof(F14NodeMap<int, int>)>::type objectBuffer;
   } u_;
 };