Avoid allocations for empty maps

Empty FixedStringMaps would do a malloc of at least 9 bytes,
and IndexedStringMap would do a 1 byte malloc, and a malloc of at least 9
bytes.

We have a lot of typically empty maps in our data structures.

hphp/runtime/vm/fixed_string_map.cpp

@@ -14,8 +14,8 @@
    +----------------------------------------------------------------------+
 */
 
-#include "hphp/runtime/vm/core_types.h"
 #include "hphp/runtime/vm/fixed_string_map.h"
+#include "hphp/runtime/vm/core_types.h"
 #include "hphp/runtime/base/macros.h"
 
 namespace HPHP {
@@ -26,6 +26,8 @@ TRACE_SET_MOD(runtime);
 
 class Func;
 
+static const StringData* null_key;
+
 inline bool strEqual(bool case_sensitive,
                      const StringData* sd1, const StringData* sd2) {
   if (sd1 == sd2) return true;
@@ -35,8 +37,21 @@ inline bool strEqual(bool case_sensitive,
       bstrcaseeq(sd1->data(), sd2->data(), sd1->size());
 }
 
+template <typename V, bool case_sensitive>
+FixedStringMap<V, case_sensitive>::~FixedStringMap() {
+  if (m_table != (Elm*)&null_key) {
+    free(m_table);
+  }
+}
+
 template <typename V, bool case_sensitive>
 void FixedStringMap<V, case_sensitive>::init(int num) {
+  if (!num) {
+    m_table = (Elm*)&null_key;
+    m_mask = 0;
+    return;
+  }
+
   static const double kLoadFactor = 0.80;
   int capac = 1;
   while (num >= kLoadFactor * capac) {

hphp/runtime/vm/fixed_string_map.h

@@ -27,7 +27,7 @@ template <typename V, bool case_sensitive> class FixedStringMap {
 public:
   explicit FixedStringMap(int num) : m_table(0) { init(num); }
   FixedStringMap() : m_mask(0), m_table(0) {}
-  ~FixedStringMap() { free(m_table); }
+  ~FixedStringMap();
 
   void init(int num);
   void add(const StringData* s, const V& v);

hphp/runtime/vm/indexed_string_map.h

@@ -56,6 +56,13 @@ struct IndexedStringMap {
   void create(const Builder& b) {
     assert(!m_size && "IndexedStringMap::create called more than once");
     m_map.init(b.size());
+    if (!b.size()) {
+      assert(!m_vec);
+      // note that we have to initialize m_map even though its zero
+      // sized (an empty FixedStringMap isn't quite empty - see
+      // FixedStringMap::init).
+      return;
+    }
     m_vec = new T[b.size()];
     m_size = b.size();