Optimize SharedVariant refcounting

Elide refcounting for non-refcounted TVs in SharedVariant.
Additionally for apc_store, if TV is a static string, use
the static version insted of the given one.

hphp/runtime/base/shared/shared_variant.cpp

@@ -48,6 +48,12 @@ SharedVariant::SharedVariant(CVarRef source, bool serialized,
       break;
     }
   case KindOfStaticString:
+    {
+      if (serialized) goto StringCase;
+      m_data.str = source.getStringData();
+      break;
+    }
+StringCase:
   case KindOfString:
     {
       String s = source.toString();
@@ -57,6 +63,12 @@ SharedVariant::SharedVariant(CVarRef source, bool serialized,
         // for unserialization.
         s = apc_reserialize(s);
       }
+      StringData* st = StringData::LookupStaticString(s.get());
+      if (st) {
+        m_data.str = st;
+        m_type = KindOfStaticString;
+        break;
+      }
       m_data.str = s->copy(true);
       break;
     }

hphp/runtime/base/shared/shared_variant.h

@@ -66,12 +66,18 @@ public:
   }
 
   void incRef() {
+    assert(IS_REFCOUNTED_TYPE(m_type));
     atomic_inc(m_count);
   }
 
   void decRef() {
     assert(m_count);
-    if (atomic_dec(m_count) == 0) {
+    if (IS_REFCOUNTED_TYPE(m_type)) {
+      if (atomic_dec(m_count) == 0) {
+        delete this;
+      }
+    } else {
+      assert(m_count == 1);
       delete this;
     }
   }

hphp/runtime/base/string_data.cpp

@@ -103,6 +103,15 @@ StringData *StringData::GetStaticString(const StringData *str) {
   return const_cast<StringData*>(pair.first->first);
 }
 
+StringData *StringData::LookupStaticString(const StringData *str) {
+  if (UNLIKELY(!s_stringDataMap)) return nullptr;
+  StringDataMap::const_iterator it = s_stringDataMap->find(str);
+  if (it != s_stringDataMap->end()) {
+    return const_cast<StringData*>(it->first);
+  }
+  return nullptr;
+}
+
 StringData* StringData::GetStaticString(const String& str) {
   assert(!str.isNull());
   return GetStaticString(str.get());

hphp/runtime/base/string_data.h

@@ -352,6 +352,10 @@ public:
   static StringData *GetStaticString(const String& str);
   static StringData *GetStaticString(const char* str);
   static StringData *GetStaticString(char c);
+
+  /* check if a static string exists that is the same as str
+   * and if so, return it. Else, return nullptr. */
+  static StringData *LookupStaticString(const StringData* str);
   static size_t GetStaticStringCount();
   static uint32_t GetCnsHandle(const StringData* cnsName);
   static uint32_t DefCnsHandle(const StringData* cnsName, bool persistent);