Remove the SharedVariant pointer from StringData.

We only use it to lazily grab the hashcode.  So just grab the hashcode
eagerly when we create the string.

hphp/runtime/base/string_data.cpp

@@ -300,10 +300,9 @@ HOT_FUNC
 StringData::StringData(SharedVariant *shared)
   : _count(0) {
   assert(shared && size_t(shared->stringLength()) <= size_t(MaxSize));
-  m_hash = 0;
+  m_hash = shared->stringHash();
   m_len = shared->stringLength();
   m_cdata = shared->stringData();
-  m_big.shared = shared;
   m_big.cap = m_len | IsShared;
 }
 
@@ -914,8 +913,8 @@ int StringData::compare(const StringData *v2) const {
 
 HOT_FUNC
 strhash_t StringData::hashHelper() const {
-  strhash_t h = isShared() ? m_big.shared->stringHash() :
-                             hash_string_inline(m_data, m_len);
+  assert(!isShared());
+  strhash_t h = hash_string_inline(m_data, m_len);
   assert(h >= 0);
   m_hash |= h;
   return h;

hphp/runtime/base/string_data.h

@@ -87,7 +87,7 @@ enum CopyMallocMode { CopyMalloc };
  * small: m_data:8, _count:4, m_len:4, m_hash:4,
  *        m_small[44]
  * big:   m_data:8, _count:4, m_len:4, m_hash:4,
- *        junk[12], node:16, shared:8, cap:8
+ *        junk[36], cap:8
  *
  * If the format is IsLiteral or IsShared, we always use the "big" layout.
  * resemblences to fbstring are not accidental.
@@ -137,7 +137,6 @@ class StringData {
   void destruct() const { if (!isStatic()) delete this; }
 
   StringData() : m_data(m_small), _count(0), m_len(0), m_hash(0) {
-    m_big.shared = 0;
     m_big.cap = IsSmall;
     m_small[0] = 0;
   }
@@ -301,11 +300,6 @@ public:
   double toDouble () const;
   DataType toNumeric(int64_t &lval, double &dval) const;
 
-  strhash_t getPrecomputedHash() const {
-    assert(!isShared());
-    return m_hash & STRHASH_MASK;
-  }
-
   strhash_t hash() const {
     strhash_t h = m_hash & STRHASH_MASK;
     return h ? h : hashHelper();
@@ -375,10 +369,8 @@ public:
     struct __attribute__((__packed__)) {
       // Calculate padding so that node, shared, and cap are pointer aligned,
       // and ensure cap overlaps the last byte of m_small.
-      static const size_t kPadding = sizeof(m_small) -
-        sizeof(SharedVariant*) - sizeof(uint64_t);
+      static const size_t kPadding = sizeof(m_small) - sizeof(uint64_t);
       char junk[kPadding];
-      SharedVariant *shared;
       uint64_t cap;
     } m_big;
   };