Treadmill for shared variants

Avoids atomic incRefs and decRefs. StringData no longer needs to
be sweepable for SharedVariants.

hphp/runtime/base/string_data.cpp

@@ -212,41 +212,6 @@ void StringData::initLiteral(const char* data, int len) {
   assert(checkSane());
 }
 
-void StringData::enlist() {
-  assert(isShared());
-  SweepNode& head = MemoryManager::TheMemoryManager()->m_strings;
-  // insert after head
-  SweepNode* next = head.next;
-  assert(uintptr_t(next) != kMallocFreeWord);
-  m_big.node.next = next;
-  m_big.node.prev = &head;
-  next->prev = head.next = &m_big.node;
-}
-
-void StringData::delist() {
-  assert(isShared());
-  SweepNode* next = m_big.node.next;
-  SweepNode* prev = m_big.node.prev;
-  assert(uintptr_t(next) != kMallocFreeWord);
-  assert(uintptr_t(prev) != kMallocFreeWord);
-  next->prev = prev;
-  prev->next = next;
-}
-
-void StringData::sweepAll() {
-  SweepNode& head = MemoryManager::TheMemoryManager()->m_strings;
-  for (SweepNode *next, *n = head.next; n != &head; n = next) {
-    next = n->next;
-    assert(next && uintptr_t(next) != kSmartFreeWord);
-    assert(next && uintptr_t(next) != kMallocFreeWord);
-    StringData* s = (StringData*)(uintptr_t(n) -
-                                  offsetof(StringData, m_big.node));
-    assert(s->isShared());
-    s->m_big.shared->decRef();
-  }
-  head.next = head.prev = &head;
-}
-
 HOT_FUNC
 void StringData::initAttach(const char* data) {
   return initAttach(data, strlen(data));
@@ -335,13 +300,11 @@ HOT_FUNC
 StringData::StringData(SharedVariant *shared)
   : _count(0) {
   assert(shared && size_t(shared->stringLength()) <= size_t(MaxSize));
-  shared->incRef();
   m_hash = 0;
   m_len = shared->stringLength();
   m_cdata = shared->stringData();
   m_big.shared = shared;
   m_big.cap = m_len | IsShared;
-  enlist();
 }
 
 HOT_FUNC
@@ -353,8 +316,6 @@ void StringData::releaseData() {
     break;
   case IsShared:
     assert(checkSane());
-    m_big.shared->decRef();
-    delist();
     break;
   case IsSmart:
     assert(checkSane());
@@ -434,10 +395,6 @@ void StringData::append(const char *s, int len) {
     // buffer is immutable, don't modify it.
     StringSlice r = slice();
     char* newdata = smart_concat(r.ptr, r.len, s, len);
-    if (isShared()) {
-      m_big.shared->decRef();
-      delist();
-    }
     m_len = newlen;
     m_data = newdata;
     m_big.cap = newlen | IsSmart;