Merge pull request #65 from microsoft/queue_of_slabs

Use a queue of slabs for free lists

src/ds/dllist.h

@@ -63,6 +63,7 @@ namespace snmalloc
       std::is_same<decltype(T::next), T*>::value, "T->next must be a T*");
 
     T* head = Terminator();
+    T* tail = Terminator();
 
   public:
     bool is_empty()
@@ -96,13 +97,35 @@ namespace snmalloc
 
       if (head != Terminator())
         head->prev = item;
+      else
+        tail = item;
 
       head = item;
 #ifndef NDEBUG
       debug_check();
 #endif
     }
 
+    void insert_back(T* item)
+    {
+#ifndef NDEBUG
+      debug_check_not_contains(item);
+#endif
+
+      item->prev = tail;
+      item->next = Terminator();
+
+      if (tail != Terminator())
+        tail->next = item;
+      else
+        head = item;
+
+      tail = item;
+#ifndef NDEBUG
+      debug_check();
+#endif
+    }
+
     void remove(T* item)
     {
 #ifndef NDEBUG
@@ -111,6 +134,8 @@ namespace snmalloc
 
       if (item->next != Terminator())
         item->next->prev = item->prev;
+      else
+        tail = item->prev;
 
       if (item->prev != Terminator())
         item->prev->next = item->next;

src/mem/alloc.h

@@ -1114,7 +1114,7 @@ namespace snmalloc
         if ((allow_reserve == NoReserve) && (slab == nullptr))
           return nullptr;
 
-        sl.insert(slab->get_link());
+        sl.insert_back(slab->get_link());
       }
       auto& ffl = small_fast_free_lists[sizeclass];
       return slab->alloc<zero_mem>(

src/mem/slab.h

@@ -215,7 +215,7 @@ namespace snmalloc
         meta.link = index;
 
         // Push on the list of slabs for this sizeclass.
-        sl->insert(meta.get_link(this));
+        sl->insert_back(meta.get_link(this));
         meta.debug_slab_invariant(is_short(), this);
         return Superslab::NoSlabReturn;
       }