Initial commit of snmalloc

History squashed from internal development.

Internal history has commit hash:
  e27a0e485c44a5003a802de2661ce3b21e120316

src/ds/mpmcstack.h

@@ -0,0 +1,75 @@
+#pragma once
+
+#include "aba.h"
+
+namespace snmalloc
+{
+  template<class T, Construction c = RequiresInit>
+  class MPMCStack
+  {
+    using ABA = ABA<T, c>;
+
+  private:
+    static_assert(
+      std::is_same<decltype(((T*)0)->next), std::atomic<T*>>::value,
+      "T->next must be a std::atomic<T*>");
+
+    ABA stack;
+
+  public:
+    void push(T* item)
+    {
+      return push(item, item);
+    }
+
+    void push(T* first, T* last)
+    {
+      // Pushes an item on the stack.
+      auto cmp = stack.read();
+
+      do
+      {
+        T* top = ABA::load(cmp);
+        last->next.store(top, std::memory_order_release);
+      } while (!stack.compare_exchange(cmp, first));
+    }
+
+    T* pop()
+    {
+      // Returns the next item. If the returned value is decommitted, it is
+      // possible for the read of top->next to segfault.
+      auto cmp = stack.read();
+      T* top;
+      T* next;
+
+      do
+      {
+        top = ABA::load(cmp);
+
+        if (top == nullptr)
+          break;
+
+        next = top->next.load(std::memory_order_acquire);
+      } while (!stack.compare_exchange(cmp, next));
+
+      return top;
+    }
+
+    T* pop_all()
+    {
+      // Returns all items as a linked list, leaving an empty stack.
+      auto cmp = stack.read();
+      T* top;
+
+      do
+      {
+        top = ABA::load(cmp);
+
+        if (top == nullptr)
+          break;
+      } while (!stack.compare_exchange(cmp, nullptr));
+
+      return top;
+    }
+  };
+}