diff -r 000000000000 -r 6474c204b198 mfbt/Maybe.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mfbt/Maybe.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,163 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A class for lazily constructing an object without sticking it on the heap. */
+
+#ifndef mozilla_Maybe_h
+#define mozilla_Maybe_h
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Assertions.h"
+
+// For placement new
+#include <new>
+
+namespace mozilla {
+
+/*
+ * Small utility for lazily constructing objects without using dynamic storage.
+ * When a Maybe<T> is constructed, it is |empty()|, i.e., no value of T has
+ * been constructed and no T destructor will be called when the Maybe<T> is
+ * destroyed. Upon calling |construct|, a T object will be constructed with the
+ * given arguments and that object will be destroyed when the owning Maybe<T>
+ * is destroyed.
+ *
+ * N.B. GCC seems to miss some optimizations with Maybe and may generate extra
+ * branches/loads/stores. Use with caution on hot paths.
+ */
+template<class T>
+class Maybe
+{
+    AlignedStorage2<T> storage;
+    bool constructed;
+
+    T& asT() { return *storage.addr(); }
+
+  public:
+    Maybe() { constructed = false; }
+    ~Maybe() { if (constructed) asT().~T(); }
+
+    bool empty() const { return !constructed; }
+
+    void construct() {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T();
+      constructed = true;
+    }
+
+    template<class T1>
+    void construct(const T1& t1) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1);
+      constructed = true;
+    }
+
+    template<class T1, class T2>
+    void construct(const T1& t1, const T2& t2) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3>
+    void construct(const T1& t1, const T2& t2, const T3& t3) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5,
+             class T6>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
+                   const T6& t6) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5,
+             class T6, class T7>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
+                   const T6& t6, const T7& t7) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5,
+             class T6, class T7, class T8>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
+                   const T6& t6, const T7& t7, const T8& t8) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5,
+             class T6, class T7, class T8, class T9>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
+                   const T6& t6, const T7& t7, const T8& t8, const T9& t9) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9);
+      constructed = true;
+    }
+
+    template<class T1, class T2, class T3, class T4, class T5,
+             class T6, class T7, class T8, class T9, class T10>
+    void construct(const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5,
+                   const T6& t6, const T7& t7, const T8& t8, const T9& t9, const T10& t10) {
+      MOZ_ASSERT(!constructed);
+      ::new (storage.addr()) T(t1, t2, t3, t4, t5, t6, t7, t8, t9, t10);
+      constructed = true;
+    }
+
+    T* addr() {
+      MOZ_ASSERT(constructed);
+      return &asT();
+    }
+
+    T& ref() {
+      MOZ_ASSERT(constructed);
+      return asT();
+    }
+
+    const T& ref() const {
+      MOZ_ASSERT(constructed);
+      return const_cast<Maybe*>(this)->asT();
+    }
+
+    void destroy() {
+      ref().~T();
+      constructed = false;
+    }
+
+    void destroyIfConstructed() {
+      if (!empty())
+        destroy();
+    }
+
+  private:
+    Maybe(const Maybe& other) MOZ_DELETE;
+    const Maybe& operator=(const Maybe& other) MOZ_DELETE;
+};
+
+} // namespace mozilla
+
+#endif /* mozilla_Maybe_h */