Issue #2166 - Part 2: Use alignas/alignof in MaybeOneOf

Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1338374

1 files changed, 27 insertions, 8 deletions

diff --git a/mfbt/MaybeOneOf.h b/mfbt/MaybeOneOf.h
index ea8124772d..e4f298ec51 100644
--- a/mfbt/MaybeOneOf.h
+++ b/mfbt/MaybeOneOf.h
@@ -3,15 +3,20 @@
  * 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 storing one of two optional value types that supports
+ * in-place lazy construction.
+ */
+
 #ifndef mozilla_MaybeOneOf_h
 #define mozilla_MaybeOneOf_h
 
-#include "mozilla/Alignment.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/Move.h"
 #include "mozilla/TemplateLib.h"
 
-#include <new>    // For placement new
+#include <new>      // for placement new
+#include <stddef.h> // for size_t
 
 namespace mozilla {
 
@@ -23,11 +28,25 @@ namespace mozilla {
  * |construct<T2>()|, a T1 or T2 object will be constructed with the given
  * arguments and that object will be destroyed when the owning MaybeOneOf is
  * destroyed.
+ *
+ * Because MaybeOneOf must be aligned suitable to hold any value stored within
+ * it, and because |alignas| requirements don't affect platform ABI with respect
+ * to how parameters are laid out in memory, MaybeOneOf can't be used as the
+ * type of a function parameter.  Pass MaybeOneOf to functions by pointer or
+ * reference instead.
  */
 template<class T1, class T2>
-class MaybeOneOf
+class MOZ_NON_PARAM MaybeOneOf
 {
-  AlignedStorage<tl::Max<sizeof(T1), sizeof(T2)>::value> storage;
+  static constexpr size_t StorageAlignment =
+    tl::Max<alignof(T1), alignof(T2)>::value;
+  static constexpr size_t StorageSize =
+    tl::Max<sizeof(T1), sizeof(T2)>::value;
+
+  alignas(StorageAlignment) unsigned char storage_[StorageSize];
+
+  void* storage() { return storage_; }
+  const void* storage() const { return storage_; }
 
   enum State { None, SomeT1, SomeT2 } state;
   template <class T, class Ignored = void> struct Type2State {};
@@ -36,14 +55,14 @@ class MaybeOneOf
   T& as()
   {
     MOZ_ASSERT(state == Type2State<T>::result);
-    return *(T*)storage.addr();
+    return *static_cast<T*>(storage());
   }
 
   template <class T>
   const T& as() const
   {
     MOZ_ASSERT(state == Type2State<T>::result);
-    return *(T*)storage.addr();
+    return *static_cast<const T*>(storage());
   }
 
 public:
@@ -65,7 +84,7 @@ public:
     }
   }
 
-  MaybeOneOf &operator=(MaybeOneOf&& rhs)
+  MaybeOneOf& operator=(MaybeOneOf&& rhs)
   {
     MOZ_ASSERT(this != &rhs, "Self-move is prohibited");
     this->~MaybeOneOf();
@@ -83,7 +102,7 @@ public:
   {
     MOZ_ASSERT(state == None);
     state = Type2State<T>::result;
-    ::new (storage.addr()) T(Forward<Args>(aArgs)...);
+    ::new (storage()) T(Forward<Args>(aArgs)...);
   }
 
   template <class T>