1 files changed, 154 insertions, 0 deletions
diff --git a/mfbt/Alignment.h b/mfbt/Alignment.h
new file mode 100644
index 0000000000..4fcda4f3e7
--- /dev/null
+++ b/mfbt/Alignment.h
@@ -0,0 +1,154 @@
+/* -*- 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/. */
+
+/* Functionality related to memory alignment. */
+
+#ifndef mozilla_Alignment_h
+#define mozilla_Alignment_h
+
+#include "mozilla/Attributes.h"
+#include <stddef.h>
+#include <stdint.h>
+
+namespace mozilla {
+
+/*
+ * This class, and the corresponding macro MOZ_ALIGNOF, figures out how many
+ * bytes of alignment a given type needs.
+ */
+template<typename T>
+class AlignmentFinder
+{
+  struct Aligner
+  {
+    char mChar;
+    T mT;
+  };
+
+public:
+  static const size_t alignment = sizeof(Aligner) - sizeof(T);
+};
+
+#define MOZ_ALIGNOF(T) mozilla::AlignmentFinder<T>::alignment
+
+/*
+ * Declare the MOZ_ALIGNED_DECL macro for declaring aligned types.
+ *
+ * For instance,
+ *
+ *   MOZ_ALIGNED_DECL(char arr[2], 8);
+ *
+ * will declare a two-character array |arr| aligned to 8 bytes.
+ */
+
+#if defined(__GNUC__)
+#  define MOZ_ALIGNED_DECL(_type, _align) \
+     _type __attribute__((aligned(_align)))
+#elif defined(_MSC_VER)
+#  define MOZ_ALIGNED_DECL(_type, _align) \
+     __declspec(align(_align)) _type
+#else
+#  warning "We don't know how to align variables on this compiler."
+#  define MOZ_ALIGNED_DECL(_type, _align) _type
+#endif
+
+/*
+ * AlignedElem<N> is a structure whose alignment is guaranteed to be at least N
+ * bytes.
+ *
+ * We support 1, 2, 4, 8, and 16-bit alignment.
+ */
+template<size_t Align>
+struct AlignedElem;
+
+/*
+ * We have to specialize this template because GCC doesn't like
+ * __attribute__((aligned(foo))) where foo is a template parameter.
+ */
+
+template<>
+struct AlignedElem<1>
+{
+  MOZ_ALIGNED_DECL(uint8_t elem, 1);
+};
+
+template<>
+struct AlignedElem<2>
+{
+  MOZ_ALIGNED_DECL(uint8_t elem, 2);
+};
+
+template<>
+struct AlignedElem<4>
+{
+  MOZ_ALIGNED_DECL(uint8_t elem, 4);
+};
+
+template<>
+struct AlignedElem<8>
+{
+  MOZ_ALIGNED_DECL(uint8_t elem, 8);
+};
+
+template<>
+struct AlignedElem<16>
+{
+  MOZ_ALIGNED_DECL(uint8_t elem, 16);
+};
+
+/*
+ * This utility pales in comparison to Boost's aligned_storage. The utility
+ * simply assumes that uint64_t is enough alignment for anyone. This may need
+ * to be extended one day...
+ *
+ * As an important side effect, pulling the storage into this template is
+ * enough obfuscation to confuse gcc's strict-aliasing analysis into not giving
+ * false negatives when we cast from the char buffer to whatever type we've
+ * constructed using the bytes.
+ */
+template<size_t Nbytes>
+struct AlignedStorage
+{
+  union U
+  {
+    char mBytes[Nbytes];
+    uint64_t mDummy;
+  } u;
+
+  const void* addr() const { return u.mBytes; }
+  void* addr() { return u.mBytes; }
+
+  AlignedStorage() = default;
+
+  // AlignedStorage is non-copyable: the default copy constructor violates
+  // strict aliasing rules, per bug 1269319.
+  AlignedStorage(const AlignedStorage&) = delete;
+  void operator=(const AlignedStorage&) = delete;
+};
+
+template<typename T>
+struct MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS AlignedStorage2
+{
+  union U
+  {
+    char mBytes[sizeof(T)];
+    uint64_t mDummy;
+  } u;
+
+  const T* addr() const { return reinterpret_cast<const T*>(u.mBytes); }
+  T* addr() { return static_cast<T*>(static_cast<void*>(u.mBytes)); }
+
+  AlignedStorage2() = default;
+
+  // AlignedStorage2 is non-copyable: the default copy constructor violates
+  // strict aliasing rules, per bug 1269319.
+  AlignedStorage2(const AlignedStorage2&) = delete;
+  void operator=(const AlignedStorage2&) = delete;
+};
+
+} /* namespace mozilla */
+
+#endif /* mozilla_Alignment_h */