Use C++11 alignment primitives

base/memory/singleton_unittest.cc

Index: base/memory/singleton_unittest.cc
diff --git a/base/memory/singleton_unittest.cc b/base/memory/singleton_unittest.cc
index a15145c8747145cb3f6e6774890307cc0fa46794..e1292151fab3659bf865b4aa42eea3c936f6d79a 100644
--- a/base/memory/singleton_unittest.cc
+++ b/base/memory/singleton_unittest.cc
@@ -16,6 +16,14 @@ static_assert(DefaultSingletonTraits<int>::kRegisterAtExit == true,
 
 typedef void (*CallbackFunc)();
 
+template <size_t alignment>
+class AlignedData {
+ public:
+  AlignedData() {}
+  ~AlignedData() {}
+  alignas(alignment) char data_[alignment];
+};
+
 class IntSingleton {
  public:
   static IntSingleton* GetInstance() {
@@ -269,19 +277,17 @@ TEST_F(SingletonTest, Basic) {
     EXPECT_EQ(0u, reinterpret_cast<uintptr_t>(ptr) & (align - 1))
 
 TEST_F(SingletonTest, Alignment) {
-  using base::AlignedMemory;
-
   // Create some static singletons with increasing sizes and alignment
   // requirements. By ordering this way, the linker will need to do some work to
   // ensure proper alignment of the static data.
   AlignedTestSingleton<int32_t>* align4 =
       AlignedTestSingleton<int32_t>::GetInstance();
-  AlignedTestSingleton<AlignedMemory<32, 32> >* align32 =
-      AlignedTestSingleton<AlignedMemory<32, 32> >::GetInstance();
-  AlignedTestSingleton<AlignedMemory<128, 128> >* align128 =
-      AlignedTestSingleton<AlignedMemory<128, 128> >::GetInstance();
-  AlignedTestSingleton<AlignedMemory<4096, 4096> >* align4096 =
-      AlignedTestSingleton<AlignedMemory<4096, 4096> >::GetInstance();
+  AlignedTestSingleton<AlignedData<32>>* align32 =
+      AlignedTestSingleton<AlignedData<32>>::GetInstance();
+  AlignedTestSingleton<AlignedData<128>>* align128 =
+      AlignedTestSingleton<AlignedData<128>>::GetInstance();
+  AlignedTestSingleton<AlignedData<4096>>* align4096 =
+      AlignedTestSingleton<AlignedData<4096>>::GetInstance();
 
   EXPECT_ALIGNED(align4, 4);

[danakj, 2017/06/12 22:22:05]

Is this legit? I realize AlignedData has only one member, but couldn't that
member be aligned differently than the AlignedData struct itself, with some
padding? Or is that not possible.

[brettw, 2017/06/12 22:28:50]

I wondered this also. I had even changed it at one point to reference the data
inside here. But even if there was padding the outer structure would always have
to be aligned at the same interval as the inner one to guarantee that the inner
one is aligned properly. And I don't think it makes sense to have a struct
aligned at "8-byte offsets starting at 4" (if this makes sense). So I think the
only reasonable thing is for the compilers to align the outer struct the same.

   EXPECT_ALIGNED(align32, 32);