Properly align small_vector.

Summary:
Previously the alignment of small_vector did not take into account the alignment of the value_type. Since the value_type is stored directly internally, this is problematic.

This change does increase the sizeof small_vector in a couple cases, but I don't think the previous size was legitimate as sizeof(small_vector<value_type>) was not an even multiple of alignof(value_type).

Reviewed By: yfeldblum

Differential Revision: D7969483

fbshipit-source-id: 968c78bf03d7c0ddfe1338754f12f97be8d0f771

folly/small_vector.h

@@ -1118,13 +1118,9 @@ class small_vector : public detail::small_vector_base<
     }
   } FOLLY_SV_PACK_ATTR;
 
-#if (FOLLY_X64 || FOLLY_PPC64)
-  typedef unsigned char InlineStorageDataType[sizeof(value_type) * MaxInline];
-#else
   typedef typename std::aligned_storage<
       sizeof(value_type) * MaxInline,
       alignof(value_type)>::type InlineStorageDataType;
-#endif
 
   typedef typename std::conditional<
       sizeof(value_type) * MaxInline != 0,
@@ -1188,8 +1184,8 @@ class small_vector : public detail::small_vector_base<
       auto vp = detail::pointerFlagClear(pdata_.heap_);
       free(vp);
     }
-  } FOLLY_SV_PACK_ATTR u;
-} FOLLY_SV_PACK_ATTR;
+  } u;
+};
 FOLLY_SV_PACK_POP
 
 //////////////////////////////////////////////////////////////////////

folly/test/small_vector_test.cpp

@@ -48,12 +48,22 @@ static_assert(sizeof(small_vector<int,10>) ==
 static_assert(sizeof(small_vector<int32_t,1,uint32_t>) ==
                 8 + 4,
               "small_vector<int32_t,1,uint32_t> is wrong size");
-static_assert(sizeof(small_vector<int32_t,1,uint16_t>) ==
-                8 + 2,
+
+// Extra 2 bytes needed for alignment.
+static_assert(
+    sizeof(small_vector<int32_t, 1, uint16_t>) == 8 + 2 + 2,
     "small_vector<int32_t,1,uint32_t> is wrong size");
-static_assert(sizeof(small_vector<int32_t,1,uint8_t>) ==
-                8 + 1,
+static_assert(
+    alignof(small_vector<int32_t, 1, uint16_t>) >= 4,
+    "small_vector not aligned correctly");
+
+// Extra 3 bytes needed for alignment.
+static_assert(
+    sizeof(small_vector<int32_t, 1, uint8_t>) == 8 + 1 + 3,
     "small_vector<int32_t,1,uint32_t> is wrong size");
+static_assert(
+    alignof(small_vector<int32_t, 1, uint8_t>) >= 4,
+    "small_vector not aligned correctly");
 
 static_assert(sizeof(small_vector<int16_t,4,uint16_t>) == 10,
               "Sizeof unexpectedly large");
@@ -63,6 +73,10 @@ static_assert(sizeof(small_vector<int16_t,4,uint16_t>) == 10,
 static_assert(!FOLLY_IS_TRIVIALLY_COPYABLE(std::unique_ptr<int>),
               "std::unique_ptr<> is trivially copyable");
 
+static_assert(
+    alignof(small_vector<std::aligned_storage<32, 32>::type, 4>) == 32,
+    "small_vector not aligned correctly");
+
 namespace {
 
 template <typename Key, typename Value, size_t N>