[libcxx-commits] [PATCH] D61301: Support overaligned types in `aligned_storage`.

[Eric Fiselier via Phabricator via libcxx-commits]

EricWF created this revision.
EricWF added reviewers: rsmith, ldionne.
Herald added a subscriber: dexonsmith.
EricWF added a reviewer: dlj.

The current implementation of aligned storage was written before we had `alignas`, so it used a list of builtin types to force the alignment. But this doesn't work overaligned requests.

This patch adds a fallback case supporting over-alignment. It only affects case that were previously ill-formed.


Repository:
  rCXX libc++

https://reviews.llvm.org/D61301

Files:
  include/type_traits
  test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp


Index: test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
===================================================================
--- test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
+++ test/std/utilities/meta/meta.trans/meta.trans.other/aligned_storage.pass.cpp
@@ -285,6 +285,14 @@
     static_assert(std::alignment_of<T1>::value == 8, "");
     static_assert(sizeof(T1) == 16, "");
     }
+  {
+    const int Align = 65536;
+    typedef typename std::aligned_storage<1, Align>::type T1;
+    static_assert(std::is_trivial<T1>::value, "");
+    static_assert(std::is_standard_layout<T1>::value, "");
+    static_assert(std::alignment_of<T1>::value == Align, "");
+    static_assert(sizeof(T1) == Align, "");
+  }
 
   return 0;
 }
Index: include/type_traits
===================================================================
--- include/type_traits
+++ include/type_traits
@@ -1768,6 +1768,9 @@
     __nat
     > > > > > > > > > > __all_types;
 
+template <size_t _Align>
+struct _ALIGNAS(_Align) __fallback_overaligned {};
+
 template <class _TL, size_t _Align> struct __find_pod;
 
 template <class _Hp, size_t _Align>
@@ -1776,7 +1779,7 @@
     typedef typename conditional<
                              _Align == _Hp::value,
                              typename _Hp::type,
-                             void
+                             __fallback_overaligned<_Align>
                          >::type type;
 };
 
@@ -1813,7 +1816,6 @@
 struct _LIBCPP_TEMPLATE_VIS aligned_storage
 {
     typedef typename __find_pod<__all_types, _Align>::type _Aligner;
-    static_assert(!is_void<_Aligner>::value, "");
     union type
     {
         _Aligner __align;


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