[libc++] Increase atomic_ref's required alignment for small types (#99654)

This patch increases the alignment requirement for std::atomic_ref
such that we can guarantee lockfree operations more often. Specifically,
we require types that are 1, 2, 4, 8, or 16 bytes in size to be aligned
to at least their size to be used with std::atomic_ref.

This is the case for most types, however a notable exception is
`long long` on x86, which is 8 bytes in length but has an alignment
of 4.

As a result of this patch, one has to be more careful about the
alignment of objects used with std::atomic_ref. Failure to provide
a properly-aligned object to std::atomic_ref is a precondition
violation and is technically UB. On the flipside, this allows us
to provide an atomic_ref that is actually lockfree more often,
which is an important QOI property.

More information in the discussion at #99570 (comment).

Co-authored-by: Louis Dionne <[email protected]>
(cherry picked from commit 59ca618)

Author: 2 people

libcxx/include/__atomic/atomic_ref.h

@@ -57,11 +57,6 @@ struct __get_aligner_instance {
 
 template <class _Tp>
 struct __atomic_ref_base {
-protected:
-  _Tp* __ptr_;
-
-  _LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}
-
 private:
   _LIBCPP_HIDE_FROM_ABI static _Tp* __clear_padding(_Tp& __val) noexcept {
     _Tp* __ptr = std::addressof(__val);
@@ -108,10 +103,14 @@ struct __atomic_ref_base {
 
   friend struct __atomic_waitable_traits<__atomic_ref_base<_Tp>>;
 
+  // require types that are 1, 2, 4, 8, or 16 bytes in length to be aligned to at least their size to be potentially
+  // used lock-free
+  static constexpr size_t __min_alignment = (sizeof(_Tp) & (sizeof(_Tp) - 1)) || (sizeof(_Tp) > 16) ? 0 : sizeof(_Tp);
+
 public:
   using value_type = _Tp;
 
-  static constexpr size_t required_alignment = alignof(_Tp);
+  static constexpr size_t required_alignment = alignof(_Tp) > __min_alignment ? alignof(_Tp) : __min_alignment;
 
   // The __atomic_always_lock_free builtin takes into account the alignment of the pointer if provided,
   // so we create a fake pointer with a suitable alignment when querying it. Note that we are guaranteed
@@ -218,6 +217,12 @@ struct __atomic_ref_base {
   }
   _LIBCPP_HIDE_FROM_ABI void notify_one() const noexcept { std::__atomic_notify_one(*this); }
   _LIBCPP_HIDE_FROM_ABI void notify_all() const noexcept { std::__atomic_notify_all(*this); }
+
+protected:
+  typedef _Tp _Aligned_Tp __attribute__((aligned(required_alignment)));
+  _Aligned_Tp* __ptr_;
+
+  _LIBCPP_HIDE_FROM_ABI __atomic_ref_base(_Tp& __obj) : __ptr_(std::addressof(__obj)) {}
 };
 
 template <class _Tp>

libcxx/test/std/atomics/atomics.ref/is_always_lock_free.pass.cpp

@@ -54,7 +54,7 @@ void check_always_lock_free(std::atomic_ref<T> const& a) {
 #define CHECK_ALWAYS_LOCK_FREE(T)                                                                                      \
   do {                                                                                                                 \
     typedef T type;                                                                                                    \
-    type obj{};                                                                                                        \
+    alignas(std::atomic_ref<type>::required_alignment) type obj{};                                                     \
     std::atomic_ref<type> a(obj);                                                                                      \
     check_always_lock_free(a);                                                                                         \
   } while (0)