diff --git a/libcxx/docs/ReleaseNotes.rst b/libcxx/docs/ReleaseNotes.rst
--- a/libcxx/docs/ReleaseNotes.rst
+++ b/libcxx/docs/ReleaseNotes.rst
@@ -85,6 +85,14 @@
 
 ABI Affecting Changes
 ---------------------
+- In freestanding mode, ``atomic<small enum class>`` does not contain a lock byte anymore if the platform
+  can implement lockfree atomics for that size. More specifically, in LLVM <= 11.0.1, an ``atomic<small enum class>``
+  would not contain a lock byte. This was broken in LLVM >= 12.0.0, where it started including a lock byte despite
+  the platform supporting lockfree atomics for that size. Starting in LLVM 15.0.1, the ABI for these types has been
+  restored to what it used to be (no lock byte), which is the most efficient implementation.
+
+  This ABI break only affects users that compile with ``-ffreestanding``, and only for ``atomic<T>`` where ``T``
+  is a non-builtin type that could be lockfree on the platform. See https://llvm.org/D133377 for more details.
 
 Build System Changes
 --------------------
diff --git a/libcxx/include/atomic b/libcxx/include/atomic
--- a/libcxx/include/atomic
+++ b/libcxx/include/atomic
@@ -1109,6 +1109,12 @@
 # define ATOMIC_POINTER_LOCK_FREE   __GCC_ATOMIC_POINTER_LOCK_FREE
 #endif
 
+template <class _Tp>
+struct __libcpp_is_always_lock_free {
+  // __atomic_always_lock_free is available in all Standard modes
+  static const bool __value = __atomic_always_lock_free(sizeof(_Tp), 0);
+};
+
 #ifdef _LIBCPP_ATOMIC_ONLY_USE_BUILTINS
 
 template<typename _Tp>
@@ -1400,42 +1406,8 @@
   return __old;
 }
 
-#ifdef __cpp_lib_atomic_is_always_lock_free
-
-template<typename _Tp> struct __cxx_is_always_lock_free {
-    enum { __value = __atomic_always_lock_free(sizeof(_Tp), 0) }; };
-
-#else
-
-template<typename _Tp> struct __cxx_is_always_lock_free { enum { __value = false }; };
-// Implementations must match the C ATOMIC_*_LOCK_FREE macro values.
-template<> struct __cxx_is_always_lock_free<bool> { enum { __value = 2 == ATOMIC_BOOL_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<char> { enum { __value = 2 == ATOMIC_CHAR_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<signed char> { enum { __value = 2 == ATOMIC_CHAR_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<unsigned char> { enum { __value = 2 == ATOMIC_CHAR_LOCK_FREE }; };
-#ifndef _LIBCPP_HAS_NO_CHAR8_T
-template<> struct __cxx_is_always_lock_free<char8_t> { enum { __value = 2 == ATOMIC_CHAR8_T_LOCK_FREE }; };
-#endif
-template<> struct __cxx_is_always_lock_free<char16_t> { enum { __value = 2 == ATOMIC_CHAR16_T_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<char32_t> { enum { __value = 2 == ATOMIC_CHAR32_T_LOCK_FREE }; };
-#ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
-template<> struct __cxx_is_always_lock_free<wchar_t> { enum { __value = 2 == ATOMIC_WCHAR_T_LOCK_FREE }; };
-#endif
-template<> struct __cxx_is_always_lock_free<short> { enum { __value = 2 == ATOMIC_SHORT_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<unsigned short> { enum { __value = 2 == ATOMIC_SHORT_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<int> { enum { __value = 2 == ATOMIC_INT_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<unsigned int> { enum { __value = 2 == ATOMIC_INT_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<long> { enum { __value = 2 == ATOMIC_LONG_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<unsigned long> { enum { __value = 2 == ATOMIC_LONG_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<long long> { enum { __value = 2 == ATOMIC_LLONG_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<unsigned long long> { enum { __value = 2 == ATOMIC_LLONG_LOCK_FREE }; };
-template<typename _Tp> struct __cxx_is_always_lock_free<_Tp*> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
-template<> struct __cxx_is_always_lock_free<std::nullptr_t> { enum { __value = 2 == ATOMIC_POINTER_LOCK_FREE }; };
-
-#endif //__cpp_lib_atomic_is_always_lock_free
-
 template <typename _Tp,
-          typename _Base = typename conditional<__cxx_is_always_lock_free<_Tp>::__value,
+          typename _Base = typename conditional<__libcpp_is_always_lock_free<_Tp>::__value,
                                                 __cxx_atomic_base_impl<_Tp>,
                                                 __cxx_atomic_lock_impl<_Tp> >::type>
 #else
@@ -1542,7 +1514,7 @@
     mutable __cxx_atomic_impl<_Tp> __a_;
 
 #if defined(__cpp_lib_atomic_is_always_lock_free)
-  static _LIBCPP_CONSTEXPR bool is_always_lock_free = __atomic_always_lock_free(sizeof(__a_), 0);
+  static _LIBCPP_CONSTEXPR bool is_always_lock_free = __libcpp_is_always_lock_free<__cxx_atomic_impl<_Tp> >::__value;
 #endif
 
     _LIBCPP_INLINE_VISIBILITY
@@ -2645,7 +2617,7 @@
 // atomic_*_lock_free : prefer the contention type most highly, then the largest lock-free type
 
 #ifdef __cpp_lib_atomic_is_always_lock_free
-# define _LIBCPP_CONTENTION_LOCK_FREE __atomic_always_lock_free(sizeof(__cxx_contention_t), 0)
+# define _LIBCPP_CONTENTION_LOCK_FREE ::std::__libcpp_is_always_lock_free<__cxx_contention_t>::__value
 #else
 # define _LIBCPP_CONTENTION_LOCK_FREE false
 #endif
diff --git a/libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp b/libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp
--- a/libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp
+++ b/libcxx/test/libcxx/atomics/atomics.align/align.pass.cpp
@@ -105,6 +105,7 @@
   CHECK_ALIGNMENT(struct LLIArr16 { long long int i[16]; });
   CHECK_ALIGNMENT(struct Padding { char c; /* padding */ long long int i; });
   CHECK_ALIGNMENT(union IntFloat { int i; float f; });
+  CHECK_ALIGNMENT(enum class StrongEnum { foo });
 
   return 0;
 }
diff --git a/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp b/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
--- a/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
+++ b/libcxx/test/std/atomics/atomics.lockfree/isalwayslockfree.pass.cpp
@@ -17,13 +17,12 @@
 
 #include "test_macros.h"
 
-#if !defined(__cpp_lib_atomic_is_always_lock_free)
-# error Feature test macro missing.
-#endif
-
-template <typename T> void checkAlwaysLockFree() {
-  if (std::atomic<T>::is_always_lock_free)
+template <typename T>
+void checkAlwaysLockFree() {
+  if (std::atomic<T>::is_always_lock_free) {
+    LIBCPP_ASSERT(sizeof(std::atomic<T>) == sizeof(T)); // technically not required, but libc++ does it that way
     assert(std::atomic<T>().is_lock_free());
+  }
 }
 
 void run()
@@ -85,10 +84,13 @@
     CHECK_ALWAYS_LOCK_FREE(struct LLIArr16 { long long int i[16]; });
     CHECK_ALWAYS_LOCK_FREE(struct Padding { char c; /* padding */ long long int i; });
     CHECK_ALWAYS_LOCK_FREE(union IntFloat { int i; float f; });
+    CHECK_ALWAYS_LOCK_FREE(enum class CharEnumClass : char { foo });
 
     // C macro and static constexpr must be consistent.
+    enum class CharEnumClass : char { foo };
     static_assert(std::atomic<bool>::is_always_lock_free == (2 == ATOMIC_BOOL_LOCK_FREE), "");
     static_assert(std::atomic<char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
+    static_assert(std::atomic<CharEnumClass>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
     static_assert(std::atomic<signed char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
     static_assert(std::atomic<unsigned char>::is_always_lock_free == (2 == ATOMIC_CHAR_LOCK_FREE), "");
 #if TEST_STD_VER > 17 && defined(__cpp_char8_t)