[libcxx-commits] [PATCH] D133377: [libc++] Always query the compiler to find whether a type is always lockfree

[Louis Dionne via Phabricator via libcxx-commits]

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In https://llvm.org/D56913, we added an emulation for the __atomic_always_lock_free
compiler builtin when compiling in Freestanding mode. However, the emulation
did (and could not) give exactly the same answer as the compiler builtin,
which led to a potential ABI break for e.g. enum classes.

After speaking to the original author of D56913 <https://reviews.llvm.org/D56913>, we agree that the correct
behavior is to instead always use the compiler builtin, since that provides
a more accurate answer, and __atomic_always_lock_free is a purely front-end
builtin which doesn't require any runtime support. Furthermore, it is
available regardless of the Standard mode (see https://godbolt.org/z/cazf3ssYY).

However, this patch does constitute an ABI break. As shown by https://godbolt.org/z/1eoex6zdK:

- In LLVM <= 11.0.1, an atomic<enum class with 1 byte> would not contain a lock byte.
- In LLVM >= 12.0.0, an atomic<enum class with 1 byte> would contain a lock byte.

This patch breaks the ABI again to bring it back to 1 byte, which seems
like the correct thing to do.

Fixes #57440


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D133377

Files:
  libcxx/include/atomic


Index: libcxx/include/atomic
===================================================================
--- libcxx/include/atomic
+++ libcxx/include/atomic
@@ -1400,42 +1400,13 @@
   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 <class _Tp>
+struct __libcpp_is_always_lock_free {
+  static const bool __value = __atomic_always_lock_free(sizeof(_Tp), 0);
+};
 
 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


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