[C2x] Remove the ATOMIC_VAR_INIT macro from stdatomic.h

[aaron.ballman]

Summary

This implements WG14 N2886 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2886.htm) which removed the macro entirely. (NB the macro was deprecated in C17.) As the paper is not particularly clear on what alternative was picked, here are my notes from the May 2022 meeting:

Does WG14 wish to adopt variant 1, change 3.2, 3.3, and 3.4 from N2886 into C23? 14/2/2 (consensus).
Does WG14 want to exchange Variant 1 with Variant 2 in N2886 in C23? 9/3/6 (consensus).
(There was no sentiment in the room for either Variant 3 or Variant 4 so those were not voted on.)
Does WG14 want to integrate change 3.5 in N2886 into C23? 8/1/9 (consensus).
Does WG14 want to integrate change 3.6 in N2886 into C23? 2/5/9 (no consensus).

I've added the libc++ reviewers to ensure this doesn't negatively impact <atomic> and the clang-vendors group for early awareness about a potentially breaking change. Any code that is broken by the removal can remove the use of ATOMIC_VAR_INIT and use regular initialization instead.

[cor3ntin]

clang/docs/ReleaseNotes.rst

97

Just confirming we want to pluralize standards here? It kind of look weird to my non native eyes.

clang/lib/Headers/stdatomic.h

50

C++ uses the date of the meeting where the change was accepted, I assume C is different?

61

Should we add a message informing people it's remove in C23?

[aaron.ballman]

clang/docs/ReleaseNotes.rst

97

I'll reword it to be less awkward because I can't convince myself one way or the other. :-D

clang/lib/Headers/stdatomic.h

50

__STDC_VERSION__ is akin to __cplusplus, so it's the date of publication (which we don't have yet, which is why I'm using a placeholder).

61

Eh, that gets annoying because it's removed in C23 but still present in C++23. I think the current wording is likely fine.

[cor3ntin]

Sorry it took me a while to reply to you. I think you convinced me this is fine as-is! Thanks

clang/lib/Headers/stdatomic.h

50

202000L is extra arbitrary, but.. there is a fixme so i guess it's fine!

61

Fair enough. It's likely C++ will want to remove it too when rebased on C23 though

[ldionne]

Thanks for the heads up. As noted in another comment, I don't think this impacts libc++ because C++23 (unfortunately) still provides the macro AFAICT. We define it ourselves in libcxx/include/__atomic/atomic_init.h.

clang/docs/ReleaseNotes.rst

- Support for outputs from asm goto statements along indirect edges has been
   added. This fixes
   `Issue 53562 <https://github.com/llvm/llvm-project/issues/53562>`_.
 
 C2x Feature Support
 ^^^^^^^^^^^^^^^^^^^
 - Implemented the ``unreachable`` macro in freestanding ``<stddef.h>`` for
   `WG14 N2826 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2826.pdf>`_
 
+- Removed the ``ATOMIC_VAR_INIT`` macro in C2x and later standards modes, which
+  implements `WG14 N2886 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2886.htm>`_
+
 Non-comprehensive list of changes in this release
 -------------------------------------------------
 - Clang now saves the address of ABI-indirect function parameters on the stack,
   improving the debug information available in programs compiled without
   optimizations.
 - Clang now supports ``__builtin_nondeterministic_value`` that returns a
   nondeterministic value of the same type as the provided argument.
 

clang/lib/Headers/stdatomic.h

 #define ATOMIC_WCHAR_T_LOCK_FREE    __CLANG_ATOMIC_WCHAR_T_LOCK_FREE
 #define ATOMIC_SHORT_LOCK_FREE      __CLANG_ATOMIC_SHORT_LOCK_FREE
 #define ATOMIC_INT_LOCK_FREE        __CLANG_ATOMIC_INT_LOCK_FREE
 #define ATOMIC_LONG_LOCK_FREE       __CLANG_ATOMIC_LONG_LOCK_FREE
 #define ATOMIC_LLONG_LOCK_FREE      __CLANG_ATOMIC_LLONG_LOCK_FREE
 #define ATOMIC_POINTER_LOCK_FREE    __CLANG_ATOMIC_POINTER_LOCK_FREE
 
 /* 7.17.2 Initialization */
-
+/* FIXME: This is using the placeholder dates Clang produces for these macros
+   in C2x mode; switch to the correct values once they've been published. */
+#if (defined(__STDC_VERSION__) && __STDC_VERSION__ < 202000L) ||               \
+    defined(__cplusplus)
+/* ATOMIC_VAR_INIT was removed in C2x, but still remains in C++2b. */
 #define ATOMIC_VAR_INIT(value) (value)
-#if ((defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201710L) ||             \
+#endif
+
+#if ((defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201710L &&              \
+      __STDC_VERSION__ < 202000L) ||                                           \
      (defined(__cplusplus) && __cplusplus >= 202002L)) &&                      \
     !defined(_CLANG_DISABLE_CRT_DEPRECATION_WARNINGS)
 /* ATOMIC_VAR_INIT was deprecated in C17 and C++20. */
 #pragma clang deprecated(ATOMIC_VAR_INIT)
 #endif
 #define atomic_init __c11_atomic_init
 
 /* 7.17.3 Order and consistency */
 
 typedef enum memory_order {
   memory_order_relaxed = __ATOMIC_RELAXED,
   memory_order_consume = __ATOMIC_CONSUME,

clang/test/C/C2x/n2886.c

+// RUN: %clang_cc1 -verify=okay -std=c11 -ffreestanding %s
+// RUN: %clang_cc1 -verify -std=c17 -ffreestanding %s
+// RUN: %clang_cc1 -verify -std=c2x -ffreestanding %s
+
+/* WG14 N2886: yes
+ * Remove ATOMIC_VAR_INIT v2
+ */
+
+/* okay-no-diagnostics */
+#include <stdatomic.h>
+
+_Atomic int a = ATOMIC_VAR_INIT(0); /* #diag */
+#if __STDC_VERSION__ <= 201710L
+/* expected-warning@#diag {{macro 'ATOMIC_VAR_INIT' has been marked as deprecated}}
+   expected-note@stdatomic.h:* {{macro marked 'deprecated' here}}
+*/
+#else
+/* expected-error@#diag {{use of undeclared identifier 'ATOMIC_VAR_INIT'}} */
+#endif
+

clang/www/c_status.html

</tr>
     <tr>
       <td>Indeterminate Values and Trap Representations</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2861.pdf">N2861</a></td>
       <td class="full" align="center">Yes</td>
     </tr>
     <tr>
       <td>Remove ATOMIC_VAR_INIT v2</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2886.htm">N2886</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 17</td>
     </tr>
     <tr>
       <td>Require exact-width integer type interfaces v2</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2888.htm">N2888</a></td>
       <td class="full" align="center">Yes</td>
     </tr>
     <tr>
       <td>Wording Change for Variably-Modified Types</td>