[clang] Define ATOMIC_FLAG_INIT correctly for C++.

[chrisnc]

No description provided.

[llvmbot]

@llvm/pr-subscribers-backend-x86

@llvm/pr-subscribers-clang

Author: Chris Copeland (chrisnc)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/97534.diff

2 Files Affected:

• (modified) clang/docs/ReleaseNotes.rst (+2)
• (modified) clang/lib/Headers/stdatomic.h (+4)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 1537eaaba0c66..3d85065a7ff8a 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -750,6 +750,8 @@ Bug Fixes in This Version
 
 - Fixed `static_cast` to array of unknown bound. Fixes (#GH62863).
 
+- Fixed the definition of ATOMIC_FLAG_INIT in stdatomic.h so it can be used in C++.
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
diff --git a/clang/lib/Headers/stdatomic.h b/clang/lib/Headers/stdatomic.h
index 9c103d98af8c5..88172038be5c4 100644
--- a/clang/lib/Headers/stdatomic.h
+++ b/clang/lib/Headers/stdatomic.h
@@ -166,7 +166,11 @@ typedef _Atomic(uintmax_t)          atomic_uintmax_t;
 
 typedef struct atomic_flag { atomic_bool _Value; } atomic_flag;
 
+#ifdef __cplusplus
+#define ATOMIC_FLAG_INIT { false }
+#else
 #define ATOMIC_FLAG_INIT { 0 }
+#endif
 
 /* These should be provided by the libc implementation. */
 #ifdef __cplusplus

[chrisnc]

Without this change, trying to use ATOMIC_FLAG_INIT in a constant initialization context in C++ yields the following:

https://godbolt.org/z/sTvMs5a95

<source>:3:17: error: non-constant-expression cannot be narrowed from type 'int' to 'atomic_bool' (aka '_Atomic(bool)') in initializer list [-Wc++11-narrowing]
    3 | atomic_flag x = ATOMIC_FLAG_INIT;
      |                 ^~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-trunk-20240703/lib/clang/19/include/stdatomic.h:169:28: note: expanded from macro 'ATOMIC_FLAG_INIT'
  169 | #define ATOMIC_FLAG_INIT { 0 }
      |                            ^
<source>:3:17: note: insert an explicit cast to silence this issue
    3 | atomic_flag x = ATOMIC_FLAG_INIT;
      |                 ^~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-trunk-20240703/lib/clang/19/include/stdatomic.h:169:28: note: expanded from macro 'ATOMIC_FLAG_INIT'
  169 | #define ATOMIC_FLAG_INIT { 0 }
      |                            ^
1 error generated.
Compiler returned: 1

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[chrisnc]

ping

[chrisnc]

ping

[AaronBallman]

Thank you for catching this! Can you also add some tests to clang/test/Headers/? We have some coverage for stdatomic.h in C, but not in C++ (I'd recommend trying to add a RUN line to the .c file with -x c++ so it's tested in C++ mode and then ensure there's sufficient coverage for ATOMIC_FLAG_INIT.)

[chrisnc]

Rebased and added a test case. I confirmed that the test fails using the current definition of ATOMIC_FLAG_INIT:

error: 'expected-error' diagnostics seen but not expected:
  Line 38: non-constant-expression cannot be narrowed from type 'int' to 'atomic_bool' (aka '_Atomic(bool)') in initializer list
error: 'expected-note' diagnostics seen but not expected:
  Line 38: insert an explicit cast to silence this issue
2 errors generated.

[AaronBallman]

LGTM!

[AaronBallman]

Btw, do you need someone to merge this on your behalf?

[chrisnc]

@AaronBallman yes, that would be great! I'm just a contributor, not a collaborator.

[AaronBallman]

I think this should be picked for the Clang 19 branch, so I filed #100362

[ldionne]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

[llvmbot]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

Error: Command failed due to missing milestone.

[ldionne]

It seems like the bot looks for the command anywhere in any comment :)

[AaronBallman]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

That's good to know! I was going off the documentation at: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches because it says "issue" specifically. I'll post a patch to update those docs, thanks!

[AaronBallman]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

That's good to know! I was going off the documentation at: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches because it says "issue" specifically. I'll post a patch to update those docs, thanks!

#100401

[llvmbot]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

That's good to know! I was going off the documentation at: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches because it says "issue" specifically. I'll post a patch to update those docs, thanks!

Error: Command failed due to missing milestone.

[llvmbot]

@AaronBallman Just FYI you can also add the Milestone to the PR directly and use the /cherry-pick command right here. This is a bit simpler.

That's good to know! I was going off the documentation at: https://www.llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches because it says "issue" specifically. I'll post a patch to update those docs, thanks!

#100401

Error: Command failed due to missing milestone.