GCC 12 warning: Invalid memory model

[riptl]

Describe the bug

Compiler warning invalid-memory-model triggered on atomic_base.h.

To Reproduce

Compile with GCC 12.

Expected behavior

No warnings

Environment (please complete the following information):

• Operating System and Version: macOS 12.4, amd64, Bazel, Homebrew GCC 12.1.0_1, C++17 mode

Logs

In member function 'void std::__atomic_base<_IntTp>::store(__int_type, std::memory_order) [with _ITp = int]',
    inlined from 'void Bloomberg::quantum::Task::SuspensionGuard::set(int)' at external/com_bloomberg_quantum/quantum/quantum_task.h:131:34:
/usr/local/Cellar/gcc@12/12.1.0_1/bin/../lib/gcc/12/gcc/x86_64-apple-darwin21/12/../../../../../../include/c++/12/bits/atomic_base.h:464:25: warning: invalid memory model 'memory_order_acq_rel' for 'void __atomic_store_4(volatile void*, unsigned int, int)' [-Winvalid-memory-model]
  464 |         __atomic_store_n(&_M_i, __i, int(__m));
      |         ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~
/usr/local/Cellar/gcc@12/12.1.0_1/bin/../lib/gcc/12/gcc/x86_64-apple-darwin21/12/../../../../../../include/c++/12/bits/atomic_base.h:464:25: note: valid models are 'memory_order_relaxed', 'memory_order_seq_cst', 'memory_order_release'

[rajumark]

Suggested solution from my end

In the file [quantum/quantum_task.h](https://github.com/bloomberg/quantum/blob/master/quantum/quantum_task.h), the fix is located on line 131.

What to change:

Find this line:

_status.store(value, std::memory_order_acq_rel);

Replace it with:

_status.store(value, std::memory_order_release);

Context:

This line is inside the SuspensionGuard::set(int value) function.

Why this fix?
As discussed in the issue, std::memory_order_acq_rel is an invalid memory model for a store operation. A store can only "release" or be "relaxed" (or sequentially consistent). GCC 12 and newer versions flag this as a warning/error (-Winvalid-memory-model) because acq_rel implies an "acquire" (read) which doesn't happen during a simple store(). Changing it to std::memory_order_release preserves the intended thread synchronization while complying with the C++ standard.