Re: using C11's _Atomic()

On Mon, Jun 01, 2026 at 11:24:11AM +0100, Dave Mitchell wrote:
> On Thu, May 07, 2026 at 01:10:53AM +0200, Leon Timmermans wrote:
> > Strictly it's probably UB, but practically any lock-free atomic type should
> > compile down to its underlying type as its representation in either case,
> > so I'm not overly worried until I've seen proof otherwise.
> 
> So, with PR #24439, I've now got atomics (apparently) working where the
> system's ability to use atomics is manually specified via
> 
>     Configure -Accflags=-DPERL_USE_ATOMIC
> 
> The next step is to convert this to a Configure probe. This is an area I'm
> very unfamiliar with, so I would like advice as to whether the proposal
> below is technically sound, and also advice on how to add it to Configure,
> including what naming conventions to use for all the Configure/config
> variables and defines.
> 
> I envisage a combined test C/C++ file like the following:
> 
>     #ifdef __cplusplus
>     #  include <atomic>
>     #  define PERL_ATOMIC(atype) std::atomic<atype>
>     #else
>     #  include <stdatomic.h>
>      /* 2 indicates guaranteed to be lock-free */
>     #  if ATOMIC_INT_LOCK_FREE == 2 && ATOMIC_POINTER_LOCK_FREE == 2
>     #    define PERL_ATOMIC(atype) _Atomic(atype)
>     #  endif
>     #endif

C++ <atomic> defines the ATOMIC_INT_LOCK_FREE and
ATOMIC_POINTER_LOCK_FREE macros with the same values as C does in
<stdatomic.h>, so those could be tested here too.

> This would be compiled and run *twice*, once using the C compiler, and
> once using the C++ compiler (does Configure locate a C++ compiler?)

Configure doesn't.

When working on cpphdrcheck.t I found at least one working C compiler
whose C++ friend didn't build trivial C++11 code, so I'm worried this
may prevent Configure succeeding if the C++ compiler is broken.

It might be better to just make the C++ side `#error` if
ATOMIC_INT_LOCK_FREE and ATOMIC_POINTER_LOCK_FREE don't have
appropriate values, which seems unlikely.

Or static assert sizeof(int) == sizeof(PERL_ATOMIC(int)) and similarly
for pointers.

Tony