Re: [RFA] choosing __platform_wait_t on targets without lock-free 64 atomics

[Thomas Rodgers <trodgers@redhat.com>]

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I agree with this change.

On Thu, Jan 5, 2023 at 4:22 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> How about this?
>
> I don't think we should worry about targets without atomic int, so don't
> bother using types smaller than int.
>
>
> -- >8 --
>
> For non-futex targets the __platform_wait_t type is currently uint64_t,
> but that requires a lock in libatomic for some 32-bit targets. We don't
> really need a 64-bit type, so use unsigned long if that is lock-free,
> and int otherwise. This should mean it's lock-free on a wider set of
> targets.
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/atomic_wait.h (__detail::__platform_wait_t):
>         Define as unsigned long if always lock-free, and unsigned int
>         otherwise.
> ---
>  libstdc++-v3/include/bits/atomic_wait.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/atomic_wait.h
> b/libstdc++-v3/include/bits/atomic_wait.h
> index bd1ed56d157..46f39f10cbc 100644
> --- a/libstdc++-v3/include/bits/atomic_wait.h
> +++ b/libstdc++-v3/include/bits/atomic_wait.h
> @@ -64,7 +64,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  // and __platform_notify() if there is a more efficient primitive
> supported
>  // by the platform (e.g. __ulock_wait()/__ulock_wake()) which is better
> than
>  // a mutex/condvar based wait.
> -    using __platform_wait_t = uint64_t;
> +# if  ATOMIC_LONG_LOCK_FREE == 2
> +    using __platform_wait_t = unsigned long;
> +# else
> +    using __platform_wait_t = unsigned int;
> +# endif
>      inline constexpr size_t __platform_wait_alignment
>        = __alignof__(__platform_wait_t);
>  #endif
> --
> 2.39.0
>
>