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

[Jonathan Wakely]

On Thu, 12 Jan 2023 at 01:27, Thomas Rodgers wrote:
>
> I agree with this change.

Thanks, pushed to trunk.

>
> 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
>>