[PATCH 4/5] barrier: use int instead of unsigned char for the phase state

[Thomas Rodgers]

On 2021-03-01 10:06, Thiago Macieira wrote:

> On Monday, 1 March 2021 09:38:58 PST Thomas Rodgers wrote: And 
> _M_phase, despite being non-atomic, is never accessed without the
> atomic_ref, aside from the constructor. Both arrive() and wait() start
> off by
> creating the atomic_ref.
> If it's non-atomic, then how is wait() supposed to wait on it,
> atomically?

> Hey, it's your code :-)
> 
> It is using atomics to operate on the value. It's just that the type on 
> the
> structure isn't an atomic by itself.

> Why, I don't know. Olivier's original code did use atomics
> <https://github.com/ogiroux/atomic_wait/blob/master/include/barrier#L55-L56>:
>    std::atomic<ptrdiff_t>             expected_adjustment;
>    std::atomic<__phase_t>             phase;

It is atomic, in that it is always an atomic_ref. This change was made 
at
Jonathan's request.

> And I am not disagreeing with that. I am, however saying, that I know
> this particular implementation (well the upstream one it is based on)
> has been extensively tested by the original author (ogiroux) including
> time on Summit. If we are going to start changing his design decisions
> (beyond the largely cosmetic, not algorithmic, ones that I have made as
> per Jonathan's request), they should be motivated by more than a 'well
> we feel int's would be better here because Futex' justification.

> That's a reasonable request.
> 
> I'd like to see the benchmark results of using a directly-futexable 
> type vs
> using unsigned char. But even the timing results need to be weighed 
> against
> the increased memory use for std::barrier, which means it's not a 
> directly-
> objective conclusion. And given we *may* get 8-bit futexes, it might be 
> worth
> keeping them this way and just tell people with large contentions to 
> upgrade.

We may also want to introduce a central barrier type for memory 
constrained environments. I specifically
removed that from this patch because it -

1) wasn't clear how we'd go about making that decision
2) this support in GCC11 is experimental

> That of course rests on the contended atomic_wait being out-of-line.