libthr: Early return in thr_once

[aokblast]

Summary

As once function slow path can only happen once, it makes sense to do
a pruning on ONCE_DONE state.

[aokblast]

lib/libthr/thread/thr_once.c

69 ↗

(On Diff #191562)

Not sure whether likely is a good idea or not?

[kib]

lib/libthr/thread/thr_once.c

69 ↗

(On Diff #191562)

Exactly which idea? The only thing that is avoided by the change is the _thr_check_init() call, which is a single load and test. So I really do not see what do we win there.

If doing this, organize the code in a way it is done in the loop, namely, use the fence if the observed state is ONCE_DONE.

Also, it might be it is more useful to add the compiler hint to _thr_check_init() if() condition instead of this.

[aokblast]

Reflect kib@'s comment

[aokblast]

lib/libthr/thread/thr_once.c

69 ↗

(On Diff #191562)

As thr_once will exactly enter slow path at the first time, after that, we exactly enter ONCE_DONE state and never goes slow path. It makes sense to avoid any _thr_check_init test if possible.

But yes, make init unlikely is a much better solution. I original think that it is much clear to distinguish slow path and fast path. But it is obvious that we make some redundancy here.

[kib]

lib/libthr/thread/thr_once.c

91 ↗

(On Diff #191570)

This is s/space*8/tab/, right? It should be a separate commit.