From: Nicholas Piggin
> Sent: 16 September 2016 09:58
> Implementing busy wait loops with cpu_relax() in callers poses
> some difficulties for powerpc.
> First, we want to put our SMT thread into a low priority mode for the
> duration of the loop, but then return to normal priority after exiting
> the loop. Dependong on the CPU design, 'HMT_low() ; HMT_medium();' as
> cpu_relax() does may have HMT_medium take effect before HMT_low made
> any (or much) difference.
> Second, it can be beneficial for some implementations to spin on the
> exit condition with a statically predicted-not-taken branch (i.e.,
> always predict the loop will exit).
> This is a quick RFC with a couple of users converted to see what
> people think. I don't use a C branch with hints, because we don't want
> the compiler moving the loop body out of line, which makes it a bit
> messy unfortunately. If there's a better way to do it, I'm all ears.
I think it will still all go wrong if the conditional isn't trivial.
In particular if the condition contains || or && it is likely to
have a branch - which could invert the loop.