On Fri, 16 Sep 2016 22:06:35 +1000 Nicholas Piggin <npig...@gmail.com> wrote:
> On Fri, 16 Sep 2016 11:57:37 +0000 > David Laight <david.lai...@aculab.com> wrote: > > > From: Nicholas Piggin > > > Sent: 16 September 2016 12:52 > > > On Fri, 16 Sep 2016 11:30:58 +0000 > > > David Laight <david.lai...@aculab.com> wrote: > > > > > > > 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. > > > > > > I don't know that it will. > > > > > > Yes, if we have exit condition that requires more branches in order to > > > be computed then we lose our nice property of never taking a branch > > > miss on loop exit. But we still avoid *this* branch miss, and still > > > prevent multiple iterations of the wait loop being speculatively > > > executed concurrently when there's no work to be done. > > > > > > And C doesn't know about the loop, so it can't do any transformation > > > except to compute the final condition. > > > > > > Or have I missed something? > > > > Try putting the code inside a conditional or at the bottom of a loop. > > gcc can replicate code to remove a branch. > > > > So: > > for (;;) { > > a; > > if (b) > > c; > > d; > > } > > That's not what this patch does though. The loop is purely asm. gcc has > no idea about it. Only thing gcc knows is to evaluate the condition and > put it in a register. Oh you're right course -- can't branch to random location. Sorry, I didn't know what you meant at first. It does need to use asm goto I guess. Thanks, Nick