Re: [PATCH][RFC] Implement arch primitives for busywait loops

[Nicholas Piggin <npiggin@gmail.com>]

On Fri, 16 Sep 2016 11:57:37 +0000
David Laight <David.Laight@ACULAB.COM> wrote:

> From: Nicholas Piggin
> > Sent: 16 September 2016 12:52
> > On Fri, 16 Sep 2016 11:30:58 +0000
> > David Laight <David.Laight@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.

Thanks,
Nick