[PATCH] [RFC] Lame fix for PR20376

[Peter Collingbourne]

On Tue, Jul 22, 2014 at 10:26:29AM +0000, Tim Northover wrote:
> We actually do have a last-ditch EFLAGS copy available (though with worrying implications for the red-zone, if we ever start using it). It's used by test/CodeGen/Generic/2011-07-07-ScheduleDAGCrash.ll for example.
> 
> Enabling it more generally is probably just a matter of tweaking getCopyCost. Which might be an idea for robustness anyway.

I see. That might be reasonable if we don't expect this situation to come
up much.

> But I'm not sure that actually affects the best solution here. I rather suspect performing the comparison again would be quicker in general. I suppose the question is how common and performance-critical such fragments are.

I don't understand this point. Compare exchanges are not idempotent.

Thanks,
-- 
Peter