On 09/12/2018 11:55 AM, Matthew Wilcox wrote: > On Wed, Sep 12, 2018 at 11:49:22AM -0400, Waiman Long wrote: >>> unless our macrology has got too clever for the compilre to see through >>> it. In which case, the right answer is to simplify the percpu code, >>> not to force the compiler to optimise the code in the way that makes >>> sense for your current microarchitecture. >>> >> I had actually looked at the x86 object file generated to verify that it >> did use cmove with the patch and use branch without. It is possible that >> there are other twists to make that happen with the above expression. I >> will need to run some experiments to figure it out. In the mean time, I >> am fine with dropping this patch as it is a micro-optimization that >> doesn't change the behavior at all. > I don't understand why you included it, to be honest. But it did get > me looking at the percpu code to see if it was too clever. And that > led to the resubmission of rth's patch from two years ago that I cc'd > you on earlier. > > With that patch applied, gcc should be able to choose to use the > cmov if it feels that would be a better optimisation. It already > makes one different decision in dcache.o, namely that it uses addq > $0x1,%gs:0x0(%rip) instead of incq %gs:0x0(%rip). Apparently this > performs better on some CPUs. > > So I wouldn't spend any more time on this patch. Thank for looking into that. Well I am not going to look further into this unless I have no other thing to do which is unlikely. Cheers, Longman
