Re: [Xen-devel] code question?

["M.A. Williamson" <maw48@xxxxxxxxx>]

> Doing some janitorial (you cleaning the flooded toilets and such) work
> today. I have come across this line of code that really I'm not sure
> what the intent was..in xen/include/sched.h
>
> #define hypercall_preempt_check() (unlikely(            \
>        softirq_pending(smp_processor_id()) |           \
>        (!!current->vcpu_info->evtchn_upcall_pending &  \
>          !current->vcpu_info->evtchn_upcall_mask)      \
>    ))
>
> the part where we have !!current->vcpu_info_evtchen_upcall pending
> should this be..should the "!! just be "!"?
I think that:
!!a & !b === (a != 0 ) && ( b == 0 )

It's just more efficient to do it directly using bitwise ops than to use 
the full logical operators (can gcc really not optimise this sort of 
thing???).
Cheers,
Mark

> And if so shouldn't this just be changed to 
>
> #define hypercall_preempt_check() (unlikely( \
>         softirq_pending(smp_processor_id()) |           \
>         (!(current->vcpu_info->evtchn_upcall_pending & \
>           current->vcpu_info->evtchn_upcall_mask)) \
>         ))
>         
>
> In a lot of the code in Xen we are using the "!" operator with bitwise
> operations..this is one of those examples.
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel