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Re: [Xen-devel] code question?
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.
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