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Re: [Xen-devel] code question?

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] code question?
From: David Hopwood <david.nospam.hopwood@xxxxxxxxxxxxxxxx>
Date: Thu, 11 Aug 2005 17:55:53 +0100
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Jerone Young wrote:
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 "!"?

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)) \

Seems more likely to have been intended as:

  #define hypercall_preempt_check() (unlikely(           \
          softirq_pending(smp_processor_id()) ||         \
          (current->vcpu_info->evtchn_upcall_pending &&  \
            !current->vcpu_info->evtchn_upcall_mask)     \

Keir wrote:
> Forming compound predicates for bitwise operators can be faster than using the
> logical operators because they are non 'short circuiting'. This means you end
> up with fewer branches in the generated code and end up with nice 
> code that should execute fast.

If that's true on a given platform, the compiler can optimize it. Note that
in this case "(current->vcpu_info->evtchn_upcall_pending &&
!current->vcpu_info->evtchn_upcall_mask)" has no side effects (unless current
or current->vcpu_info are not valid pointers, which is undefined behaviour so
still optimizable).

David Hopwood <david.nospam.hopwood@xxxxxxxxxxxxxxxx>

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