Re: How to use "nothing" in pipeline description?

["Vladimir N. Makarov" <vmakarov at redhat dot com>]

H. J. Lu wrote:

> If an instruction has latency 3 and throughput 1, should I write it as
>
> (define_insn_reservation "simple" 3
>  (eq_attr "memory" "none")
>  "p0")
>
> or
>
> (define_insn_reservation "simple" 3
>  (eq_attr "memory" "none")
>  "p0,nothing*2")
>
> Are they equivalent?
Yes.

> What happens when there are fewer reservation
> cycles than default latency?
>
>  
They are different things.  Latency is used for finding data ready 
delays (priorities used to order insns according their critical path 
length).  When input data are ready for an insn, the reservations are 
checked to find that there are enough functional units to execute the 
insn.  So there is no correlation between latency time and reservations 
with the compiler point of view.

Frequently, reservation is shorter than the latency because after a few 
cycles there will be never reservation hazards because further execution 
is fully pipelined (in other words we always have enough functional 
units for the further execution).  In this case we can omit reservation 
of the corresponding functional units.  Latency time less than the 
reservation length is also permitted but usually it has no sense 
(although we can describe all execution pipeline stages like prefetching 
and decoding.  Latency times usually are given only for actual operation 
execution without such stages).

Vlad