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Re: [RFC] P6 microarch (Pentium 2/3) DFA scheduler description

On Thursday 26 February 2004 11:42, Jan Hubicka wrote:
> > Hello,
> Hi,
> just few quick comments:
> ! ;; The goal is to schedule suck that we have few-one-one uops sequences
> ! ;; in each cycle to decode as many instructions per cycle as possible.
> ! (define_cpu_unit "decoder0" "ppro_decoder")
> ! (define_cpu_unit "decoder1" "ppro_decoder")
> ! (define_cpu_unit "decoder2" "ppro_decoder")
> !
> ! ;; All simple instructions can be decoded on each of the three decoders.
> ! (define_reservation "decodern" "(decoder0|decoder1|decoder2)")
> I think you need exclusion/presecne sets here in order to ensure that
> decoder1 is not allocated before decoder0 (they are always fed in order)
> But for deterministic automaton this perhaps don't matter, don't know
> for sure.

You are right, and it does matter.  I had hard-coded this kind of
re-ordering in i386.c and it helped a bit but not enough to justify
such ugly code.  I was wondering how I could force the DFA to first
feed decoder0, and you've just provided me with a possible answer.

> You need to model that operands needed for memory are needed earlier
> than operand needed just for computation.  (...)  I think you need to
> increase the latencies by one cycle for operations loading from memory.

Okey, I'll experiment with that.

Thanks :-)


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