Re: DFA scheduler for i386.md

[Vladimir Makarov <vmakarov at redhat dot com>]

law@redhat.com wrote:
> 
> In message <3CC03CC7.2E51C9D7@redhat.com>, Vladimir Makarov writes:
>  > Jan Hubicka wrote:
>  > >
>  > > Hi,
>  > > Yesterday I started work on rewriting the scheduler descriptions in i386.m
>  > d to
>  > > DFA scheduler in hope that new code will be smaller and easier to understa
>  > nd.
>  > > So far I am about 70% done with pentium description and it apperas to work
>  >  well
>  > > and be much cleaner than older code.  I hope to fix all the MD_SCHED macro
>  > s
>  > > usage soonish so we will be able to kill it later.  Also the pentium descr
>  > iption
>  > > is far the larges from i386 ones that is about the larges in GCC, so I wan
>  > t
>  > > to give DFA some testing.
>  > >
>  >
>  >   Thanks, Jan.  But i686 is out of order execution, speculative
>  > processor.  Moreover, the situation is complicated because x86 insns are
>  > translated into smaller RISC like insns and you don't know this
>  > translation.  Even all insn scheduling practically gives nothing to
>  > improve the code.  So it will be a miracle if you have an improvement by
>  > usage of the dfa description.
> I disagree.  Vlad, why don't you read some of the literature which
> discusses scheduling for out of order execution machines?
> 
> The basic concept is that you do not schedule for latency, but instead
> for the various units such as issue/retirement.
> 
> For example, scheduling on the PA8000 (out of order) wins about 6% over not
> scheduling.  This is due to presenting instructions to the processor in such
> a way as to improve the utilization of the issue and retirement units.

  I meant i686.  Sure, even out-of-order execution/speculative
processors has some bottlenecks which can not resolved by insn
scheduler.  But as for i686,  I ran Speck for P4 with and without
scheduling several months ago.  There is a very small difference (as I
remember only 1-2%).

Vlad