Re: DFA for PPro, P2, P3

[law at redhat dot com]

 In message <20020502223945.B5496@redhat.com>, Richard Henderson writes:
 > On Thu, May 02, 2002 at 08:28:48PM -0600, law@redhat.com wrote:
 > > That's one of the things that is (to me at least) extremely unclear about
 > > the PPro/P2/P3 pipeline -- ordering/dependency of uops.
 > 
 > Well it's quite clear that a load-execute x86 insn (as your example
 > must have been)
Actually my example was a shift of a register by a constant.  Those
are incorrectly marked as using p0 and p01 in the current pipeline
description.  If it was a load-execute it would have used p2 in some
way.  I didn't figure it was worth confusing folks with this tidbit
of information :-)


 > will consist of a load uop plus an execute uop, and
 > it is quite clear that the two must be data dependent.
It's certainly true that some can be derived; it's the obvious thing
to do once the base description is working.  Particularly since one of
the strengths of the DFA model is that we can handle/schedule for
resources that are used during different stages of an instruction's
execution.


 > Now, the execute uop may not be executed immediately after the load
 > uop, but I think for all practical purposes on an OOO machine
 > reserving the resource is good enough, since that pretty much
 > guarantees that at least as many resources as you reserved will
 > be available, so it won't run any slower.
Right.  

 > But (p0|p1)+p0 makes no sense no matter how you slice it.
I know.  I just haven't gotten around to dealing with dependencies between
the uops yet.  Trust me, it's on my todo list.

jeff