Re: Scheduling automaton question

[Alex Turjan <aturjan at yahoo dot com>]

Hi,
According to me at this moment the scheduler does not support your needs.

I was confronted with a similar problem as yours and I solved it by implementing the TARGET_SCHED_DFA_NEW_CYCLE hook. Inside of the function which supports this hook I choose/set the insn reservation that makes  possible to fit as many other insn as possible in the same cycle.

During this process I also update the ready list with insns that become ready as a result of scheduling the current insn ( like in your example -insns that are anti-dependent on the current insn and which therefore can be scheduled in the current cycle). Thus the best insn reservation makes possible scheduling antidependent insns of cost zero in the same cycle by avoiding resource conflicts. 

Alex


few changes in the gcc mainline sources

--- On Fri, 2/11/11, Bernd Schmidt <bernds@codesourcery.com> wrote:

> From: Bernd Schmidt <bernds@codesourcery.com>
> Subject: Scheduling automaton question
> To: "GCC List" <gcc@gcc.gnu.org>
> Cc: "Vladimir N. Makarov" <vmakarov@redhat.com>
> Date: Friday, February 11, 2011, 2:33 PM
> Suppose I have two insns, one
> reserving (A|B|C), and the other reserving
> A. I'm observing that when the first one is scheduled in an
> otherwise
> empty state, it reserves the A unit and blocks the second
> one from being
> scheduled in the same cycle. This is a problem when there's
> an
> anti-dependence of cost 0 between the two instructions.
> 
> Vlad - two questions. Is this behaviour what you would
> expect to happen,
> and how much work do you think would be involved to fix it
> (i.e. make
> the first one transition to a state where we can still
> reserve any two
> out of the three units)?
> 
> 
> Bernd
>