Re: Fwd: Questions on PA machine description?

["Jerry Quinn" <jquinn@nortelnetworks.com>]

Jeffrey A Law wrote:
> 
>   In message < 36F811ED.32912006@americasm01.nt.com >you write:
>   > I started trying to look at the haifa_sched.c file.  I can't really see
>   > where insns dependent on the current one in the ready queue are
>   > removed.  I'm looking in the loop on ready[] in schedule_block().  My
>   > guess is that somehow schedule_insn does this, but I'm not sure.
> Yes.
> 
> schedule_insn walks over the INSN_DEPEND list and decrements the dependency
> count for all the instructions which are dependent on INSN.  If the dependency
> count goes to zero, then the dependent insn is added to the ready queue.
> 
>   for (link = INSN_DEPEND (insn); link != 0; link = XEXP (link, 1))
>     {
>       rtx next = XEXP (link, 0);
>       int cost = insn_cost (insn, link, next);
> 
>       INSN_TICK (next) = MAX (INSN_TICK (next), clock + cost);
> 
>       if ((INSN_DEP_COUNT (next) -= 1) == 0)
>         {
> [ ... ]
>           /* Adjust the priority of NEXT and either put it on the ready
>              list or queue it.  */
>           adjust_priority (next);
>           if (effective_cost <= 1)
>             ready[n_ready++] = next;
>           else
>             queue_insn (next, effective_cost);
>         }

So the ready list for a cycle starts out with insns with no
dependencies.  Then when we pick an insn off the ready list, it's placed
into the scheduled chain and schedule_insn is called.

OK, now I'm confused, because in my head it looks like the code should
already do what you want.  schedule_insn is called as soon as we
schedule an insn from the ready list.  So, if we have a dependent insn,
such as the lo_sum following a sethi, the dependency would now be
reduced to 0, and it becomes eligible for the lower part of the code. 
So as long as effective_cost is OK, it would be added to the ready
queue.  INSN_DEPEND is the list of insns that depend on the one being
scheduled, right?

Since this doesn't happen, I'm obviously missing something.

If the alu has 0 delay, insn_cost would return 0?  Then, INSN_TICK would
be unchanged.  And effective_cost would end up 0, causing the dependent
insn to be placed into ready.

I'm babbling again and confused.  Enough for today :-)

Jerry


-- 
Jerry Quinn                             Tel: (514) 761-8737
jquinn@nortelnetworks.com               Fax: (514) 761-8505
Speech Recognition Research