Re: PATCH: MIPS 74K load/store scheduling tweak (take 2)

[Richard Sandiford <richard@codesourcery.com>]

David Ung <davidu@mips.com> writes:
> Richard Sandiford wrote:
>> David Ung <davidu@mips.com> writes:
>>> This is the multi-issue problem.  ready_sort is called more than once
>>> during each cycle.  The 1st instruction chosen during that cycle may
>>> not be an AGEN one (eg a floating point insn or an ALU insn), which
>>> means mips_sched_reorder2 needs to be implemented.  So instead of
>>> doing the same thing in mips_sched_reorder2, just changing the
>>> INSN_PRIORITY gets the same effect done automatically by ready_sort.
>> 
>> But the problem with that is that the priority sticks.  If we change
>> the priority of a load L1 (say) from A to B, the scheduler may later
>> insert a newly-ready load L2 with a priority between A and B.
>> We would then wrongly treat L1 as having a higher priority than L2.
>
> The priority sticking is not really a problem, because if there are
> insn in the ready queue, it mean that they are good to go at the
> current cycle.  A newly-ready load L2 you talked about is not going to
> happen during the current cycle, and the now-modified load/store would
> have been issued.

It means that they are "good to go" from a data-dependence point of view,
but it doesn't mean that one load or store will issue this cycle.
Consider the case where a conditional move has been issued on the
previous cycle, and unit restrictions mean no agen insn can be issued
on this one.  A new, higher-priority load may then become ready on
the next cycle.

A similar situation would occur if we're in the wake of a "multi" insn.

>> I think defining TARGET_SCHED_REORDER2 is the right thing.  Let's
>> move the "cycle == 0" stuff in mips_sched_reorg into Sandra's new
>> mips_sched_init function.  I think mips_sched_reorder will then be
>> suitable for both TARGET_SCHED_REORDER and TARGET_SCHED_REORDER2.
>
> I have no objection to doing it again in TARGET_SCHED_REORDER2.  The
> only worry is that TARGET_SCHED_REORDER2 will get called multiple
> times in the same cycle (upto 3 for 74k?)!!

True, but remember that we're doing an O(n log n) sort each time
we call one of these hooks.  A linear walk isn't much overhead
in comparison.

Richard