[llvm-dev] How to correctly model resource constraints for in-order scheduling model

Ziqiang Patrick Huang via llvm-dev llvm-dev at lists.llvm.org
Fri Aug 28 08:04:50 PDT 2015


I'm experimenting with the MachineScheduler by providing a subtarget
scheduling model.

Say I have a 2-wide in-order machine that has 2 load/store unit, so I can
issue 2 independent loads per cycle.

By referencing TargetSchedule.td,

it seems like I should do

def MyTargetLdStUnit : ProcResource<2> { let BufferSize = 0 ;}

But right now the scheduler only schedule 1 load/cycle even if there is
another independent load, so I noticed that in

void SchedBoundary::bumpNode(SUnit *SU) in MachineScheduler.cpp

When the scheduler reserve the resource for an instruction, it does not
take the number of units into account,

ReservedCycles[PIdx] = isTop() ? NextCycle + PI->Cycles

Later when the scheduler tries to issue the second load in the same cycle,
 it calls SchedBoundary::checkHazard and reads ReservedCycles[PIdx] which
is next cycle, so it takes the load out from the available queue and push
it into the pending queue.

Is this a bug or I did not model things right

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