llvm-project/llvm/lib/Target/PowerPC/PPCMachineScheduler.cpp

//===- PPCMachineScheduler.cpp - MI Scheduler for PowerPC -------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#include "PPCMachineScheduler.h"
#include "MCTargetDesc/PPCMCTargetDesc.h"

using namespace llvm;

static cl::opt<bool> 
DisableAddiLoadHeuristic("disable-ppc-sched-addi-load",
                         cl::desc("Disable scheduling addi instruction before" 
                                  "load for ppc"), cl::Hidden);
static cl::opt<bool>
    EnableAddiHeuristic("ppc-postra-bias-addi",
                        cl::desc("Enable scheduling addi instruction as early"
                                 "as possible post ra"),
                        cl::Hidden, cl::init(true));

static bool isADDIInstr(const GenericScheduler::SchedCandidate &Cand) {
  return Cand.SU->getInstr()->getOpcode() == PPC::ADDI ||
         Cand.SU->getInstr()->getOpcode() == PPC::ADDI8;
}

bool PPCPreRASchedStrategy::biasAddiLoadCandidate(SchedCandidate &Cand,
                                                  SchedCandidate &TryCand,
                                                  SchedBoundary &Zone) const {
  if (DisableAddiLoadHeuristic)
    return false;

  SchedCandidate &FirstCand = Zone.isTop() ? TryCand : Cand;
  SchedCandidate &SecondCand = Zone.isTop() ? Cand : TryCand;
  if (isADDIInstr(FirstCand) && SecondCand.SU->getInstr()->mayLoad()) {
    TryCand.Reason = Stall;
    return true;
  }
  if (FirstCand.SU->getInstr()->mayLoad() && isADDIInstr(SecondCand)) {
    TryCand.Reason = NoCand;
    return true;
  }

  return false;
}

void PPCPreRASchedStrategy::tryCandidate(SchedCandidate &Cand,
                                         SchedCandidate &TryCand,
                                         SchedBoundary *Zone) const {
  // From GenericScheduler::tryCandidate

  // Initialize the candidate if needed.
  if (!Cand.isValid()) {
    TryCand.Reason = NodeOrder;
    return;
  }

  // Bias PhysReg Defs and copies to their uses and defined respectively.
  if (tryGreater(biasPhysReg(TryCand.SU, TryCand.AtTop),
                 biasPhysReg(Cand.SU, Cand.AtTop), TryCand, Cand, PhysReg))
    return;

  // Avoid exceeding the target's limit.
  if (DAG->isTrackingPressure() &&
      tryPressure(TryCand.RPDelta.Excess, Cand.RPDelta.Excess, TryCand, Cand,
                  RegExcess, TRI, DAG->MF))
    return;

  // Avoid increasing the max critical pressure in the scheduled region.
  if (DAG->isTrackingPressure() &&
      tryPressure(TryCand.RPDelta.CriticalMax, Cand.RPDelta.CriticalMax,
                  TryCand, Cand, RegCritical, TRI, DAG->MF))
    return;

  // We only compare a subset of features when comparing nodes between
  // Top and Bottom boundary. Some properties are simply incomparable, in many
  // other instances we should only override the other boundary if something
  // is a clear good pick on one boundary. Skip heuristics that are more
  // "tie-breaking" in nature.
  bool SameBoundary = Zone != nullptr;
  if (SameBoundary) {
    // For loops that are acyclic path limited, aggressively schedule for
    // latency. Within an single cycle, whenever CurrMOps > 0, allow normal
    // heuristics to take precedence.
    if (Rem.IsAcyclicLatencyLimited && !Zone->getCurrMOps() &&
        tryLatency(TryCand, Cand, *Zone))
      return;

    // Prioritize instructions that read unbuffered resources by stall cycles.
    if (tryLess(Zone->getLatencyStallCycles(TryCand.SU),
                Zone->getLatencyStallCycles(Cand.SU), TryCand, Cand, Stall))
      return;
  }

  // Keep clustered nodes together to encourage downstream peephole
  // optimizations which may reduce resource requirements.
  //
  // This is a best effort to set things up for a post-RA pass. Optimizations
  // like generating loads of multiple registers should ideally be done within
  // the scheduler pass by combining the loads during DAG postprocessing.
  const SUnit *CandNextClusterSU =
      Cand.AtTop ? DAG->getNextClusterSucc() : DAG->getNextClusterPred();
  const SUnit *TryCandNextClusterSU =
      TryCand.AtTop ? DAG->getNextClusterSucc() : DAG->getNextClusterPred();
  if (tryGreater(TryCand.SU == TryCandNextClusterSU,
                 Cand.SU == CandNextClusterSU, TryCand, Cand, Cluster))
    return;

  if (SameBoundary) {
    // Weak edges are for clustering and other constraints.
    if (tryLess(getWeakLeft(TryCand.SU, TryCand.AtTop),
                getWeakLeft(Cand.SU, Cand.AtTop), TryCand, Cand, Weak))
      return;
  }

  // Avoid increasing the max pressure of the entire region.
  if (DAG->isTrackingPressure() &&
      tryPressure(TryCand.RPDelta.CurrentMax, Cand.RPDelta.CurrentMax, TryCand,
                  Cand, RegMax, TRI, DAG->MF))
    return;

  if (SameBoundary) {
    // Avoid critical resource consumption and balance the schedule.
    TryCand.initResourceDelta(DAG, SchedModel);
    if (tryLess(TryCand.ResDelta.CritResources, Cand.ResDelta.CritResources,
                TryCand, Cand, ResourceReduce))
      return;
    if (tryGreater(TryCand.ResDelta.DemandedResources,
                   Cand.ResDelta.DemandedResources, TryCand, Cand,
                   ResourceDemand))
      return;

    // Avoid serializing long latency dependence chains.
    // For acyclic path limited loops, latency was already checked above.
    if (!RegionPolicy.DisableLatencyHeuristic && TryCand.Policy.ReduceLatency &&
        !Rem.IsAcyclicLatencyLimited && tryLatency(TryCand, Cand, *Zone))
      return;

    // Fall through to original instruction order.
    if ((Zone->isTop() && TryCand.SU->NodeNum < Cand.SU->NodeNum) ||
        (!Zone->isTop() && TryCand.SU->NodeNum > Cand.SU->NodeNum)) {
      TryCand.Reason = NodeOrder;
    }
  }

  // GenericScheduler::tryCandidate end

  // Add powerpc specific heuristic only when TryCand isn't selected or
  // selected as node order.
  if (TryCand.Reason != NodeOrder && TryCand.Reason != NoCand)
    return;

  // There are some benefits to schedule the ADDI before the load to hide the
  // latency, as RA may create a true dependency between the load and addi.
  if (SameBoundary) {
    if (biasAddiLoadCandidate(Cand, TryCand, *Zone))
      return;
  }
}

bool PPCPostRASchedStrategy::biasAddiCandidate(SchedCandidate &Cand,
                                               SchedCandidate &TryCand) const {
  if (!EnableAddiHeuristic)
    return false;

  if (isADDIInstr(TryCand) && !isADDIInstr(Cand)) {
    TryCand.Reason = Stall;
    return true;
  }
  return false;
}

void PPCPostRASchedStrategy::tryCandidate(SchedCandidate &Cand,
                                          SchedCandidate &TryCand) {
  // From PostGenericScheduler::tryCandidate

  // Initialize the candidate if needed.
  if (!Cand.isValid()) {
    TryCand.Reason = NodeOrder;
    return;
  }

  // Prioritize instructions that read unbuffered resources by stall cycles.
  if (tryLess(Top.getLatencyStallCycles(TryCand.SU),
              Top.getLatencyStallCycles(Cand.SU), TryCand, Cand, Stall))
    return;

  // Keep clustered nodes together.
  if (tryGreater(TryCand.SU == DAG->getNextClusterSucc(),
                 Cand.SU == DAG->getNextClusterSucc(), TryCand, Cand, Cluster))
    return;

  // Avoid critical resource consumption and balance the schedule.
  if (tryLess(TryCand.ResDelta.CritResources, Cand.ResDelta.CritResources,
              TryCand, Cand, ResourceReduce))
    return;
  if (tryGreater(TryCand.ResDelta.DemandedResources,
                 Cand.ResDelta.DemandedResources, TryCand, Cand,
                 ResourceDemand))
    return;

  // Avoid serializing long latency dependence chains.
  if (Cand.Policy.ReduceLatency && tryLatency(TryCand, Cand, Top)) {
    return;
  }

  // Fall through to original instruction order.
  if (TryCand.SU->NodeNum < Cand.SU->NodeNum)
    TryCand.Reason = NodeOrder;

  // PostGenericScheduler::tryCandidate end

  // Add powerpc post ra specific heuristic only when TryCand isn't selected or
  // selected as node order.
  if (TryCand.Reason != NodeOrder && TryCand.Reason != NoCand)
    return;

  // There are some benefits to schedule the ADDI as early as possible post ra
  // to avoid stalled by vector instructions which take up all the hw units.
  // And ADDI is usually used to post inc the loop indvar, which matters the
  // performance.
  if (biasAddiCandidate(Cand, TryCand))
    return;
}

void PPCPostRASchedStrategy::enterMBB(MachineBasicBlock *MBB) {
  // Custom PPC PostRA specific behavior here.
  PostGenericScheduler::enterMBB(MBB);
}

void PPCPostRASchedStrategy::leaveMBB() {
  // Custom PPC PostRA specific behavior here.
  PostGenericScheduler::leaveMBB();
}

void PPCPostRASchedStrategy::initialize(ScheduleDAGMI *Dag) {
  // Custom PPC PostRA specific initialization here.
  PostGenericScheduler::initialize(Dag);
}

SUnit *PPCPostRASchedStrategy::pickNode(bool &IsTopNode) {
  // Custom PPC PostRA specific scheduling here.
  return PostGenericScheduler::pickNode(IsTopNode);
}