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[AMDGPU] Examine instructions in pending queues during scheduling #147653

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Oct 16, 2025
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[AMDGPU] Examine instructions in pending queues during scheduling #147653

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180 changes: 163 additions & 17 deletions llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,12 @@ static cl::opt<bool> GCNTrackers(
cl::desc("Use the AMDGPU specific RPTrackers during scheduling"),
cl::init(false));

static cl::opt<unsigned> PendingQueueLimit(
"amdgpu-scheduler-pending-queue-limit", cl::Hidden,
cl::desc(
"Max (Available+Pending) size to inspect pending queue (0 disables)"),
cl::init(256));

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
#define DUMP_MAX_REG_PRESSURE
static cl::opt<bool> PrintMaxRPRegUsageBeforeScheduler(
Expand Down Expand Up @@ -335,17 +341,52 @@ void GCNSchedStrategy::initCandidate(SchedCandidate &Cand, SUnit *SU,
}
}

static bool shouldCheckPending(SchedBoundary &Zone,
const TargetSchedModel *SchedModel) {
bool HasBufferedModel =
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Do we need this check?

SchedModel->hasInstrSchedModel() && SchedModel->getMicroOpBufferSize();
unsigned Combined = Zone.Available.size() + Zone.Pending.size();
return Combined <= PendingQueueLimit && HasBufferedModel;
}

static SUnit *pickOnlyChoice(SchedBoundary &Zone,
const TargetSchedModel *SchedModel) {
// pickOnlyChoice() releases pending instructions and checks for new hazards.
SUnit *OnlyChoice = Zone.pickOnlyChoice();
if (!shouldCheckPending(Zone, SchedModel) || Zone.Pending.empty())
return OnlyChoice;

return nullptr;
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We can probably pick the only pending instruction so long as we handle the cycle.

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I added then removed this since it requires a bunch of bookkeeping from the default pickOnlyChoice() to be reimplemented here for a very small gain in rare instances.

}

void GCNSchedStrategy::printCandidateDecision(const SchedCandidate &Current,
const SchedCandidate &Preferred) {
LLVM_DEBUG({
dbgs() << "Prefer:\t\t";
DAG->dumpNode(*Preferred.SU);

if (Current.SU) {
dbgs() << "Not:\t";
DAG->dumpNode(*Current.SU);
}

dbgs() << "Reason:\t\t";
traceCandidate(Preferred);
});
}

// This function is mostly cut and pasted from
// GenericScheduler::pickNodeFromQueue()
void GCNSchedStrategy::pickNodeFromQueue(SchedBoundary &Zone,
const CandPolicy &ZonePolicy,
const RegPressureTracker &RPTracker,
SchedCandidate &Cand,
SchedCandidate &Cand, bool &IsPending,
bool IsBottomUp) {
const SIRegisterInfo *SRI = static_cast<const SIRegisterInfo *>(TRI);
ArrayRef<unsigned> Pressure = RPTracker.getRegSetPressureAtPos();
unsigned SGPRPressure = 0;
unsigned VGPRPressure = 0;
IsPending = false;
if (DAG->isTrackingPressure()) {
if (!GCNTrackers) {
SGPRPressure = Pressure[AMDGPU::RegisterPressureSets::SReg_32];
Expand All @@ -358,8 +399,9 @@ void GCNSchedStrategy::pickNodeFromQueue(SchedBoundary &Zone,
VGPRPressure = T->getPressure().getArchVGPRNum();
}
}
ReadyQueue &Q = Zone.Available;
for (SUnit *SU : Q) {
LLVM_DEBUG(dbgs() << "Available Q:\n");
ReadyQueue &AQ = Zone.Available;
for (SUnit *SU : AQ) {

SchedCandidate TryCand(ZonePolicy);
initCandidate(TryCand, SU, Zone.isTop(), RPTracker, SRI, SGPRPressure,
Expand All @@ -371,40 +413,70 @@ void GCNSchedStrategy::pickNodeFromQueue(SchedBoundary &Zone,
// Initialize resource delta if needed in case future heuristics query it.
if (TryCand.ResDelta == SchedResourceDelta())
TryCand.initResourceDelta(Zone.DAG, SchedModel);
LLVM_DEBUG(printCandidateDecision(Cand, TryCand));
Cand.setBest(TryCand);
LLVM_DEBUG(traceCandidate(Cand));
} else {
printCandidateDecision(TryCand, Cand);
}
}

if (!shouldCheckPending(Zone, SchedModel))
return;

LLVM_DEBUG(dbgs() << "Pending Q:\n");
ReadyQueue &PQ = Zone.Pending;
for (SUnit *SU : PQ) {

SchedCandidate TryCand(ZonePolicy);
initCandidate(TryCand, SU, Zone.isTop(), RPTracker, SRI, SGPRPressure,
VGPRPressure, IsBottomUp);
// Pass SchedBoundary only when comparing nodes from the same boundary.
SchedBoundary *ZoneArg = Cand.AtTop == TryCand.AtTop ? &Zone : nullptr;
tryPendingCandidate(Cand, TryCand, ZoneArg);
if (TryCand.Reason != NoCand) {
// Initialize resource delta if needed in case future heuristics query it.
if (TryCand.ResDelta == SchedResourceDelta())
TryCand.initResourceDelta(Zone.DAG, SchedModel);
LLVM_DEBUG(printCandidateDecision(Cand, TryCand));
IsPending = true;
Cand.setBest(TryCand);
} else {
printCandidateDecision(TryCand, Cand);
}
}
}

// This function is mostly cut and pasted from
// GenericScheduler::pickNodeBidirectional()
SUnit *GCNSchedStrategy::pickNodeBidirectional(bool &IsTopNode) {
SUnit *GCNSchedStrategy::pickNodeBidirectional(bool &IsTopNode,
bool &PickedPending) {
// Schedule as far as possible in the direction of no choice. This is most
// efficient, but also provides the best heuristics for CriticalPSets.
if (SUnit *SU = Bot.pickOnlyChoice()) {
if (SUnit *SU = pickOnlyChoice(Bot, SchedModel)) {
IsTopNode = false;
return SU;
}
if (SUnit *SU = Top.pickOnlyChoice()) {
if (SUnit *SU = pickOnlyChoice(Top, SchedModel)) {
IsTopNode = true;
return SU;
}
// Set the bottom-up policy based on the state of the current bottom zone and
// the instructions outside the zone, including the top zone.
// Set the bottom-up policy based on the state of the current bottom zone
// and the instructions outside the zone, including the top zone.
CandPolicy BotPolicy;
setPolicy(BotPolicy, /*IsPostRA=*/false, Bot, &Top);
// Set the top-down policy based on the state of the current top zone and
// the instructions outside the zone, including the bottom zone.
CandPolicy TopPolicy;
setPolicy(TopPolicy, /*IsPostRA=*/false, Top, &Bot);

bool BotPending = false;
// See if BotCand is still valid (because we previously scheduled from Top).
LLVM_DEBUG(dbgs() << "Picking from Bot:\n");
if (!BotCand.isValid() || BotCand.SU->isScheduled ||
BotCand.Policy != BotPolicy) {
BotCand.reset(CandPolicy());
pickNodeFromQueue(Bot, BotPolicy, DAG->getBotRPTracker(), BotCand,
BotPending,
/*IsBottomUp=*/true);
assert(BotCand.Reason != NoCand && "failed to find the first candidate");
} else {
Expand All @@ -414,19 +486,22 @@ SUnit *GCNSchedStrategy::pickNodeBidirectional(bool &IsTopNode) {
SchedCandidate TCand;
TCand.reset(CandPolicy());
pickNodeFromQueue(Bot, BotPolicy, DAG->getBotRPTracker(), TCand,
BotPending,
/*IsBottomUp=*/true);
assert(TCand.SU == BotCand.SU &&
"Last pick result should correspond to re-picking right now");
}
#endif
}

bool TopPending = false;
// Check if the top Q has a better candidate.
LLVM_DEBUG(dbgs() << "Picking from Top:\n");
if (!TopCand.isValid() || TopCand.SU->isScheduled ||
TopCand.Policy != TopPolicy) {
TopCand.reset(CandPolicy());
pickNodeFromQueue(Top, TopPolicy, DAG->getTopRPTracker(), TopCand,
TopPending,
/*IsBottomUp=*/false);
assert(TopCand.Reason != NoCand && "failed to find the first candidate");
} else {
Expand All @@ -436,6 +511,7 @@ SUnit *GCNSchedStrategy::pickNodeBidirectional(bool &IsTopNode) {
SchedCandidate TCand;
TCand.reset(CandPolicy());
pickNodeFromQueue(Top, TopPolicy, DAG->getTopRPTracker(), TCand,
TopPending,
/*IsBottomUp=*/false);
assert(TCand.SU == TopCand.SU &&
"Last pick result should correspond to re-picking right now");
Expand All @@ -446,12 +522,21 @@ SUnit *GCNSchedStrategy::pickNodeBidirectional(bool &IsTopNode) {
// Pick best from BotCand and TopCand.
LLVM_DEBUG(dbgs() << "Top Cand: "; traceCandidate(TopCand);
dbgs() << "Bot Cand: "; traceCandidate(BotCand););
SchedCandidate Cand = BotCand;
TopCand.Reason = NoCand;
tryCandidate(Cand, TopCand, nullptr);
if (TopCand.Reason != NoCand) {
Cand.setBest(TopCand);
SchedCandidate Cand = BotPending ? TopCand : BotCand;
SchedCandidate TryCand = BotPending ? BotCand : TopCand;
PickedPending = BotPending && TopPending;

TryCand.Reason = NoCand;
if (BotPending || TopPending) {
PickedPending |= tryPendingCandidate(Cand, TopCand, nullptr);
} else {
tryCandidate(Cand, TryCand, nullptr);
}

if (TryCand.Reason != NoCand) {
Cand.setBest(TryCand);
}

LLVM_DEBUG(dbgs() << "Picking: "; traceCandidate(Cand););

IsTopNode = Cand.AtTop;
Expand All @@ -466,35 +551,55 @@ SUnit *GCNSchedStrategy::pickNode(bool &IsTopNode) {
Bot.Available.empty() && Bot.Pending.empty() && "ReadyQ garbage");
return nullptr;
}
bool PickedPending;
SUnit *SU;
do {
PickedPending = false;
if (RegionPolicy.OnlyTopDown) {
SU = Top.pickOnlyChoice();
SU = pickOnlyChoice(Top, SchedModel);
if (!SU) {
CandPolicy NoPolicy;
TopCand.reset(NoPolicy);
pickNodeFromQueue(Top, NoPolicy, DAG->getTopRPTracker(), TopCand,
PickedPending,
/*IsBottomUp=*/false);
assert(TopCand.Reason != NoCand && "failed to find a candidate");
SU = TopCand.SU;
}
IsTopNode = true;
} else if (RegionPolicy.OnlyBottomUp) {
SU = Bot.pickOnlyChoice();
SU = pickOnlyChoice(Bot, SchedModel);
if (!SU) {
CandPolicy NoPolicy;
BotCand.reset(NoPolicy);
pickNodeFromQueue(Bot, NoPolicy, DAG->getBotRPTracker(), BotCand,
PickedPending,
/*IsBottomUp=*/true);
assert(BotCand.Reason != NoCand && "failed to find a candidate");
SU = BotCand.SU;
}
IsTopNode = false;
} else {
SU = pickNodeBidirectional(IsTopNode);
SU = pickNodeBidirectional(IsTopNode, PickedPending);
}
} while (SU->isScheduled);

if (PickedPending) {
unsigned ReadyCycle = IsTopNode ? SU->TopReadyCycle : SU->BotReadyCycle;
SchedBoundary &Zone = IsTopNode ? Top : Bot;
unsigned CurrentCycle = Zone.getCurrCycle();
if (ReadyCycle > CurrentCycle)
Zone.bumpCycle(ReadyCycle);

// FIXME: checkHazard() doesn't give information about which cycle the
// hazard will resolve so just keep bumping the cycle by 1. This could be
// made more efficient if checkHazard() returned more details.
while (Zone.checkHazard(SU))
Zone.bumpCycle(Zone.getCurrCycle() + 1);

Zone.releasePending();
}

if (SU->isTopReady())
Top.removeReady(SU);
if (SU->isBottomReady())
Expand Down Expand Up @@ -540,6 +645,47 @@ GCNSchedStageID GCNSchedStrategy::getNextStage() const {
return *std::next(CurrentStage);
}

bool GCNSchedStrategy::tryPendingCandidate(SchedCandidate &Cand,
SchedCandidate &TryCand,
SchedBoundary *Zone) const {
// Initialize the candidate if needed.
if (!Cand.isValid()) {
TryCand.Reason = NodeOrder;
return true;
}

// 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 TryCand.Reason != NoCand;

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

// 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 TryCand.Reason != NoCand;

bool SameBoundary = Zone != nullptr;
if (SameBoundary) {
TryCand.initResourceDelta(DAG, SchedModel);
if (tryLess(TryCand.ResDelta.CritResources, Cand.ResDelta.CritResources,
TryCand, Cand, ResourceReduce))
return TryCand.Reason != NoCand;
if (tryGreater(TryCand.ResDelta.DemandedResources,
Cand.ResDelta.DemandedResources, TryCand, Cand,
ResourceDemand))
return TryCand.Reason != NoCand;
}

return false;
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Seems like there may be some cases which would benefit from using a stall cycle heuristic. Maybe as a follow-up.

}

GCNMaxOccupancySchedStrategy::GCNMaxOccupancySchedStrategy(
const MachineSchedContext *C, bool IsLegacyScheduler)
: GCNSchedStrategy(C) {
Expand Down
21 changes: 19 additions & 2 deletions llvm/lib/Target/AMDGPU/GCNSchedStrategy.h
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Original file line number Diff line number Diff line change
Expand Up @@ -44,17 +44,34 @@ raw_ostream &operator<<(raw_ostream &OS, const GCNSchedStageID &StageID);
/// heuristics to determine excess/critical pressure sets.
class GCNSchedStrategy : public GenericScheduler {
protected:
SUnit *pickNodeBidirectional(bool &IsTopNode);
SUnit *pickNodeBidirectional(bool &IsTopNode, bool &PickedPending);

void pickNodeFromQueue(SchedBoundary &Zone, const CandPolicy &ZonePolicy,
const RegPressureTracker &RPTracker,
SchedCandidate &Cand, bool IsBottomUp);
SchedCandidate &Cand, bool &IsPending,
bool IsBottomUp);

void initCandidate(SchedCandidate &Cand, SUnit *SU, bool AtTop,
const RegPressureTracker &RPTracker,
const SIRegisterInfo *SRI, unsigned SGPRPressure,
unsigned VGPRPressure, bool IsBottomUp);

/// Evaluates instructions in the pending queue using a subset of scheduling
/// heuristics.
///
/// Instructions that cannot be issued due to hardware constraints are placed
/// in the pending queue rather than the available queue, making them normally
/// invisible to scheduling heuristics. However, in certain scenarios (such as
/// avoiding register spilling), it may be beneficial to consider scheduling
/// these not-yet-ready instructions.
bool tryPendingCandidate(SchedCandidate &Cand, SchedCandidate &TryCand,
SchedBoundary *Zone) const;

#ifndef NDEBUG
void printCandidateDecision(const SchedCandidate &Current,
const SchedCandidate &Preferred);
#endif

std::vector<unsigned> Pressure;

std::vector<unsigned> MaxPressure;
Expand Down
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