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[SCCP] Improve worklist management #146321

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Merged
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Jun 30, 2025
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[SCCP] Improve worklist management #146321

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[SCCP] Improve worklist management
nikic Jun 29, 2025
0750ab4
count -> contains
nikic Jun 30, 2025
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167 changes: 66 additions & 101 deletions llvm/lib/Transforms/Utils/SCCPSolver.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -426,7 +426,10 @@ void SCCPSolver::inferArgAttributes() const {
class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
const DataLayout &DL;
std::function<const TargetLibraryInfo &(Function &)> GetTLI;
SmallPtrSet<BasicBlock *, 8> BBExecutable; // The BBs that are executable.
/// Basic blocks that are executable (but may not have been visited yet).
SmallPtrSet<BasicBlock *, 8> BBExecutable;
/// Basic blocks that are executable and have been visited at least once.
SmallPtrSet<BasicBlock *, 8> BBVisited;
DenseMap<Value *, ValueLatticeElement>
ValueState; // The state each value is in.

Expand Down Expand Up @@ -466,15 +469,14 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
/// constants.
SmallPtrSet<Function *, 16> TrackingIncomingArguments;

/// The reason for two worklists is that overdefined is the lowest state
/// on the lattice, and moving things to overdefined as fast as possible
/// makes SCCP converge much faster.
///
/// By having a separate worklist, we accomplish this because everything
/// possibly overdefined will become overdefined at the soonest possible
/// point.
SmallVector<Value *, 64> OverdefinedInstWorkList;
SmallVector<Value *, 64> InstWorkList;
/// Worklist of instructions to re-visit. This only includes instructions
/// in blocks that have already been visited at least once.
SmallSetVector<Instruction *, 16> InstWorkList;

/// Current instruction while visiting a block for the first time, used to
/// avoid unnecessary instruction worklist insertions. Null if an instruction
/// is visited outside a whole-block visitation.
Instruction *CurI = nullptr;

// The BasicBlock work list
SmallVector<BasicBlock *, 64> BBWorkList;
Expand All @@ -497,12 +499,15 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
return dyn_cast_or_null<ConstantInt>(getConstant(IV, Ty));
}

// pushToWorkList - Helper for markConstant/markOverdefined
void pushToWorkList(ValueLatticeElement &IV, Value *V);
/// Push instruction \p I to the worklist.
void pushToWorkList(Instruction *I);

/// Push users of value \p V to the worklist.
void pushUsersToWorkList(Value *V);

// Helper to push \p V to the worklist, after updating it to \p IV. Also
// prints a debug message with the updated value.
void pushToWorkListMsg(ValueLatticeElement &IV, Value *V);
/// Like pushUsersToWorkList(), but also prints a debug message with the
/// updated value.
void pushUsersToWorkListMsg(ValueLatticeElement &IV, Value *V);

// markConstant - Make a value be marked as "constant". If the value
// is not already a constant, add it to the instruction work list so that
Expand Down Expand Up @@ -661,46 +666,9 @@ class SCCPInstVisitor : public InstVisitor<SCCPInstVisitor> {
// successors are reachable from a given terminator instruction.
void getFeasibleSuccessors(Instruction &TI, SmallVectorImpl<bool> &Succs);

// OperandChangedState - This method is invoked on all of the users of an
// instruction that was just changed state somehow. Based on this
// information, we need to update the specified user of this instruction.
void operandChangedState(Instruction *I) {
if (BBExecutable.count(I->getParent())) // Inst is executable?
visit(*I);
}

// Add U as additional user of V.
void addAdditionalUser(Value *V, User *U) { AdditionalUsers[V].insert(U); }

// Mark I's users as changed, including AdditionalUsers.
void markUsersAsChanged(Value *I) {
// Functions include their arguments in the use-list. Changed function
// values mean that the result of the function changed. We only need to
// update the call sites with the new function result and do not have to
// propagate the call arguments.
if (isa<Function>(I)) {
for (User *U : I->users()) {
if (auto *CB = dyn_cast<CallBase>(U))
handleCallResult(*CB);
}
} else {
for (User *U : I->users())
if (auto *UI = dyn_cast<Instruction>(U))
operandChangedState(UI);
}

auto Iter = AdditionalUsers.find(I);
if (Iter != AdditionalUsers.end()) {
// Copy additional users before notifying them of changes, because new
// users may be added, potentially invalidating the iterator.
SmallVector<Instruction *, 2> ToNotify;
for (User *U : Iter->second)
if (auto *UI = dyn_cast<Instruction>(U))
ToNotify.push_back(UI);
for (Instruction *UI : ToNotify)
operandChangedState(UI);
}
}
void handleCallOverdefined(CallBase &CB);
void handleCallResult(CallBase &CB);
void handleCallArguments(CallBase &CB);
Expand Down Expand Up @@ -985,27 +953,49 @@ bool SCCPInstVisitor::markBlockExecutable(BasicBlock *BB) {
return true;
}

void SCCPInstVisitor::pushToWorkList(ValueLatticeElement &IV, Value *V) {
if (IV.isOverdefined()) {
if (OverdefinedInstWorkList.empty() || OverdefinedInstWorkList.back() != V)
OverdefinedInstWorkList.push_back(V);
void SCCPInstVisitor::pushToWorkList(Instruction *I) {
// If we're currently visiting a block, do not push any instructions in the
// same blocks that are after the current one, as they will be visited
// anyway. We do have to push updates to earlier instructions (e.g. phi
// nodes or loads of tracked globals).
if (CurI && I->getParent() == CurI->getParent() && !I->comesBefore(CurI))
return;
// Only push instructions in already visited blocks. Otherwise we'll handle
// it when we visit the block for the first time.
if (BBVisited.contains(I->getParent()))
InstWorkList.insert(I);
}

void SCCPInstVisitor::pushUsersToWorkList(Value *V) {
for (User *U : V->users())
if (auto *UI = dyn_cast<Instruction>(U))
pushToWorkList(UI);
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Is it still valuable to have a separate worklist for insts that contain an operand just marked as Overdefined?

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I see a negative effects from using a separate overdefined worklist: https://llvm-compile-time-tracker.com/compare.php?from=8f1fac228d7ebbddd8a2aa00e47ee3f5712db125&to=715c96e212becce2bb05cffd7a468891547ef992&stat=instructions:u


auto Iter = AdditionalUsers.find(V);
if (Iter != AdditionalUsers.end()) {
// Copy additional users before notifying them of changes, because new
// users may be added, potentially invalidating the iterator.
SmallVector<Instruction *, 2> ToNotify;
for (User *U : Iter->second)
if (auto *UI = dyn_cast<Instruction>(U))
ToNotify.push_back(UI);
for (Instruction *UI : ToNotify)
pushToWorkList(UI);
}
if (InstWorkList.empty() || InstWorkList.back() != V)
InstWorkList.push_back(V);
}

void SCCPInstVisitor::pushToWorkListMsg(ValueLatticeElement &IV, Value *V) {
void SCCPInstVisitor::pushUsersToWorkListMsg(ValueLatticeElement &IV,
Value *V) {
LLVM_DEBUG(dbgs() << "updated " << IV << ": " << *V << '\n');
pushToWorkList(IV, V);
pushUsersToWorkList(V);
}

bool SCCPInstVisitor::markConstant(ValueLatticeElement &IV, Value *V,
Constant *C, bool MayIncludeUndef) {
if (!IV.markConstant(C, MayIncludeUndef))
return false;
LLVM_DEBUG(dbgs() << "markConstant: " << *C << ": " << *V << '\n');
pushToWorkList(IV, V);
pushUsersToWorkList(V);
return true;
}

Expand All @@ -1014,7 +1004,7 @@ bool SCCPInstVisitor::markNotConstant(ValueLatticeElement &IV, Value *V,
if (!IV.markNotConstant(C))
return false;
LLVM_DEBUG(dbgs() << "markNotConstant: " << *C << ": " << *V << '\n');
pushToWorkList(IV, V);
pushUsersToWorkList(V);
return true;
}

Expand All @@ -1023,7 +1013,7 @@ bool SCCPInstVisitor::markConstantRange(ValueLatticeElement &IV, Value *V,
if (!IV.markConstantRange(CR))
return false;
LLVM_DEBUG(dbgs() << "markConstantRange: " << CR << ": " << *V << '\n');
pushToWorkList(IV, V);
pushUsersToWorkList(V);
return true;
}

Expand All @@ -1036,7 +1026,7 @@ bool SCCPInstVisitor::markOverdefined(ValueLatticeElement &IV, Value *V) {
<< "Function '" << F->getName() << "'\n";
else dbgs() << *V << '\n');
// Only instructions go on the work list
pushToWorkList(IV, V);
pushUsersToWorkList(V);
return true;
}

Expand Down Expand Up @@ -1144,7 +1134,7 @@ bool SCCPInstVisitor::mergeInValue(ValueLatticeElement &IV, Value *V,
ValueLatticeElement MergeWithV,
ValueLatticeElement::MergeOptions Opts) {
if (IV.mergeIn(MergeWithV, Opts)) {
pushToWorkList(IV, V);
pushUsersToWorkList(V);
LLVM_DEBUG(dbgs() << "Merged " << MergeWithV << " into " << *V << " : "
<< IV << "\n");
return true;
Expand All @@ -1164,7 +1154,7 @@ bool SCCPInstVisitor::markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest) {
<< " -> " << Dest->getName() << '\n');

for (PHINode &PN : Dest->phis())
visitPHINode(PN);
pushToWorkList(&PN);
}
return true;
}
Expand Down Expand Up @@ -1540,7 +1530,7 @@ void SCCPInstVisitor::visitSelectInst(SelectInst &I) {
bool Changed = State.mergeIn(TVal);
Changed |= State.mergeIn(FVal);
if (Changed)
pushToWorkListMsg(State, &I);
pushUsersToWorkListMsg(State, &I);
}

// Handle Unary Operators.
Expand Down Expand Up @@ -2039,53 +2029,28 @@ void SCCPInstVisitor::handleCallResult(CallBase &CB) {

void SCCPInstVisitor::solve() {
// Process the work lists until they are empty!
while (!BBWorkList.empty() || !InstWorkList.empty() ||
!OverdefinedInstWorkList.empty()) {
// Process the overdefined instruction's work list first, which drives other
// things to overdefined more quickly.
while (!OverdefinedInstWorkList.empty()) {
Value *I = OverdefinedInstWorkList.pop_back_val();
Invalidated.erase(I);

LLVM_DEBUG(dbgs() << "\nPopped off OI-WL: " << *I << '\n');

// "I" got into the work list because it either made the transition from
// bottom to constant, or to overdefined.
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined
// Update all of the users of this instruction's value.
//
markUsersAsChanged(I);
}

while (!BBWorkList.empty() || !InstWorkList.empty()) {
// Process the instruction work list.
while (!InstWorkList.empty()) {
Value *I = InstWorkList.pop_back_val();
Instruction *I = InstWorkList.pop_back_val();
Invalidated.erase(I);

LLVM_DEBUG(dbgs() << "\nPopped off I-WL: " << *I << '\n');

// "I" got into the work list because it made the transition from undef to
// constant.
//
// Anything on this worklist that is overdefined need not be visited
// since all of its users will have already been marked as overdefined.
// Update all of the users of this instruction's value.
//
if (I->getType()->isStructTy() || !getValueState(I).isOverdefined())
markUsersAsChanged(I);
visit(I);
}

// Process the basic block work list.
while (!BBWorkList.empty()) {
BasicBlock *BB = BBWorkList.pop_back_val();
BBVisited.insert(BB);

LLVM_DEBUG(dbgs() << "\nPopped off BBWL: " << *BB << '\n');

// Notify all instructions in this basic block that they are newly
// executable.
visit(BB);
for (Instruction &I : *BB) {
CurI = &I;
visit(I);
}
CurI = nullptr;
}
}
}
Expand Down
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