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[ConstraintElimination] Remove ConstraintListTy (NFCI).
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This patch simplifies constraint handling by removing the
ConstraintListTy wrapper struct and moving the Preconditions directly
into ConstraintTy. This reduces the amount of memory needed for managing
constraints.

The only use case for ConstraintListTy was adding 2 constraints to model
ICMP_EQ conditions. But this can be handled by adding an IsEq flag. When
adding an equality constraint, we need to add the constraint and the
inverted constraint.
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@fhahn
fhahn committed Feb 18, 2022
1 parent 3ce2ee2 commit 00ab91b
Showing 1 changed file with 62 additions and 64 deletions.
126 changes: 62 additions & 64 deletions llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
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Expand Up @@ -89,53 +89,32 @@ struct PreconditionTy {

struct ConstraintTy {
SmallVector<int64_t, 8> Coefficients;
SmallVector<PreconditionTy, 2> Preconditions;

bool IsSigned;
bool IsSigned = false;
bool IsEq = false;

ConstraintTy() = default;

ConstraintTy(SmallVector<int64_t, 8> Coefficients, bool IsSigned)
: Coefficients(Coefficients), IsSigned(IsSigned) {}

unsigned size() const { return Coefficients.size(); }
};

/// Struct to manage a list of constraints with pre-conditions that must be
/// satisfied before using the constraints.
struct ConstraintListTy {
SmallVector<ConstraintTy, 4> Constraints;
SmallVector<PreconditionTy, 4> Preconditions;

ConstraintListTy() = default;

ConstraintListTy(ArrayRef<ConstraintTy> Constraints,
ArrayRef<PreconditionTy> Preconditions)
: Constraints(Constraints.begin(), Constraints.end()),
Preconditions(Preconditions.begin(), Preconditions.end()) {}

void mergeIn(const ConstraintListTy &Other) {
append_range(Constraints, Other.Constraints);
// TODO: Do smarter merges here, e.g. exclude duplicates.
append_range(Preconditions, Other.Preconditions);
}

unsigned size() const { return Constraints.size(); }

unsigned empty() const { return Constraints.empty(); }
unsigned empty() const { return Coefficients.empty(); }

/// Returns true if any constraint has a non-zero coefficient for any of the
/// newly added indices. Zero coefficients for new indices are removed. If it
/// returns true, no new variable need to be added to the system.
bool needsNewIndices(const DenseMap<Value *, unsigned> &NewIndices) {
assert(size() == 1);
for (unsigned I = 0; I < NewIndices.size(); ++I) {
int64_t Last = get(0).Coefficients.pop_back_val();
int64_t Last = Coefficients.pop_back_val();
if (Last != 0)
return true;
}
return false;
}

ConstraintTy &get(unsigned I) { return Constraints[I]; }

/// Returns true if all preconditions for this list of constraints are
/// satisfied given \p CS and the corresponding \p Value2Index mapping.
bool isValid(const ConstraintInfo &Info) const;
Expand Down Expand Up @@ -249,10 +228,11 @@ decompose(Value *V, SmallVector<PreconditionTy, 4> &Preconditions,
/// Turn a condition \p CmpI into a vector of constraints, using indices from \p
/// Value2Index. Additional indices for newly discovered values are added to \p
/// NewIndices.
static ConstraintListTy
static ConstraintTy
getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
const DenseMap<Value *, unsigned> &Value2Index,
DenseMap<Value *, unsigned> &NewIndices) {
bool IsEq = false;
// Try to convert Pred to one of ULE/SLT/SLE/SLT.
switch (Pred) {
case CmpInst::ICMP_UGT:
Expand All @@ -267,12 +247,8 @@ getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,
if (match(Op1, m_Zero())) {
Pred = CmpInst::ICMP_ULE;
} else {
auto A =
getConstraint(CmpInst::ICMP_UGE, Op0, Op1, Value2Index, NewIndices);
auto B =
getConstraint(CmpInst::ICMP_ULE, Op0, Op1, Value2Index, NewIndices);
A.mergeIn(B);
return A;
IsEq = true;
Pred = CmpInst::ICMP_ULE;
}
break;
case CmpInst::ICMP_NE:
Expand Down Expand Up @@ -330,7 +306,11 @@ getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,

// Build result constraint, by first adding all coefficients from A and then
// subtracting all coefficients from B.
SmallVector<int64_t, 8> R(Value2Index.size() + NewIndices.size() + 1, 0);
ConstraintTy Res(
SmallVector<int64_t, 8>(Value2Index.size() + NewIndices.size() + 1, 0),
IsSigned);
Res.IsEq = IsEq;
auto &R = Res.Coefficients;
for (const auto &KV : VariablesA)
R[GetOrAddIndex(KV.second)] += KV.first;

Expand All @@ -339,27 +319,30 @@ getConstraint(CmpInst::Predicate Pred, Value *Op0, Value *Op1,

R[0] = Offset1 + Offset2 +
(Pred == (IsSigned ? CmpInst::ICMP_SLT : CmpInst::ICMP_ULT) ? -1 : 0);
return {{{R, IsSigned}}, Preconditions};
Res.Preconditions = std::move(Preconditions);
return Res;
}

static ConstraintListTy getConstraint(CmpInst *Cmp, ConstraintInfo &Info,
DenseMap<Value *, unsigned> &NewIndices) {
static ConstraintTy getConstraint(CmpInst *Cmp, ConstraintInfo &Info,
DenseMap<Value *, unsigned> &NewIndices) {
return getConstraint(
Cmp->getPredicate(), Cmp->getOperand(0), Cmp->getOperand(1),
Info.getValue2Index(CmpInst::isSigned(Cmp->getPredicate())), NewIndices);
}

bool ConstraintListTy::isValid(const ConstraintInfo &Info) const {
return all_of(Preconditions, [&Info](const PreconditionTy &C) {
DenseMap<Value *, unsigned> NewIndices;
auto R = getConstraint(C.Pred, C.Op0, C.Op1,
Info.getValue2Index(CmpInst::isSigned(C.Pred)),
NewIndices);
// TODO: properly check NewIndices.
return NewIndices.empty() && R.Preconditions.empty() && R.size() == 1 &&
Info.getCS(CmpInst::isSigned(C.Pred))
.isConditionImplied(R.get(0).Coefficients);
});
bool ConstraintTy::isValid(const ConstraintInfo &Info) const {
return Coefficients.size() > 0 &&
all_of(Preconditions, [&Info](const PreconditionTy &C) {
DenseMap<Value *, unsigned> NewIndices;
auto R = getConstraint(
C.Pred, C.Op0, C.Op1,
Info.getValue2Index(CmpInst::isSigned(C.Pred)), NewIndices);
// TODO: properly check NewIndices.
return NewIndices.empty() && R.Preconditions.empty() && !R.IsEq &&
R.size() >= 2 &&
Info.getCS(CmpInst::isSigned(C.Pred))
.isConditionImplied(R.Coefficients);
});
}

namespace {
Expand Down Expand Up @@ -553,11 +536,12 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT) {

DenseMap<Value *, unsigned> NewIndices;
auto R = getConstraint(Cmp, Info, NewIndices);
if (!R.isValidSingle(Info) || R.needsNewIndices(NewIndices))
if (R.IsEq || R.size() < 2 || R.needsNewIndices(NewIndices) ||
!R.isValid(Info))
continue;

auto &CSToUse = Info.getCS(R.get(0).IsSigned);
if (CSToUse.isConditionImplied(R.get(0).Coefficients)) {
auto &CSToUse = Info.getCS(R.IsSigned);
if (CSToUse.isConditionImplied(R.Coefficients)) {
if (!DebugCounter::shouldExecute(EliminatedCounter))
continue;

Expand All @@ -578,7 +562,7 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT) {
Changed = true;
}
if (CSToUse.isConditionImplied(
ConstraintSystem::negate(R.get(0).Coefficients))) {
ConstraintSystem::negate(R.Coefficients))) {
if (!DebugCounter::shouldExecute(EliminatedCounter))
continue;

Expand Down Expand Up @@ -626,23 +610,37 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT) {

LLVM_DEBUG(dbgs() << "Adding " << *CB.Condition << " " << CB.Not << "\n");
bool Added = false;
for (auto &E : R.Constraints) {
auto &CSToUse = Info.getCS(E.IsSigned);
if (E.Coefficients.empty())
continue;
assert(CmpInst::isSigned(CB.Condition->getPredicate()) == R.IsSigned &&
"condition and constraint signs must match");
auto &CSToUse = Info.getCS(R.IsSigned);
if (R.Coefficients.empty())
continue;

Added |= CSToUse.addVariableRowFill(R.Coefficients);

// If R has been added to the system, queue it for removal once it goes
// out-of-scope.
if (Added) {
for (auto &KV : NewIndices)
Info.getValue2Index(R.IsSigned).insert(KV);

LLVM_DEBUG({
dbgs() << " constraint: ";
dumpWithNames(E, Info.getValue2Index(E.IsSigned));
dumpWithNames(R, Info.getValue2Index(R.IsSigned));
});

Added |= CSToUse.addVariableRowFill(E.Coefficients);
DFSInStack.emplace_back(CB.NumIn, CB.NumOut, CB.Condition, CB.Not,
R.IsSigned);

if (R.IsEq) {
// Also add the inverted constraint for equality constraints.
for (auto &Coeff : R.Coefficients)
Coeff *= -1;
CSToUse.addVariableRowFill(R.Coefficients);

// If R has been added to the system, queue it for removal once it goes
// out-of-scope.
if (Added)
DFSInStack.emplace_back(CB.NumIn, CB.NumOut, CB.Condition, CB.Not,
E.IsSigned);
R.IsSigned);
}
}
}

Expand Down

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