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[SCEV] Improve handling of divisibility information from loop guards. #163021

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[SCEV] Improve handling of divisibility information from loop guards. #163021

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8 changes: 4 additions & 4 deletions llvm/include/llvm/Analysis/ScalarEvolution.h
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Original file line number Diff line number Diff line change
Expand Up @@ -763,6 +763,10 @@ class ScalarEvolution {
getUMinFromMismatchedTypes(SmallVectorImpl<const SCEV *> &Ops,
bool Sequential = false);

/// Try to match the pattern generated by getURemExpr(A, B). If successful,
/// Assign A and B to LHS and RHS, respectively.
LLVM_ABI bool matchURem(const SCEV *Expr, const SCEV *&LHS, const SCEV *&RHS);

/// Transitively follow the chain of pointer-type operands until reaching a
/// SCEV that does not have a single pointer operand. This returns a
/// SCEVUnknown pointer for well-formed pointer-type expressions, but corner
Expand Down Expand Up @@ -2316,10 +2320,6 @@ class ScalarEvolution {
/// an add rec on said loop.
void getUsedLoops(const SCEV *S, SmallPtrSetImpl<const Loop *> &LoopsUsed);

/// Try to match the pattern generated by getURemExpr(A, B). If successful,
/// Assign A and B to LHS and RHS, respectively.
LLVM_ABI bool matchURem(const SCEV *Expr, const SCEV *&LHS, const SCEV *&RHS);

/// Look for a SCEV expression with type `SCEVType` and operands `Ops` in
/// `UniqueSCEVs`. Return if found, else nullptr.
SCEV *findExistingSCEVInCache(SCEVTypes SCEVType, ArrayRef<const SCEV *> Ops);
Expand Down
290 changes: 165 additions & 125 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15557,6 +15557,123 @@ void ScalarEvolution::LoopGuards::collectFromPHI(
}
}

// Checks whether Expr is a non-negative constant, and Divisor is a positive
// constant, and returns their APInt in ExprVal and in DivisorVal.
static bool getNonNegExprAndPosDivisor(const SCEV *Expr, const SCEV *Divisor,
APInt &ExprVal, APInt &DivisorVal) {
auto *ConstExpr = dyn_cast<SCEVConstant>(Expr);
auto *ConstDivisor = dyn_cast<SCEVConstant>(Divisor);
if (!ConstExpr || !ConstDivisor)
return false;
ExprVal = ConstExpr->getAPInt();
DivisorVal = ConstDivisor->getAPInt();
return ExprVal.isNonNegative() && !DivisorVal.isNonPositive();
}

// Return a new SCEV that modifies \p Expr to the closest number divisible by
// \p Divisor and less than or equal to Expr.
// For now, only handle constant Expr and Divisor.
static const SCEV *getPreviousSCEVDivisibleByDivisor(const SCEV *Expr,
const SCEV *Divisor,
ScalarEvolution &SE) {
APInt ExprVal;
APInt DivisorVal;
if (!getNonNegExprAndPosDivisor(Expr, Divisor, ExprVal, DivisorVal))
return Expr;
APInt Rem = ExprVal.urem(DivisorVal);
// return the SCEV: Expr - Expr % Divisor
return SE.getConstant(ExprVal - Rem);
}

// Return a new SCEV that modifies \p Expr to the closest number divisible by
// \p Divisor and greater than or equal to Expr.
// For now, only handle constant Expr and Divisor.
static const SCEV *getNextSCEVDivisibleByDivisor(const SCEV *Expr,
const SCEV *Divisor,
ScalarEvolution &SE) {
APInt ExprVal;
APInt DivisorVal;
if (!getNonNegExprAndPosDivisor(Expr, Divisor, ExprVal, DivisorVal))
return Expr;
APInt Rem = ExprVal.urem(DivisorVal);
if (!Rem.isZero())
// return the SCEV: Expr + Divisor - Expr % Divisor
return SE.getConstant(ExprVal + DivisorVal - Rem);
return Expr;
}

static bool collectDivisibilityInformation(
ICmpInst::Predicate Predicate, const SCEV *LHS, const SCEV *RHS,
DenseMap<const SCEV *, const SCEV *> &DivInfo,
DenseMap<const SCEV *, const SCEV *> &Multiples, ScalarEvolution &SE) {
// If we have LHS == 0, check if LHS is computing a property of some unknown
// SCEV %v which we can rewrite %v to express explicitly.
if (Predicate != CmpInst::ICMP_EQ || !match(RHS, m_scev_Zero()))
return false;
// If LHS is A % B, i.e. A % B == 0, rewrite A to (A /u B) * B to
// explicitly express that.
const SCEV *URemLHS = nullptr;
const SCEV *URemRHS = nullptr;
if (!SE.matchURem(LHS, URemLHS, URemRHS))
return false;
if (const SCEVUnknown *LHSUnknown = dyn_cast<SCEVUnknown>(URemLHS)) {
const auto *Multiple = SE.getMulExpr(SE.getUDivExpr(LHS, URemRHS), URemRHS);
DivInfo[LHSUnknown] = Multiple;
Multiples[LHSUnknown] = URemRHS;
return true;
}
return false;
}

// Check if the condition is a divisibility guard (A % B == 0).
static bool isDivisibilityGuard(const SCEV *LHS, const SCEV *RHS,
ScalarEvolution &SE) {
const SCEV *X, *Y;
return SE.matchURem(LHS, X, Y) && RHS->isZero();
}

// Apply divisibility by \p Divisor on MinMaxExpr with constant values,
// recursively. This is done by aligning up/down the constant value to the
// Divisor.
static const SCEV *applyDivisibilityOnMinMaxExpr(const SCEV *MinMaxExpr,
const SCEV *Divisor,
ScalarEvolution &SE) {
// Return true if \p Expr is a MinMax SCEV expression with a non-negative
// constant operand. If so, return in \p SCTy the SCEV type and in \p RHS
// the non-constant operand and in \p LHS the constant operand.
auto IsMinMaxSCEVWithNonNegativeConstant =
[](const SCEV *Expr, SCEVTypes &SCTy, const SCEV *&LHS,
const SCEV *&RHS) {
if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr)) {
if (MinMax->getNumOperands() != 2)
return false;
if (auto *C = dyn_cast<SCEVConstant>(MinMax->getOperand(0))) {
if (C->getAPInt().isNegative())
return false;
SCTy = MinMax->getSCEVType();
LHS = MinMax->getOperand(0);
RHS = MinMax->getOperand(1);
return true;
}
}
return false;
};

const SCEV *MinMaxLHS = nullptr, *MinMaxRHS = nullptr;
SCEVTypes SCTy;
if (!IsMinMaxSCEVWithNonNegativeConstant(MinMaxExpr, SCTy, MinMaxLHS,
MinMaxRHS))
return MinMaxExpr;
auto IsMin = isa<SCEVSMinExpr>(MinMaxExpr) || isa<SCEVUMinExpr>(MinMaxExpr);
assert(SE.isKnownNonNegative(MinMaxLHS) && "Expected non-negative operand!");
auto *DivisibleExpr =
IsMin ? getPreviousSCEVDivisibleByDivisor(MinMaxLHS, Divisor, SE)
: getNextSCEVDivisibleByDivisor(MinMaxLHS, Divisor, SE);
SmallVector<const SCEV *> Ops = {
applyDivisibilityOnMinMaxExpr(MinMaxRHS, Divisor, SE), DivisibleExpr};
return SE.getMinMaxExpr(SCTy, Ops);
}

void ScalarEvolution::LoopGuards::collectFromBlock(
ScalarEvolution &SE, ScalarEvolution::LoopGuards &Guards,
const BasicBlock *Block, const BasicBlock *Pred,
Expand All @@ -15567,19 +15684,14 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
SmallVector<const SCEV *> ExprsToRewrite;
auto CollectCondition = [&](ICmpInst::Predicate Predicate, const SCEV *LHS,
const SCEV *RHS,
DenseMap<const SCEV *, const SCEV *>
&RewriteMap) {
DenseMap<const SCEV *, const SCEV *> &RewriteMap,
const DenseMap<const SCEV *, const SCEV *>
&DivInfo) {
// WARNING: It is generally unsound to apply any wrap flags to the proposed
// replacement SCEV which isn't directly implied by the structure of that
// SCEV. In particular, using contextual facts to imply flags is *NOT*
// legal. See the scoping rules for flags in the header to understand why.

// If LHS is a constant, apply information to the other expression.
if (isa<SCEVConstant>(LHS)) {
std::swap(LHS, RHS);
Predicate = CmpInst::getSwappedPredicate(Predicate);
}

// Check for a condition of the form (-C1 + X < C2). InstCombine will
// create this form when combining two checks of the form (X u< C2 + C1) and
// (X >=u C1).
Expand Down Expand Up @@ -15612,115 +15724,6 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
if (MatchRangeCheckIdiom())
return;

// Return true if \p Expr is a MinMax SCEV expression with a non-negative
// constant operand. If so, return in \p SCTy the SCEV type and in \p RHS
// the non-constant operand and in \p LHS the constant operand.
auto IsMinMaxSCEVWithNonNegativeConstant =
[&](const SCEV *Expr, SCEVTypes &SCTy, const SCEV *&LHS,
const SCEV *&RHS) {
if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr)) {
if (MinMax->getNumOperands() != 2)
return false;
if (auto *C = dyn_cast<SCEVConstant>(MinMax->getOperand(0))) {
if (C->getAPInt().isNegative())
return false;
SCTy = MinMax->getSCEVType();
LHS = MinMax->getOperand(0);
RHS = MinMax->getOperand(1);
return true;
}
}
return false;
};

// Checks whether Expr is a non-negative constant, and Divisor is a positive
// constant, and returns their APInt in ExprVal and in DivisorVal.
auto GetNonNegExprAndPosDivisor = [&](const SCEV *Expr, const SCEV *Divisor,
APInt &ExprVal, APInt &DivisorVal) {
auto *ConstExpr = dyn_cast<SCEVConstant>(Expr);
auto *ConstDivisor = dyn_cast<SCEVConstant>(Divisor);
if (!ConstExpr || !ConstDivisor)
return false;
ExprVal = ConstExpr->getAPInt();
DivisorVal = ConstDivisor->getAPInt();
return ExprVal.isNonNegative() && !DivisorVal.isNonPositive();
};

// Return a new SCEV that modifies \p Expr to the closest number divides by
// \p Divisor and greater or equal than Expr.
// For now, only handle constant Expr and Divisor.
auto GetNextSCEVDividesByDivisor = [&](const SCEV *Expr,
const SCEV *Divisor) {
APInt ExprVal;
APInt DivisorVal;
if (!GetNonNegExprAndPosDivisor(Expr, Divisor, ExprVal, DivisorVal))
return Expr;
APInt Rem = ExprVal.urem(DivisorVal);
if (!Rem.isZero())
// return the SCEV: Expr + Divisor - Expr % Divisor
return SE.getConstant(ExprVal + DivisorVal - Rem);
return Expr;
};

// Return a new SCEV that modifies \p Expr to the closest number divides by
// \p Divisor and less or equal than Expr.
// For now, only handle constant Expr and Divisor.
auto GetPreviousSCEVDividesByDivisor = [&](const SCEV *Expr,
const SCEV *Divisor) {
APInt ExprVal;
APInt DivisorVal;
if (!GetNonNegExprAndPosDivisor(Expr, Divisor, ExprVal, DivisorVal))
return Expr;
APInt Rem = ExprVal.urem(DivisorVal);
// return the SCEV: Expr - Expr % Divisor
return SE.getConstant(ExprVal - Rem);
};

// Apply divisibilty by \p Divisor on MinMaxExpr with constant values,
// recursively. This is done by aligning up/down the constant value to the
// Divisor.
std::function<const SCEV *(const SCEV *, const SCEV *)>
ApplyDivisibiltyOnMinMaxExpr = [&](const SCEV *MinMaxExpr,
const SCEV *Divisor) {
const SCEV *MinMaxLHS = nullptr, *MinMaxRHS = nullptr;
SCEVTypes SCTy;
if (!IsMinMaxSCEVWithNonNegativeConstant(MinMaxExpr, SCTy, MinMaxLHS,
MinMaxRHS))
return MinMaxExpr;
auto IsMin =
isa<SCEVSMinExpr>(MinMaxExpr) || isa<SCEVUMinExpr>(MinMaxExpr);
assert(SE.isKnownNonNegative(MinMaxLHS) &&
"Expected non-negative operand!");
auto *DivisibleExpr =
IsMin ? GetPreviousSCEVDividesByDivisor(MinMaxLHS, Divisor)
: GetNextSCEVDividesByDivisor(MinMaxLHS, Divisor);
SmallVector<const SCEV *> Ops = {
ApplyDivisibiltyOnMinMaxExpr(MinMaxRHS, Divisor), DivisibleExpr};
return SE.getMinMaxExpr(SCTy, Ops);
};

// If we have LHS == 0, check if LHS is computing a property of some unknown
// SCEV %v which we can rewrite %v to express explicitly.
if (Predicate == CmpInst::ICMP_EQ && match(RHS, m_scev_Zero())) {
// If LHS is A % B, i.e. A % B == 0, rewrite A to (A /u B) * B to
// explicitly express that.
const SCEV *URemLHS = nullptr;
const SCEV *URemRHS = nullptr;
if (SE.matchURem(LHS, URemLHS, URemRHS)) {
if (const SCEVUnknown *LHSUnknown = dyn_cast<SCEVUnknown>(URemLHS)) {
auto I = RewriteMap.find(LHSUnknown);
const SCEV *RewrittenLHS =
I != RewriteMap.end() ? I->second : LHSUnknown;
RewrittenLHS = ApplyDivisibiltyOnMinMaxExpr(RewrittenLHS, URemRHS);
const auto *Multiple =
SE.getMulExpr(SE.getUDivExpr(RewrittenLHS, URemRHS), URemRHS);
RewriteMap[LHSUnknown] = Multiple;
ExprsToRewrite.push_back(LHSUnknown);
return;
}
}
}

// Do not apply information for constants or if RHS contains an AddRec.
if (isa<SCEVConstant>(LHS) || SE.containsAddRecurrence(RHS))
return;
Expand Down Expand Up @@ -15751,7 +15754,11 @@ void ScalarEvolution::LoopGuards::collectFromBlock(

const SCEV *RewrittenLHS = GetMaybeRewritten(LHS);
const SCEV *DividesBy = nullptr;
const APInt &Multiple = SE.getConstantMultiple(RewrittenLHS);
// Apply divisibility information when computing the constant multiple.
LoopGuards DivGuards(SE);
DivGuards.RewriteMap = DivInfo;
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Is this copying the map?

const APInt &Multiple =
SE.getConstantMultiple(DivGuards.rewrite(RewrittenLHS));
if (!Multiple.isOne())
DividesBy = SE.getConstant(Multiple);

Expand All @@ -15775,21 +15782,23 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
[[fallthrough]];
case CmpInst::ICMP_SLT: {
RHS = SE.getMinusSCEV(RHS, One);
RHS = DividesBy ? GetPreviousSCEVDividesByDivisor(RHS, DividesBy) : RHS;
RHS = DividesBy ? getPreviousSCEVDivisibleByDivisor(RHS, DividesBy, SE)
: RHS;
break;
}
case CmpInst::ICMP_UGT:
case CmpInst::ICMP_SGT:
RHS = SE.getAddExpr(RHS, One);
RHS = DividesBy ? GetNextSCEVDividesByDivisor(RHS, DividesBy) : RHS;
RHS = DividesBy ? getNextSCEVDivisibleByDivisor(RHS, DividesBy, SE) : RHS;
break;
case CmpInst::ICMP_ULE:
case CmpInst::ICMP_SLE:
RHS = DividesBy ? GetPreviousSCEVDividesByDivisor(RHS, DividesBy) : RHS;
RHS = DividesBy ? getPreviousSCEVDivisibleByDivisor(RHS, DividesBy, SE)
: RHS;
break;
case CmpInst::ICMP_UGE:
case CmpInst::ICMP_SGE:
RHS = DividesBy ? GetNextSCEVDividesByDivisor(RHS, DividesBy) : RHS;
RHS = DividesBy ? getNextSCEVDivisibleByDivisor(RHS, DividesBy, SE) : RHS;
break;
default:
break;
Expand Down Expand Up @@ -15843,7 +15852,8 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
case CmpInst::ICMP_NE:
if (match(RHS, m_scev_Zero())) {
const SCEV *OneAlignedUp =
DividesBy ? GetNextSCEVDividesByDivisor(One, DividesBy) : One;
DividesBy ? getNextSCEVDivisibleByDivisor(One, DividesBy, SE)
: One;
To = SE.getUMaxExpr(FromRewritten, OneAlignedUp);
}
break;
Expand Down Expand Up @@ -15916,8 +15926,11 @@ void ScalarEvolution::LoopGuards::collectFromBlock(

// Now apply the information from the collected conditions to
// Guards.RewriteMap. Conditions are processed in reverse order, so the
// earliest conditions is processed first. This ensures the SCEVs with the
// earliest conditions is processed first, except guards with divisibility
// information, which are moved to the back. This ensures the SCEVs with the
// shortest dependency chains are constructed first.
SmallVector<std::tuple<CmpInst::Predicate, const SCEV *, const SCEV *>>
GuardsToProcess;
for (auto [Term, EnterIfTrue] : reverse(Terms)) {
SmallVector<Value *, 8> Worklist;
SmallPtrSet<Value *, 8> Visited;
Expand All @@ -15932,7 +15945,12 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
EnterIfTrue ? Cmp->getPredicate() : Cmp->getInversePredicate();
const auto *LHS = SE.getSCEV(Cmp->getOperand(0));
const auto *RHS = SE.getSCEV(Cmp->getOperand(1));
CollectCondition(Predicate, LHS, RHS, Guards.RewriteMap);
// If LHS is a constant, apply information to the other expression.
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And if LHS is not a constant, we make an arbitrary choice? Should we be using CompareSCEVComplexity or something?

if (isa<SCEVConstant>(LHS)) {
std::swap(LHS, RHS);
Predicate = CmpInst::getSwappedPredicate(Predicate);
}
GuardsToProcess.emplace_back(Predicate, LHS, RHS);
continue;
}

Expand All @@ -15945,6 +15963,28 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
}
}

// Process divisibility guards in reverse order to populate DivInfo early.
DenseMap<const SCEV *, const SCEV *> Multiples;
DenseMap<const SCEV *, const SCEV *> DivInfo;
for (const auto &[Predicate, LHS, RHS] : GuardsToProcess) {
if (!isDivisibilityGuard(LHS, RHS, SE))
continue;
collectDivisibilityInformation(Predicate, LHS, RHS, DivInfo, Multiples, SE);
}

for (const auto &[Predicate, LHS, RHS] : GuardsToProcess)
CollectCondition(Predicate, LHS, RHS, Guards.RewriteMap, DivInfo);

// Apply divisibility information last. This ensures it is applied to the
// outermost expression after other rewrites for the given value.
for (const auto &[K, V] : Multiples) {
Guards.RewriteMap[K] = SE.getMulExpr(
SE.getUDivExpr(applyDivisibilityOnMinMaxExpr(Guards.rewrite(K), V, SE),
V),
V);
ExprsToRewrite.push_back(K);
}

// Let the rewriter preserve NUW/NSW flags if the unsigned/signed ranges of
// the replacement expressions are contained in the ranges of the replaced
// expressions.
Expand Down
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