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[LAA,Loads] Use loop guards and max BTC if needed when checking deref. #155672

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Sep 2, 2025
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[LAA,Loads] Use loop guards and max BTC if needed when checking deref. #155672

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22 changes: 16 additions & 6 deletions llvm/include/llvm/Analysis/LoopAccessAnalysis.h
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Original file line number Diff line number Diff line change
Expand Up @@ -183,10 +183,12 @@ class MemoryDepChecker {
MemoryDepChecker(PredicatedScalarEvolution &PSE, AssumptionCache *AC,
DominatorTree *DT, const Loop *L,
const DenseMap<Value *, const SCEV *> &SymbolicStrides,
unsigned MaxTargetVectorWidthInBits)
unsigned MaxTargetVectorWidthInBits,
std::optional<ScalarEvolution::LoopGuards> &LoopGuards)
: PSE(PSE), AC(AC), DT(DT), InnermostLoop(L),
SymbolicStrides(SymbolicStrides),
MaxTargetVectorWidthInBits(MaxTargetVectorWidthInBits) {}
MaxTargetVectorWidthInBits(MaxTargetVectorWidthInBits),
LoopGuards(LoopGuards) {}

/// Register the location (instructions are given increasing numbers)
/// of a write access.
Expand Down Expand Up @@ -373,7 +375,7 @@ class MemoryDepChecker {
PointerBounds;

/// Cache for the loop guards of InnermostLoop.
std::optional<ScalarEvolution::LoopGuards> LoopGuards;
std::optional<ScalarEvolution::LoopGuards> &LoopGuards;

/// Check whether there is a plausible dependence between the two
/// accesses.
Expand Down Expand Up @@ -531,8 +533,9 @@ class RuntimePointerChecking {
AliasSetId(AliasSetId), Expr(Expr), NeedsFreeze(NeedsFreeze) {}
};

RuntimePointerChecking(MemoryDepChecker &DC, ScalarEvolution *SE)
: DC(DC), SE(SE) {}
RuntimePointerChecking(MemoryDepChecker &DC, ScalarEvolution *SE,
std::optional<ScalarEvolution::LoopGuards> &LoopGuards)
: DC(DC), SE(SE), LoopGuards(LoopGuards) {}

/// Reset the state of the pointer runtime information.
void reset() {
Expand Down Expand Up @@ -646,6 +649,9 @@ class RuntimePointerChecking {
/// Holds a pointer to the ScalarEvolution analysis.
ScalarEvolution *SE;

/// Cache for the loop guards of the loop.
std::optional<ScalarEvolution::LoopGuards> &LoopGuards;

/// Set of run-time checks required to establish independence of
/// otherwise may-aliasing pointers in the loop.
SmallVector<RuntimePointerCheck, 4> Checks;
Expand Down Expand Up @@ -821,6 +827,9 @@ class LoopAccessInfo {

Loop *TheLoop;

/// Cache for the loop guards of TheLoop.
std::optional<ScalarEvolution::LoopGuards> LoopGuards;

/// Determines whether we should generate partial runtime checks when not all
/// memory accesses could be analyzed.
bool AllowPartial;
Expand Down Expand Up @@ -938,7 +947,8 @@ LLVM_ABI std::pair<const SCEV *, const SCEV *> getStartAndEndForAccess(
const SCEV *MaxBTC, ScalarEvolution *SE,
DenseMap<std::pair<const SCEV *, Type *>,
std::pair<const SCEV *, const SCEV *>> *PointerBounds,
DominatorTree *DT, AssumptionCache *AC);
DominatorTree *DT, AssumptionCache *AC,
std::optional<ScalarEvolution::LoopGuards> &LoopGuards);

class LoopAccessInfoManager {
/// The cache.
Expand Down
29 changes: 10 additions & 19 deletions llvm/lib/Analysis/Loads.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -26,10 +26,6 @@

using namespace llvm;

static cl::opt<bool>
UseSymbolicMaxBTCForDerefInLoop("use-symbolic-maxbtc-deref-loop",
cl::init(false));

static bool isAligned(const Value *Base, Align Alignment,
const DataLayout &DL) {
return Base->getPointerAlignment(DL) >= Alignment;
Expand Down Expand Up @@ -335,18 +331,10 @@ bool llvm::isDereferenceableAndAlignedInLoop(
: SE.getBackedgeTakenCount(L);
if (isa<SCEVCouldNotCompute>(MaxBECount))
return false;

if (isa<SCEVCouldNotCompute>(BECount) && !UseSymbolicMaxBTCForDerefInLoop) {
// TODO: Support symbolic max backedge taken counts for loops without
// computable backedge taken counts.
MaxBECount =
Predicates
? SE.getPredicatedConstantMaxBackedgeTakenCount(L, *Predicates)
: SE.getConstantMaxBackedgeTakenCount(L);
}

const auto &[AccessStart, AccessEnd] = getStartAndEndForAccess(
L, PtrScev, LI->getType(), BECount, MaxBECount, &SE, nullptr, &DT, AC);
std::optional<ScalarEvolution::LoopGuards> LoopGuards;
const auto &[AccessStart, AccessEnd] =
getStartAndEndForAccess(L, PtrScev, LI->getType(), BECount, MaxBECount,
&SE, nullptr, &DT, AC, LoopGuards);
if (isa<SCEVCouldNotCompute>(AccessStart) ||
isa<SCEVCouldNotCompute>(AccessEnd))
return false;
Expand All @@ -355,10 +343,13 @@ bool llvm::isDereferenceableAndAlignedInLoop(
const SCEV *PtrDiff = SE.getMinusSCEV(AccessEnd, AccessStart);
if (isa<SCEVCouldNotCompute>(PtrDiff))
return false;
ScalarEvolution::LoopGuards LoopGuards =
ScalarEvolution::LoopGuards::collect(AddRec->getLoop(), SE);

if (!LoopGuards)
LoopGuards.emplace(
ScalarEvolution::LoopGuards::collect(AddRec->getLoop(), SE));

APInt MaxPtrDiff =
SE.getUnsignedRangeMax(SE.applyLoopGuards(PtrDiff, LoopGuards));
SE.getUnsignedRangeMax(SE.applyLoopGuards(PtrDiff, *LoopGuards));

Value *Base = nullptr;
APInt AccessSize;
Expand Down
66 changes: 39 additions & 27 deletions llvm/lib/Analysis/LoopAccessAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -193,30 +193,28 @@ RuntimeCheckingPtrGroup::RuntimeCheckingPtrGroup(
/// Returns \p A + \p B, if it is guaranteed not to unsigned wrap. Otherwise
/// return nullptr. \p A and \p B must have the same type.
static const SCEV *addSCEVNoOverflow(const SCEV *A, const SCEV *B,
ScalarEvolution &SE,
const Instruction *CtxI) {
if (!SE.willNotOverflow(Instruction::Add, /*IsSigned=*/false, A, B, CtxI))
ScalarEvolution &SE) {
if (!SE.willNotOverflow(Instruction::Add, /*IsSigned=*/false, A, B))
return nullptr;
return SE.getAddExpr(A, B);
}

/// Returns \p A * \p B, if it is guaranteed not to unsigned wrap. Otherwise
/// return nullptr. \p A and \p B must have the same type.
static const SCEV *mulSCEVOverflow(const SCEV *A, const SCEV *B,
ScalarEvolution &SE,
const Instruction *CtxI) {
if (!SE.willNotOverflow(Instruction::Mul, /*IsSigned=*/false, A, B, CtxI))
ScalarEvolution &SE) {
if (!SE.willNotOverflow(Instruction::Mul, /*IsSigned=*/false, A, B))
return nullptr;
return SE.getMulExpr(A, B);
}

/// Return true, if evaluating \p AR at \p MaxBTC cannot wrap, because \p AR at
/// \p MaxBTC is guaranteed inbounds of the accessed object.
static bool
evaluatePtrAddRecAtMaxBTCWillNotWrap(const SCEVAddRecExpr *AR,
const SCEV *MaxBTC, const SCEV *EltSize,
ScalarEvolution &SE, const DataLayout &DL,
DominatorTree *DT, AssumptionCache *AC) {
static bool evaluatePtrAddRecAtMaxBTCWillNotWrap(
const SCEVAddRecExpr *AR, const SCEV *MaxBTC, const SCEV *EltSize,
ScalarEvolution &SE, const DataLayout &DL, DominatorTree *DT,
AssumptionCache *AC,
std::optional<ScalarEvolution::LoopGuards> &LoopGuards) {
auto *PointerBase = SE.getPointerBase(AR->getStart());
auto *StartPtr = dyn_cast<SCEVUnknown>(PointerBase);
if (!StartPtr)
Expand All @@ -234,12 +232,11 @@ evaluatePtrAddRecAtMaxBTCWillNotWrap(const SCEVAddRecExpr *AR,
Type *WiderTy = SE.getWiderType(MaxBTC->getType(), Step->getType());
const SCEV *DerefBytesSCEV = SE.getConstant(WiderTy, DerefBytes);

// Context which dominates the entire loop.
auto *CtxI = L->getLoopPredecessor()->getTerminator();
// Check if we have a suitable dereferencable assumption we can use.
if (!StartPtrV->canBeFreed()) {
RetainedKnowledge DerefRK = getKnowledgeValidInContext(
StartPtrV, {Attribute::Dereferenceable}, *AC, CtxI, DT);
StartPtrV, {Attribute::Dereferenceable}, *AC,
L->getLoopPredecessor()->getTerminator(), DT);
if (DerefRK) {
DerefBytesSCEV = SE.getUMaxExpr(
DerefBytesSCEV, SE.getConstant(WiderTy, DerefRK.ArgValue));
Expand All @@ -263,23 +260,36 @@ evaluatePtrAddRecAtMaxBTCWillNotWrap(const SCEVAddRecExpr *AR,
SE.getMinusSCEV(AR->getStart(), StartPtr), WiderTy);

const SCEV *OffsetAtLastIter =
mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE, CtxI);
if (!OffsetAtLastIter)
return false;
mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
if (!OffsetAtLastIter) {
// Re-try with constant max backedge-taken count if using the symbolic one
// failed.
MaxBTC = SE.getNoopOrZeroExtend(
SE.getConstantMaxBackedgeTakenCount(AR->getLoop()), WiderTy);
OffsetAtLastIter =
mulSCEVOverflow(MaxBTC, SE.getAbsExpr(Step, /*IsNSW=*/false), SE);
Comment on lines +265 to +270
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@fhahn fhahn Aug 27, 2025

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There are some cases where we cannot apply loop guards effectively to MaxBTC (if it is an AddRec), but constant BTC computation uses them to get a tighter upper bound.

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@annamthomas annamthomas Aug 27, 2025

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why not use the CtxI above as done previously? Would that along with the mulSCEVOverflow change allow to get whats needed?

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@fhahn fhahn Aug 28, 2025

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Unfortunately that's not enough in some cases, for example https://github.com/llvm/llvm-project/blob/main/llvm/test/Transforms/LoopVectorize/single_early_exit.ll#L333 where the trip count of the inner loop depends on an induction from the outer loop

if (!OffsetAtLastIter)
return false;
}

const SCEV *OffsetEndBytes = addSCEVNoOverflow(
OffsetAtLastIter, SE.getNoopOrZeroExtend(EltSize, WiderTy), SE, CtxI);
OffsetAtLastIter, SE.getNoopOrZeroExtend(EltSize, WiderTy), SE);
if (!OffsetEndBytes)
return false;

if (IsKnownNonNegative) {
// For positive steps, check if
// (AR->getStart() - StartPtr) + (MaxBTC * Step) + EltSize <= DerefBytes,
// while making sure none of the computations unsigned wrap themselves.
const SCEV *EndBytes =
addSCEVNoOverflow(StartOffset, OffsetEndBytes, SE, CtxI);
const SCEV *EndBytes = addSCEVNoOverflow(StartOffset, OffsetEndBytes, SE);
if (!EndBytes)
return false;

if (!LoopGuards)
LoopGuards.emplace(
ScalarEvolution::LoopGuards::collect(AR->getLoop(), SE));

EndBytes = SE.applyLoopGuards(EndBytes, *LoopGuards);
return SE.isKnownPredicate(CmpInst::ICMP_ULE, EndBytes, DerefBytesSCEV);
}

Expand All @@ -296,7 +306,8 @@ std::pair<const SCEV *, const SCEV *> llvm::getStartAndEndForAccess(
const SCEV *MaxBTC, ScalarEvolution *SE,
DenseMap<std::pair<const SCEV *, Type *>,
std::pair<const SCEV *, const SCEV *>> *PointerBounds,
DominatorTree *DT, AssumptionCache *AC) {
DominatorTree *DT, AssumptionCache *AC,
std::optional<ScalarEvolution::LoopGuards> &LoopGuards) {
std::pair<const SCEV *, const SCEV *> *PtrBoundsPair;
if (PointerBounds) {
auto [Iter, Ins] = PointerBounds->insert(
Expand Down Expand Up @@ -332,7 +343,7 @@ std::pair<const SCEV *, const SCEV *> llvm::getStartAndEndForAccess(
// separately checks that accesses cannot not wrap, so unsigned max
// represents an upper bound.
if (evaluatePtrAddRecAtMaxBTCWillNotWrap(AR, MaxBTC, EltSizeSCEV, *SE, DL,
DT, AC)) {
DT, AC, LoopGuards)) {
ScEnd = AR->evaluateAtIteration(MaxBTC, *SE);
} else {
ScEnd = SE->getAddExpr(
Expand Down Expand Up @@ -381,7 +392,7 @@ void RuntimePointerChecking::insert(Loop *Lp, Value *Ptr, const SCEV *PtrExpr,
const SCEV *BTC = PSE.getBackedgeTakenCount();
const auto &[ScStart, ScEnd] = getStartAndEndForAccess(
Lp, PtrExpr, AccessTy, BTC, SymbolicMaxBTC, PSE.getSE(),
&DC.getPointerBounds(), DC.getDT(), DC.getAC());
&DC.getPointerBounds(), DC.getDT(), DC.getAC(), LoopGuards);
assert(!isa<SCEVCouldNotCompute>(ScStart) &&
!isa<SCEVCouldNotCompute>(ScEnd) &&
"must be able to compute both start and end expressions");
Expand Down Expand Up @@ -1987,13 +1998,13 @@ bool MemoryDepChecker::areAccessesCompletelyBeforeOrAfter(const SCEV *Src,
ScalarEvolution &SE = *PSE.getSE();
const auto &[SrcStart_, SrcEnd_] =
getStartAndEndForAccess(InnermostLoop, Src, SrcTy, BTC, SymbolicMaxBTC,
&SE, &PointerBounds, DT, AC);
&SE, &PointerBounds, DT, AC, LoopGuards);
if (isa<SCEVCouldNotCompute>(SrcStart_) || isa<SCEVCouldNotCompute>(SrcEnd_))
return false;

const auto &[SinkStart_, SinkEnd_] =
getStartAndEndForAccess(InnermostLoop, Sink, SinkTy, BTC, SymbolicMaxBTC,
&SE, &PointerBounds, DT, AC);
&SE, &PointerBounds, DT, AC, LoopGuards);
if (isa<SCEVCouldNotCompute>(SinkStart_) ||
isa<SCEVCouldNotCompute>(SinkEnd_))
return false;
Expand Down Expand Up @@ -3040,8 +3051,9 @@ LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
TTI->getRegisterBitWidth(TargetTransformInfo::RGK_FixedWidthVector) * 2;

DepChecker = std::make_unique<MemoryDepChecker>(
*PSE, AC, DT, L, SymbolicStrides, MaxTargetVectorWidthInBits);
PtrRtChecking = std::make_unique<RuntimePointerChecking>(*DepChecker, SE);
*PSE, AC, DT, L, SymbolicStrides, MaxTargetVectorWidthInBits, LoopGuards);
PtrRtChecking =
std::make_unique<RuntimePointerChecking>(*DepChecker, SE, LoopGuards);
if (canAnalyzeLoop())
CanVecMem = analyzeLoop(AA, LI, TLI, DT);
}
Expand Down
14 changes: 3 additions & 11 deletions llvm/lib/Analysis/ScalarEvolution.cpp
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Expand Up @@ -2338,23 +2338,15 @@ bool ScalarEvolution::willNotOverflow(Instruction::BinaryOps BinOp, bool Signed,
// Can we use context to prove the fact we need?
if (!CtxI)
return false;
// TODO: Support mul.
if (BinOp == Instruction::Mul)
return false;
auto *RHSC = dyn_cast<SCEVConstant>(RHS);
// TODO: Lift this limitation.
if (!RHSC)
return false;
APInt C = RHSC->getAPInt();
unsigned NumBits = C.getBitWidth();
if (BinOp == Instruction::Mul) {
// Multiplying by 0 or 1 never overflows
if (C.isZero() || C.isOne())
return true;
if (Signed)
return false;
APInt Limit = APInt::getMaxValue(NumBits).udiv(C);
// To avoid overflow, we need to make sure that LHS <= MAX / C.
return isKnownPredicateAt(ICmpInst::ICMP_ULE, LHS, getConstant(Limit),
CtxI);
}
Comment on lines -2347 to -2357
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@fhahn fhahn Aug 27, 2025

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Not sure if we should leave the code in, for the current test cases, we get the expected results due to #155300

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@annamthomas annamthomas Aug 27, 2025

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I had some cases downstream where I needed this code. I'll check if #155300 allows to get the test cases, but I think we can remove the code out unless it is exercised.

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@fhahn fhahn Aug 28, 2025

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That would be great. If there are, could you add them?

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@annamthomas annamthomas Sep 2, 2025

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In my cases as well, #155300 was enough. We can remove this.

bool IsSub = (BinOp == Instruction::Sub);
bool IsNegativeConst = (Signed && C.isNegative());
// Compute the direction and magnitude by which we need to check overflow.
Expand Down
2 changes: 1 addition & 1 deletion llvm/test/Transforms/LoopVectorize/vect.stats.ll
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@@ -1,4 +1,4 @@
; RUN: opt < %s -passes=loop-vectorize -force-vector-interleave=4 -force-vector-width=4 -debug-only=loop-vectorize -enable-early-exit-vectorization -use-symbolic-maxbtc-deref-loop --disable-output -stats -S 2>&1 | FileCheck %s
; RUN: opt < %s -passes=loop-vectorize -force-vector-interleave=4 -force-vector-width=4 -debug-only=loop-vectorize -enable-early-exit-vectorization --disable-output -stats -S 2>&1 | FileCheck %s
; REQUIRES: asserts

; We have 3 loops, two of them are vectorizable (with one being early-exit
Expand Down
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