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[LAA] Add initial support for non-power-of-2 store-load forwarding distance #137873

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[LAA] Add initial support for non-power-of-2 store-load forwarding distance #137873

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6142fdf
[𝘀𝗽𝗿] initial version
alexey-bataev Apr 29, 2025
2ebb2c6
Rebase
alexey-bataev Jun 17, 2025
1de7324
Rebase, address comments
alexey-bataev Jul 8, 2025
6c9e35e
Address comments, rebase
alexey-bataev Aug 4, 2025
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43 changes: 32 additions & 11 deletions llvm/include/llvm/Analysis/LoopAccessAnalysis.h
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Original file line number Diff line number Diff line change
Expand Up @@ -182,9 +182,10 @@ class MemoryDepChecker {

MemoryDepChecker(PredicatedScalarEvolution &PSE, const Loop *L,
const DenseMap<Value *, const SCEV *> &SymbolicStrides,
unsigned MaxTargetVectorWidthInBits)
unsigned MaxTargetVectorWidthInBits, bool AllowNonPow2Deps)
: PSE(PSE), InnermostLoop(L), SymbolicStrides(SymbolicStrides),
MaxTargetVectorWidthInBits(MaxTargetVectorWidthInBits) {}
MaxTargetVectorWidthInBits(MaxTargetVectorWidthInBits),
AllowNonPow2Deps(AllowNonPow2Deps) {}

/// Register the location (instructions are given increasing numbers)
/// of a write access.
Expand Down Expand Up @@ -221,17 +222,29 @@ class MemoryDepChecker {

/// Return true if there are no store-load forwarding dependencies.
bool isSafeForAnyStoreLoadForwardDistances() const {
return MaxStoreLoadForwardSafeDistanceInBits ==
std::numeric_limits<uint64_t>::max();
return MaxPowerOf2StoreLoadForwardSafeDistanceInBits ==
std::numeric_limits<uint64_t>::max() &&
MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits ==
std::numeric_limits<uint64_t>::max();
}

/// Return safe power-of-2 number of elements, which do not prevent store-load
/// forwarding, multiplied by the size of the elements in bits.
uint64_t getStoreLoadForwardSafeDistanceInBits() const {
/// Return safe number of elements, which do not prevent store-load
/// forwarding, multiplied by the size of the elements in bits (power-of-2).
uint64_t getPowerOf2StoreLoadForwardSafeDistanceInBits() const {
assert(!isSafeForAnyStoreLoadForwardDistances() &&
"Expected the distance, that prevent store-load forwarding, to be "
"set.");
return MaxStoreLoadForwardSafeDistanceInBits;
return MaxPowerOf2StoreLoadForwardSafeDistanceInBits;
}

/// Return safe number of elements, which do not prevent store-load
/// forwarding, multiplied by the size of the elements in bits
/// (non-power-of-2).
uint64_t getNonPowerOf2StoreLoadForwardSafeDistanceInBits() const {
assert(!isSafeForAnyStoreLoadForwardDistances() &&
"Expected the distance, that prevent store-load forwarding, to be "
"set.");
return MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits;
}

/// In same cases when the dependency check fails we can still
Expand Down Expand Up @@ -322,9 +335,14 @@ class MemoryDepChecker {
/// restrictive.
uint64_t MaxSafeVectorWidthInBits = -1U;

/// Maximum power-of-2 number of elements, which do not prevent store-load
/// forwarding, multiplied by the size of the elements in bits.
uint64_t MaxStoreLoadForwardSafeDistanceInBits =
/// Maximum number of elements, which do not prevent store-load forwarding,
/// multiplied by the size of the elements in bits (power-of-2).
uint64_t MaxPowerOf2StoreLoadForwardSafeDistanceInBits =
std::numeric_limits<uint64_t>::max();

/// Maximum number of elements, which do not prevent store-load forwarding,
/// multiplied by the size of the elements in bits (non-power-of-2).
uint64_t MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits =
std::numeric_limits<uint64_t>::max();
Comment on lines +355 to 363
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@artagnon artagnon May 13, 2025

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Seems redundant to have both, when only one is ever going to be used.

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@alexey-bataev alexey-bataev May 13, 2025

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We don't know before the vectorization which one is going to be used. Need to keep both


/// If we see a non-constant dependence distance we can still try to
Expand All @@ -351,6 +369,9 @@ class MemoryDepChecker {
/// backwards-vectorizable or unknown (triggering a runtime check).
unsigned MaxTargetVectorWidthInBits = 0;

/// True if current target supports non-power-of-2 dependence distances.
bool AllowNonPow2Deps = false;
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@artagnon artagnon May 13, 2025

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Shouldn't the comment say "if target supports predicated vector predicated intrinsics"?

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@alexey-bataev alexey-bataev May 13, 2025 •
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Not sure this is correct. The fact that is supports predicated intrinsics does not mean it supports non-power-of-2 dep distance.

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@fhahn fhahn Jul 2, 2025

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I am not sure the name/comment are accurate? Dependence could have any distance and still be supported, e.g. a forward dependene could have a distance of 3 which is totally fine.

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@fhahn fhahn Aug 1, 2025

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How about AllowNonPow2StoreLoadForwardDistance?

With the comment update to clarify that this only applies to computing the store-load forward distance.


/// Mapping of SCEV expressions to their expanded pointer bounds (pair of
/// start and end pointer expressions).
DenseMap<std::pair<const SCEV *, Type *>,
Expand Down
2 changes: 1 addition & 1 deletion llvm/include/llvm/Transforms/Vectorize/LoopVectorizationLegality.h
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Expand Up @@ -423,7 +423,7 @@ class LoopVectorizationLegality {
/// Return safe power-of-2 number of elements, which do not prevent store-load
/// forwarding and safe to operate simultaneously.
uint64_t getMaxStoreLoadForwardSafeDistanceInBits() const {
return LAI->getDepChecker().getStoreLoadForwardSafeDistanceInBits();
return LAI->getDepChecker().getPowerOf2StoreLoadForwardSafeDistanceInBits();
}

/// Returns true if vector representation of the instruction \p I
Expand Down
57 changes: 49 additions & 8 deletions llvm/lib/Analysis/LoopAccessAnalysis.cpp
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Expand Up @@ -1819,7 +1819,8 @@ bool MemoryDepChecker::couldPreventStoreLoadForward(uint64_t Distance,
// Maximum vector factor.
uint64_t MaxVFWithoutSLForwardIssuesPowerOf2 =
std::min(VectorizerParams::MaxVectorWidth * TypeByteSize,
MaxStoreLoadForwardSafeDistanceInBits);
MaxPowerOf2StoreLoadForwardSafeDistanceInBits);
uint64_t MaxVFWithoutSLForwardIssuesNonPowerOf2 = 0;

// Compute the smallest VF at which the store and load would be misaligned.
for (uint64_t VF = 2 * TypeByteSize;
Expand All @@ -1831,24 +1832,61 @@ bool MemoryDepChecker::couldPreventStoreLoadForward(uint64_t Distance,
break;
}
}
// If target supports active vector length, then it supports non-power-2
// vector factor. So, we iterate in a backward order to find largest VF, which
// allows aligned stores-loads or the number of iterations between conflicting
// memory addresses is not less than 8 (NumItersForStoreLoadThroughMemory).
if (AllowNonPow2Deps) {
MaxVFWithoutSLForwardIssuesNonPowerOf2 =
std::min(8 * VectorizerParams::MaxVectorWidth / TypeByteSize,
MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits);

for (uint64_t VF = MaxVFWithoutSLForwardIssuesNonPowerOf2;
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@fhahn fhahn Jul 2, 2025

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Why do we count backwards here while forwards for the power-of-2 case?

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@alexey-bataev alexey-bataev Jul 8, 2025

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The power-of-2 case tries to find the minimal supported vector factor. For non-power-of-2 it tries to find the largest (but still legal) dep distance, so it goes backwards

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@fhahn fhahn Jul 14, 2025

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Ok I see. But then it behaves different to MaxVFWithoutSLForwardIssuesPowerOf2. With MaxVFWithoutSLForwardIssuesPowerOf2 limits the Max VF we can use and we can also use any VF between 1 and MaxVF.

Is MaxVFWithoutSLForwardIssuesNonPowerOf2 a single non-power-of-2 VF we can use, but other VFs between 1 .. MaxVFWithoutSLForwardIssuesNonPowerOf2 may not be used?

Am I understanding correctly for example with, Max pow2 VF = 2, MaxNonPowOf2 VF = 9, LV can either chose 2 or 16 (with limiting VF to 9)?

If that's the case, I think it. would be good to update the comment for MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits to make this difference to the power-of-2 variant clear, as they would behave quite differently.

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Ok I see. But then it behaves different to MaxVFWithoutSLForwardIssuesPowerOf2. With MaxVFWithoutSLForwardIssuesPowerOf2 limits the Max VF we can use and we can also use any VF between 1 and MaxVF.

Not quite so. Any power-of-2 VF, but not any VF.

Is MaxVFWithoutSLForwardIssuesNonPowerOf2 a single non-power-of-2 VF we can use, but other VFs between 1 .. MaxVFWithoutSLForwardIssuesNonPowerOf2 may not be used?

Not necessary. We can use any whole divider of the MaxVFWithoutSLForwardIssuesNonPowerOf2. Say, if MaxVFWithoutSLForwardIssuesNonPowerOf2 is 9, then we can use 3 and 9. If it is 6, we can use 2, 3, 6. All these are safe.

Am I understanding correctly for example with, Max pow2 VF = 2, MaxNonPowOf2 VF = 9, LV can either chose 2 or 16 (with limiting VF to 9)?

The vector factor can be 2, 4, 8 or 16. But with non-power-of-2 we need an extra check ( or special instruction), that the number of the processed elements is limited by 9 or 3 elements only.

If that's the case, I think it. would be good to update the comment for MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits to make this difference to the power-of-2 variant clear, as they would behave quite differently.

Suggestions? Any preferences here?

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@fhahn fhahn Aug 1, 2025

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Suggestions? Any preferences here?

Thanks for the extra info. I think it would be helpful to update the comment to. include a generalized variant of the extra info you shared regarding what VFs can be picked (highlighted below)

Not necessary. We can use any whole divider of the MaxVFWithoutSLForwardIssuesNonPowerOf2. Say, if MaxVFWithoutSLForwardIssuesNonPowerOf2 is 9, then we can use 3 and 9. If it is 6, we can use 2, 3, 6. All these are safe.

I guess most RISCV HW with EVL support will support store-load forwarding with non-power-of-2 distances?

VF > MaxVFWithoutSLForwardIssuesPowerOf2; VF -= TypeByteSize) {
if (Distance % VF == 0 ||
Distance / VF >= NumItersForStoreLoadThroughMemory) {
uint64_t GCD =
isSafeForAnyStoreLoadForwardDistances()
? VF
: std::gcd(MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits,
VF);
MaxVFWithoutSLForwardIssuesNonPowerOf2 = GCD;
break;
}
}
}

if (MaxVFWithoutSLForwardIssuesPowerOf2 < 2 * TypeByteSize) {
if (MaxVFWithoutSLForwardIssuesPowerOf2 < 2 * TypeByteSize &&
MaxVFWithoutSLForwardIssuesNonPowerOf2 < 2 * TypeByteSize) {
LLVM_DEBUG(
dbgs() << "LAA: Distance " << Distance
<< " that could cause a store-load forwarding conflict\n");
return true;
}

// Handle non-power-2 store-load forwarding distance, power-of-2 distance can
// be calculated.
if (AllowNonPow2Deps && CommonStride &&
MaxVFWithoutSLForwardIssuesNonPowerOf2 <
MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits &&
MaxVFWithoutSLForwardIssuesNonPowerOf2 !=
8 * VectorizerParams::MaxVectorWidth / TypeByteSize) {
uint64_t MaxVF = MaxVFWithoutSLForwardIssuesNonPowerOf2 / CommonStride;
uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits =
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Do we need to compute this separately? Would it instead be possible to always compute the non-power-of-2 version and then have users convert it to the closest power-of-2 if that's what they need?

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@alexey-bataev alexey-bataev May 13, 2025

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Unfortunately, it won't work. I tried this, but there might be different results because of CommonStride value

std::min(MaxNonPowerOf2StoreLoadForwardSafeDistanceInBits, MaxVFInBits);
}

if (CommonStride &&
MaxVFWithoutSLForwardIssuesPowerOf2 <
MaxStoreLoadForwardSafeDistanceInBits &&
MaxPowerOf2StoreLoadForwardSafeDistanceInBits &&
MaxVFWithoutSLForwardIssuesPowerOf2 !=
VectorizerParams::MaxVectorWidth * TypeByteSize) {
uint64_t MaxVF =
bit_floor(MaxVFWithoutSLForwardIssuesPowerOf2 / CommonStride);
uint64_t MaxVFInBits = MaxVF * TypeByteSize * 8;
MaxStoreLoadForwardSafeDistanceInBits =
std::min(MaxStoreLoadForwardSafeDistanceInBits, MaxVFInBits);
MaxPowerOf2StoreLoadForwardSafeDistanceInBits =
std::min(MaxPowerOf2StoreLoadForwardSafeDistanceInBits, MaxVFInBits);
}
return false;
}
Expand Down Expand Up @@ -2985,8 +3023,9 @@ LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
MaxTargetVectorWidthInBits =
TTI->getRegisterBitWidth(TargetTransformInfo::RGK_FixedWidthVector) * 2;

DepChecker = std::make_unique<MemoryDepChecker>(*PSE, L, SymbolicStrides,
MaxTargetVectorWidthInBits);
DepChecker = std::make_unique<MemoryDepChecker>(
*PSE, L, SymbolicStrides, MaxTargetVectorWidthInBits,
TTI && TTI->hasActiveVectorLength());
PtrRtChecking = std::make_unique<RuntimePointerChecking>(*DepChecker, SE);
if (canAnalyzeLoop())
CanVecMem = analyzeLoop(AA, LI, TLI, DT);
Expand All @@ -3000,7 +3039,9 @@ void LoopAccessInfo::print(raw_ostream &OS, unsigned Depth) const {
OS << " with a maximum safe vector width of "
<< DC.getMaxSafeVectorWidthInBits() << " bits";
if (!DC.isSafeForAnyStoreLoadForwardDistances()) {
uint64_t SLDist = DC.getStoreLoadForwardSafeDistanceInBits();
uint64_t SLDist = DC.getNonPowerOf2StoreLoadForwardSafeDistanceInBits();
if (SLDist == std::numeric_limits<uint64_t>::max())
SLDist = DC.getPowerOf2StoreLoadForwardSafeDistanceInBits();
Comment on lines +3097 to +3099
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@artagnon artagnon May 13, 2025

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This logic would be eliminated by not having two different fields.

OS << ", with a maximum safe store-load forward width of " << SLDist
<< " bits";
}
Expand Down
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