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[NFCI][msan] Refactor into 'horizontalReduce' #152961

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Aug 11, 2025
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[NFCI][msan] Refactor into 'horizontalReduce' #152961

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[NFCI][msan] Refactor into 'horizontalReduce'
thurstond Aug 11, 2025
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Assert that total number of elements is divisible by reduction factor
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103 changes: 60 additions & 43 deletions llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2690,6 +2690,54 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
SC.Done(&I);
}

// Perform a bitwise OR on the horizontal pairs (or other specified grouping)
// of elements.
//
// For example, suppose we have:
// VectorA: <a1, a2, a3, a4, a5, a6>
// VectorB: <b1, b2, b3, b4, b5, b6>
// ReductionFactor: 3.
// The output would be:
// <a1|a2|a3, a4|a5|a6, b1|b2|b3, b4|b5|b6>
//
// This is convenient for instrumenting horizontal add/sub.
// For bitwise OR on "vertical" pairs, see maybeHandleSimpleNomemIntrinsic().
Value *horizontalReduce(IntrinsicInst &I, unsigned ReductionFactor,
Value *VectorA, Value *VectorB) {
assert(isa<FixedVectorType>(VectorA->getType()));
unsigned TotalNumElems =
cast<FixedVectorType>(VectorA->getType())->getNumElements();

if (VectorB) {
assert(VectorA->getType() == VectorB->getType());
TotalNumElems = TotalNumElems * 2;
}

assert(TotalNumElems % ReductionFactor == 0);

Value *Or = nullptr;

IRBuilder<> IRB(&I);
for (unsigned i = 0; i < ReductionFactor; i++) {
SmallVector<int, 16> Mask;
for (unsigned X = 0; X < TotalNumElems; X += ReductionFactor)
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I would put an example in a comment

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Done

Mask.push_back(X + i);

Value *Masked;
if (VectorB)
Masked = IRB.CreateShuffleVector(VectorA, VectorB, Mask);
else
Masked = IRB.CreateShuffleVector(VectorA, Mask);

if (Or)
Or = IRB.CreateOr(Or, Masked);
else
Or = Masked;
}

return Or;
}

/// Propagate shadow for 1- or 2-vector intrinsics that combine adjacent
/// fields.
///
Expand All @@ -2711,31 +2759,16 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
2 * ReturnType->getNumElements());

IRBuilder<> IRB(&I);
unsigned Width = ParamType->getNumElements() * I.arg_size();

// Horizontal OR of shadow
SmallVector<int, 8> EvenMask;
SmallVector<int, 8> OddMask;
for (unsigned X = 0; X < Width; X += 2) {
EvenMask.push_back(X);
OddMask.push_back(X + 1);
}

Value *FirstArgShadow = getShadow(&I, 0);
Value *EvenShadow;
Value *OddShadow;
if (I.arg_size() == 2) {
Value *SecondArgShadow = getShadow(&I, 1);
EvenShadow =
IRB.CreateShuffleVector(FirstArgShadow, SecondArgShadow, EvenMask);
OddShadow =
IRB.CreateShuffleVector(FirstArgShadow, SecondArgShadow, OddMask);
} else {
EvenShadow = IRB.CreateShuffleVector(FirstArgShadow, EvenMask);
OddShadow = IRB.CreateShuffleVector(FirstArgShadow, OddMask);
}
Value *SecondArgShadow = nullptr;
if (I.arg_size() == 2)
SecondArgShadow = getShadow(&I, 1);

Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, FirstArgShadow,
SecondArgShadow);

Value *OrShadow = IRB.CreateOr(EvenShadow, OddShadow);
OrShadow = CreateShadowCast(IRB, OrShadow, getShadowTy(&I));

setShadow(&I, OrShadow);
Expand Down Expand Up @@ -2768,23 +2801,14 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {

IRBuilder<> IRB(&I);

unsigned TotalNumElems = ParamType->getNumElements() * I.arg_size();
FixedVectorType *ReinterpretShadowTy = nullptr;
assert(isAligned(Align(ReinterpretElemWidth),
ParamType->getPrimitiveSizeInBits()));
ReinterpretShadowTy = FixedVectorType::get(
IRB.getIntNTy(ReinterpretElemWidth),
ParamType->getPrimitiveSizeInBits() / ReinterpretElemWidth);
TotalNumElems = ReinterpretShadowTy->getNumElements() * I.arg_size();

// Horizontal OR of shadow
SmallVector<int, 8> EvenMask;
SmallVector<int, 8> OddMask;
for (unsigned X = 0; X < TotalNumElems - 1; X += 2) {
EvenMask.push_back(X);
OddMask.push_back(X + 1);
}

Value *FirstArgShadow = getShadow(&I, 0);
FirstArgShadow = IRB.CreateBitCast(FirstArgShadow, ReinterpretShadowTy);

Expand All @@ -2796,22 +2820,15 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
Align(2),
cast<FixedVectorType>(FirstArgShadow->getType())->getNumElements()));

Value *EvenShadow;
Value *OddShadow;
Value *SecondArgShadow = nullptr;
if (I.arg_size() == 2) {
Value *SecondArgShadow = getShadow(&I, 1);
SecondArgShadow = getShadow(&I, 1);
SecondArgShadow = IRB.CreateBitCast(SecondArgShadow, ReinterpretShadowTy);

EvenShadow =
IRB.CreateShuffleVector(FirstArgShadow, SecondArgShadow, EvenMask);
OddShadow =
IRB.CreateShuffleVector(FirstArgShadow, SecondArgShadow, OddMask);
} else {
EvenShadow = IRB.CreateShuffleVector(FirstArgShadow, EvenMask);
OddShadow = IRB.CreateShuffleVector(FirstArgShadow, OddMask);
}

Value *OrShadow = IRB.CreateOr(EvenShadow, OddShadow);
Value *OrShadow = horizontalReduce(I, /*ReductionFactor=*/2, FirstArgShadow,
SecondArgShadow);

OrShadow = CreateShadowCast(IRB, OrShadow, getShadowTy(&I));

setShadow(&I, OrShadow);
Expand Down Expand Up @@ -3219,7 +3236,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
/// Caller guarantees that this intrinsic does not access memory.
///
/// TODO: "horizontal"/"pairwise" intrinsics are often incorrectly matched by
/// by this handler.
/// by this handler. See horizontalReduce().
[[maybe_unused]] bool
maybeHandleSimpleNomemIntrinsic(IntrinsicInst &I,
unsigned int trailingFlags) {
Expand Down