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[LAA] Document reasoning in multiple places in isDependent (NFC). #89381

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Apr 22, 2024
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[LAA] Document reasoning in multiple places in isDependent (NFC). #89381

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17 changes: 13 additions & 4 deletions llvm/lib/Analysis/LoopAccessAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2034,6 +2034,10 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(

ScalarEvolution &SE = *PSE.getSE();
auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
// If the distance between the acecsses is larger than their absolute stride
// multiplied by the backedge taken count, the accesses are independet, i.e.
// they are far enough appart that accesses won't access the same location
// across all loop ierations.
if (!isa<SCEVCouldNotCompute>(Dist) && HasSameSize &&
isSafeDependenceDistance(DL, SE, *(PSE.getBackedgeTakenCount()), *Dist,
Stride, TypeByteSize))
Expand All @@ -2049,7 +2053,8 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
const APInt &Val = C->getAPInt();
int64_t Distance = Val.getSExtValue();

// Attempt to prove strided accesses independent.
// If the distance between accesses and their strides are known constants,
// check whether the accesses interlace each other.
if (std::abs(Distance) > 0 && Stride > 1 && HasSameSize &&
areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
Expand All @@ -2059,9 +2064,13 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
// Negative distances are not plausible dependencies.
if (Val.isNegative()) {
bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
// There is no need to update MaxSafeVectorWidthInBits after call to
// couldPreventStoreLoadForward, even if it changed MinDepDistBytes,
// since a forward dependency will allow vectorization using any width.
// Check if the first access writes to a location that is read in a later
// iteration, where the distance between them is not a multiple of a vector
// factor and relatively small.
//
// NOTE: There is no need to update MaxSafeVectorWidthInBits after call to
// couldPreventStoreLoadForward, even if it changed MinDepDistBytes, since a
// forward dependency will allow vectorization using any width.
if (IsTrueDataDependence && EnableForwardingConflictDetection &&
(!HasSameSize || couldPreventStoreLoadForward(Val.abs().getZExtValue(),
TypeByteSize))) {
Expand Down