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[DA] Cache delinearization results. NFCI. #164379

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[DA] Cache delinearization results. NFCI. #164379

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6 changes: 6 additions & 0 deletions llvm/include/llvm/Analysis/DependenceAnalysis.h
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Original file line number Diff line number Diff line change
Expand Up @@ -420,6 +420,12 @@ class DependenceInfo {
Function *F;
SmallVector<const SCEVPredicate *, 4> Assumptions;

/// Cache for delinearized subscripts to avoid recomputation.
/// Maps (Instruction, Loop, AccessFn) -> Subscripts
DenseMap<std::tuple<Instruction *, Loop *, const SCEV *>,
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@kasuga-fj kasuga-fj Oct 21, 2025

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Does the key need to be a tuple? Wouldn't Instruction * or const SCEV * be sufficient?

SmallVector<const SCEV *, 4>>
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@Meinersbur Meinersbur Oct 21, 2025

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Suggested change
SmallVector<const SCEV *, 4>>
SmallVector<const SCEV *, 1>>

Consider a smaller SmallVector, maybe even SmallVector<const SCEV *, 0>. DenseMap only uses a fraction of its entries, big item and key objects waste a lot of space.

DelinearizationCache;

/// Subscript - This private struct represents a pair of subscripts from
/// a pair of potentially multi-dimensional array references. We use a
/// vector of them to guide subscript partitioning.
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36 changes: 31 additions & 5 deletions llvm/lib/Analysis/DependenceAnalysis.cpp
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Expand Up @@ -3463,11 +3463,37 @@ bool DependenceInfo::tryDelinearize(Instruction *Src, Instruction *Dst,

SmallVector<const SCEV *, 4> SrcSubscripts, DstSubscripts;

if (!tryDelinearizeFixedSize(Src, Dst, SrcAccessFn, DstAccessFn,
SrcSubscripts, DstSubscripts) &&
!tryDelinearizeParametricSize(Src, Dst, SrcAccessFn, DstAccessFn,
SrcSubscripts, DstSubscripts))
return false;
// Check cache for both Src and Dst subscripts
auto SrcCacheKey = std::make_tuple(Src, SrcLoop, SrcAccessFn);
auto DstCacheKey = std::make_tuple(Dst, DstLoop, DstAccessFn);
auto SrcCacheIt = DelinearizationCache.find(SrcCacheKey);
auto DstCacheIt = DelinearizationCache.find(DstCacheKey);
Comment on lines +3469 to +3470
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@kasuga-fj kasuga-fj Oct 21, 2025

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Consider using try_emplace to avoid repeated hash lookup.

bool SrcCached = (SrcCacheIt != DelinearizationCache.end());
bool DstCached = (DstCacheIt != DelinearizationCache.end());

if (SrcCached && DstCached) {
// Both are cached - use cached values and skip delinearization
SrcSubscripts = SrcCacheIt->second;
DstSubscripts = DstCacheIt->second;
LLVM_DEBUG(dbgs() << " Delinearization cache hit for both Src and Dst\n");
} else {
// At least one is not cached - need to compute both
if (!tryDelinearizeFixedSize(Src, Dst, SrcAccessFn, DstAccessFn,
SrcSubscripts, DstSubscripts) &&
!tryDelinearizeParametricSize(Src, Dst, SrcAccessFn, DstAccessFn,
SrcSubscripts, DstSubscripts))
return false;

// Cache the results
if (!SrcCached) {
DelinearizationCache[SrcCacheKey] = SrcSubscripts;
LLVM_DEBUG(dbgs() << " Cached Src subscripts\n");
}
if (!DstCached) {
DelinearizationCache[DstCacheKey] = DstSubscripts;
LLVM_DEBUG(dbgs() << " Cached Dst subscripts\n");
}
}
Comment on lines +3466 to +3496
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@Meinersbur Meinersbur Oct 21, 2025

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Consider refactoring into a function for lookup.

Src and Dst computation need to be disentangled or looked up together. tryDelinearizeFixedSize implementes a consistency check, eg. that both have the same dimensions. This isn't guaranteed of they are looked up independently.


assert(isLoopInvariant(SrcBase, SrcLoop) &&
isLoopInvariant(DstBase, DstLoop) &&
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