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Fix NibbleCodec for AVX3_{ZEN4,DL,SPR} #766

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Merged
copybara-service merged 1 commit into from
Nov 11, 2025
Merged

Fix NibbleCodec for AVX3_{ZEN4,DL,SPR} #766

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7 changes: 5 additions & 2 deletions compression/nuq-inl.h
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Original file line number Diff line number Diff line change
Expand Up @@ -480,9 +480,12 @@ class NibbleCodec {
static_assert(kHalf <= 1);
const size_t N = hn::Lanes(d8);
constexpr size_t kMaxN = hn::MaxLanes(d8);
constexpr bool kPermuteAcrossBlocks =
HWY_TARGET <= HWY_AVX3_DL || !HWY_ARCH_X86;
// For kHalf=1 and 512-bit vectors, kAdd would be 16, which is out of
// bounds for TableLookupBytes. We instead BroadcastBlock<1> there.
constexpr uint8_t kAdd = kMaxN < 64 ? kHalf * kMaxN / 4 : 0;
constexpr uint8_t kAdd =
kMaxN < 64 || kPermuteAcrossBlocks ? kHalf * kMaxN / 4 : 0;
// The only performance-portable op to replicate bytes is TableLookupBytes,
// but this only works if vectors are 128-bit or we first BroadcastBlock,
// which only works for <= 512-bit vectors. For scalable vectors, we
Expand All @@ -506,7 +509,7 @@ class NibbleCodec {
} else if constexpr (kMaxN <= 16) { // <= 128-bit
// No BroadcastBlock, we anyway only have one block.
return hn::TableLookupBytes(bytes, hn::Load(d8, kRep4));
} else if constexpr (HWY_TARGET <= HWY_AVX3_DL || !HWY_ARCH_X86) {
} else if constexpr (kPermuteAcrossBlocks) {
// No BroadcastBlock, can directly permute across blocks.
return hn::TableLookupLanes(bytes, hn::SetTableIndices(d8, kRep4));
} else { // 256..512-bit, no efficient TableLookupLanes
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