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AArch64: Improve sequence decoding #4813

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AArch64: Improve sequence decoding #4813

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e932526
AArch64: Improve Clang sequence decoding
davidbolvansky Oct 3, 2026
8964512
AArch64: Share decoder layout across Clang and GCC
davidbolvansky Oct 3, 2026
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53 changes: 31 additions & 22 deletions lib/decompress/zstd_decompress_block.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1241,15 +1241,8 @@ ZSTD_decodeSequence(seqState_t* seqState, const ZSTD_longOffset_e longOffsets, c
size_t prevOffset0 = seqState->prevOffset[0];
size_t prevOffset1 = seqState->prevOffset[1];
size_t prevOffset2 = seqState->prevOffset[2];
/*
* ZSTD_seqSymbol is a 64 bits wide structure.
* It can be loaded in one operation
* and its fields extracted by simply shifting or bit-extracting on aarch64.
* GCC doesn't recognize this and generates more unnecessary ldr/ldrb/ldrh
* operations that cause performance drop. This can be avoided by using this
* ZSTD_memcpy hack.
*/
# if defined(__GNUC__) && !defined(__clang__)
/* Encourage a single 64-bit load per symbol, followed by field extraction. */
# if defined(__GNUC__)
ZSTD_seqSymbol llDInfoS, mlDInfoS, ofDInfoS;
ZSTD_seqSymbol* const llDInfo = &llDInfoS;
ZSTD_seqSymbol* const mlDInfo = &mlDInfoS;
Expand Down Expand Up @@ -1323,26 +1316,42 @@ ZSTD_decodeSequence(seqState_t* seqState, const ZSTD_longOffset_e longOffsets, c
seq.offset = offset;
}

if (mlBits > 0)
seq.matchLength += BIT_readBitsFast(&seqState->DStream, mlBits/*>0*/);

if (UNLIKELY(totalBits >= STREAM_ACCUMULATOR_MIN_64-(LLFSELog+MLFSELog+OffFSELog)))
BIT_reloadDStream(&seqState->DStream);

/* Ensure there are enough bits to read the rest of data in 64-bit mode. */
/* Most symbols need no additional literal-length bits. Keep their
* match-length read separate from the uncommon two-length case. */
ZSTD_STATIC_ASSERT(16+LLFSELog+MLFSELog+OffFSELog < STREAM_ACCUMULATOR_MIN_64);

if (llBits > 0)
seq.litLength += BIT_readBitsFast(&seqState->DStream, llBits/*>0*/);
if (UNLIKELY(llBits != 0)) {
if (mlBits != 0)
seq.matchLength += BIT_readBitsFast(&seqState->DStream, mlBits);
if (UNLIKELY(totalBits >= STREAM_ACCUMULATOR_MIN_64-(LLFSELog+MLFSELog+OffFSELog)))
BIT_reloadDStream(&seqState->DStream);
seq.litLength += BIT_readBitsFast(&seqState->DStream, llBits);
} else if (mlBits != 0) {
seq.matchLength += BIT_readBitsFast(&seqState->DStream, mlBits);
if (UNLIKELY(totalBits >= STREAM_ACCUMULATOR_MIN_64-(LLFSELog+MLFSELog+OffFSELog)))
BIT_reloadDStream(&seqState->DStream);
} else if (UNLIKELY(ofBits >= STREAM_ACCUMULATOR_MIN_64-(LLFSELog+MLFSELog+OffFSELog))) {
BIT_reloadDStream(&seqState->DStream);
}

DEBUGLOG(6, "seq: litL=%u, matchL=%u, offset=%u",
(U32)seq.litLength, (U32)seq.matchLength, (U32)seq.offset);

if (!isLastSeq) {
/* Don't update FSE state for last sequence. */
ZSTD_updateFseStateWithDInfo(&seqState->stateLL, &seqState->DStream, llNext, llnbBits); /* <= 9 bits */
ZSTD_updateFseStateWithDInfo(&seqState->stateML, &seqState->DStream, mlNext, mlnbBits); /* <= 9 bits */
ZSTD_updateFseStateWithDInfo(&seqState->stateOffb, &seqState->DStream, ofNext, ofnbBits); /* <= 8 bits */
/* Compute each bit position from the same starting position to
* avoid serial updates of bitsConsumed. */
{ U32 const consumed = seqState->DStream.bitsConsumed;
U32 const llmlBits = llnbBits + mlnbBits;
U32 const allFseBits = llmlBits + ofnbBits;
BitContainerType const container = seqState->DStream.bitContainer;
seqState->stateLL.state = llNext + BIT_getMiddleBits(
container, 64 - (consumed + llnbBits), llnbBits);
seqState->stateML.state = mlNext + BIT_getMiddleBits(
container, 64 - (consumed + llmlBits), mlnbBits);
seqState->stateOffb.state = ofNext + BIT_getMiddleBits(
container, 64 - (consumed + allFseBits), ofnbBits);
seqState->DStream.bitsConsumed = consumed + allFseBits;
}
BIT_reloadDStream(&seqState->DStream);
}
}
Expand Down
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