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improve decompression speed of long variant by ~+5%

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changed strategy,
now unconditionally prefetch the first 2 cache lines,
instead of cache lines corresponding to the first and last bytes of the match.

This better corresponds to cpu expectation,
which should auto-prefetch following cachelines on detecting the sequential nature of the read.

This is globally positive, by +5%,
though exact gains depend on compiler (from -2% to +15%).
The only negative counter-example is gcc-9.
This commit is contained in:
Yann Collet 2021-05-07 11:26:14 -07:00
parent ee425faaa7
commit 1db5947591
1 changed files with 10 additions and 10 deletions
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20
lib/decompress/zstd_decompress_block.c
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@ -1215,16 +1215,16 @@ ZSTD_decompressSequences_default(ZSTD_DCtx* dctx,
#ifndef ZSTD_FORCE_DECOMPRESS_SEQUENCES_SHORT
FORCE_INLINE_TEMPLATE size_t
ZSTD_prefetchMatch(size_t prefixPos, seq_t const sequence,
ZSTD_prefetchMatch(size_t prefetchPos, seq_t const sequence,
const BYTE* const prefixStart, const BYTE* const dictEnd)
{
prefixPos += sequence.litLength;
{ const BYTE* const matchBase = (sequence.offset > prefixPos) ? dictEnd : prefixStart;
const BYTE* const match = matchBase + prefixPos - sequence.offset; /* note : this operation can overflow when seq.offset is really too large, which can only happen when input is corrupted.
* No consequence though : no memory access will occur, offset is only used for prefetching */
PREFETCH_L1(match); PREFETCH_L1(match + sequence.matchLength - 1); /* note : it's safe to invoke PREFETCH() on any memory address, including invalid ones */
prefetchPos += sequence.litLength;
{ const BYTE* const matchBase = (sequence.offset > prefetchPos) ? dictEnd : prefixStart;
const BYTE* const match = matchBase + prefetchPos - sequence.offset; /* note : this operation can overflow when seq.offset is really too large, which can only happen when input is corrupted.
* No consequence though : memory address is only used for prefetching, not for dereferencing */
PREFETCH_L1(match); PREFETCH_L1(match+CACHELINE_SIZE); /* note : it's safe to invoke PREFETCH() on any memory address, including invalid ones */
}
return prefixPos + sequence.matchLength;
return prefetchPos + sequence.matchLength;
}
/* This decoding function employs prefetching
@ -1260,7 +1260,7 @@ ZSTD_decompressSequencesLong_body(
int const seqAdvance = MIN(nbSeq, ADVANCED_SEQS);
seqState_t seqState;
int seqNb;
size_t prefixPos = (size_t)(op-prefixStart); /* track position relative to prefixStart */
size_t prefetchPos = (size_t)(op-prefixStart); /* track position relative to prefixStart */
dctx->fseEntropy = 1;
{ int i; for (i=0; i<ZSTD_REP_NUM; i++) seqState.prevOffset[i] = dctx->entropy.rep[i]; }
@ -1276,7 +1276,7 @@ ZSTD_decompressSequencesLong_body(
/* prepare in advance */
for (seqNb=0; (BIT_reloadDStream(&seqState.DStream) <= BIT_DStream_completed) && (seqNb<seqAdvance); seqNb++) {
seq_t const sequence = ZSTD_decodeSequence(&seqState, isLongOffset);
prefixPos = ZSTD_prefetchMatch(prefixPos, sequence, prefixStart, dictEnd);
prefetchPos = ZSTD_prefetchMatch(prefetchPos, sequence, prefixStart, dictEnd);
sequences[seqNb] = sequence;
}
RETURN_ERROR_IF(seqNb<seqAdvance, corruption_detected, "");
@ -1291,7 +1291,7 @@ ZSTD_decompressSequencesLong_body(
#endif
if (ZSTD_isError(oneSeqSize)) return oneSeqSize;
prefixPos = ZSTD_prefetchMatch(prefixPos, sequence, prefixStart, dictEnd);
prefetchPos = ZSTD_prefetchMatch(prefetchPos, sequence, prefixStart, dictEnd);
sequences[seqNb & STORED_SEQS_MASK] = sequence;
op += oneSeqSize;
}