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Possible zstd_fast optimization involving 6 byte matching code #4561

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Possible zstd_fast optimization involving 6 byte matching code #4561

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32 changes: 28 additions & 4 deletions lib/compress/zstd_fast.c
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Original file line number Diff line number Diff line change
Expand Up @@ -97,7 +97,7 @@ void ZSTD_fillHashTable(ZSTD_MatchState_t* ms,
}


typedef int (*ZSTD_match4Found) (const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit);
typedef int (*ZSTD_matchFound) (const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit);

static int
ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit)
Expand Down Expand Up @@ -140,6 +140,22 @@ ZSTD_match4Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 ma
return (MEM_read32(currentPtr) == mval);
}

static int
ZSTD_match6Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit)
{
/* using a branch instead of a cmov,
* because it's faster in scenarios where matchIdx >= idxLowLimit is generally true,
* aka almost all candidates are within range */
U32 mval;
if (matchIdx >= idxLowLimit) {
mval = MEM_read32(matchAddress);
} else {
mval = MEM_read32(currentPtr) ^ 1; /* guaranteed to not match. */
}

return (MEM_read32(currentPtr) == mval && MEM_read16(currentPtr+4) == MEM_read16(matchAddress+4));
}


/**
* If you squint hard enough (and ignore repcodes), the search operation at any
Expand Down Expand Up @@ -224,6 +240,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
U32 offcode;
const BYTE* match0;
size_t mLength;


/* ip0 and ip1 are always adjacent. The targetLength skipping and
* uncompressibility acceleration is applied to every other position,
Expand All @@ -232,7 +249,13 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
size_t step;
const BYTE* nextStep;
const size_t kStepIncr = (1 << (kSearchStrength - 1));
const ZSTD_match4Found matchFound = useCmov ? ZSTD_match4Found_cmov : ZSTD_match4Found_branch;

/* If we use the cmov condition, then just always do 4 byte matching.
* If we are using the branch match found, and have a hash of 6 or greater,
* then we verify we have found at least a 6 byte match before continuing,
* as the extra 2 byte compare operation is a bit faster than relying on ZSTD_count later. */
const ZSTD_matchFound matchFound = useCmov ? ZSTD_match4Found_cmov : (mls >= 6 ? ZSTD_match6Found_branch : ZSTD_match4Found_branch);
const size_t mLengthGuaranteed = (!useCmov && mls >= 6) ? 6 : 4;

DEBUGLOG(5, "ZSTD_compressBlock_fast_generic");
ip0 += (ip0 == prefixStart);
Expand Down Expand Up @@ -318,7 +341,8 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
/* Write next hash table entry, since it's already calculated */
if (step <= 4) {
/* Avoid writing an index if it's >= position where search will resume.
* The minimum possible match has length 4, so search can resume at ip0 + 4.
* The minimum possible match has length 4, so search
* can resume at ip0 + 4.
*/
hashTable[hash1] = (U32)(ip1 - base);
}
Expand Down Expand Up @@ -381,7 +405,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
rep_offset2 = rep_offset1;
rep_offset1 = (U32)(ip0-match0);
offcode = OFFSET_TO_OFFBASE(rep_offset1);
mLength = 4;
mLength = mLengthGuaranteed;

/* Count the backwards match length. */
while (((ip0>anchor) & (match0>prefixStart)) && (ip0[-1] == match0[-1])) {
Expand Down