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zstd: Improve zstd best efficiency (#784)
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* zstd: Improve best encoder by extending backwards

Before/after compared to master:

```
enwik8	zskp	4	100000000	29798035	5596	17.04
enwik8	zskp	4	100000000	29298617	5678	16.79

silesia.tar	zskp	4	211947520	59904436	10622	19.03
silesia.tar	zskp	4	211947520	59311059	10818	18.68

TS40.txt	zskp	4	400000000	123504912	22005	17.34
TS40.txt	zskp	4	400000000	121367006	23661	16.12

apache.log	zskp	4	2622574440	113537595	21239	117.76
apache.log	zskp	4	2622574440	110106407	24301	102.92

github-ranks-backup.bin	zskp	4	1862623243	377801114	75884	23.41
github-ranks-backup.bin	zskp	4	1862623243	373322075	76540	23.21
```

Doubles table size.

---------

Co-authored-by: greatroar <61184462+greatroar@users.noreply.github.com>
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@klauspost @greatroar
klauspost and greatroar authored Mar 23, 2023
1 parent 69a8ecc commit 2f99358
Showing 1 changed file with 70 additions and 99 deletions.
169 changes: 70 additions & 99 deletions zstd/enc_best.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ import (
)

const (
bestLongTableBits = 22 // Bits used in the long match table
bestLongTableBits = 23 // Bits used in the long match table
bestLongTableSize = 1 << bestLongTableBits // Size of the table
bestLongLen = 8 // Bytes used for table hash

Expand Down Expand Up @@ -188,7 +188,7 @@ encodeLoop:
panic("offset0 was 0")
}

const goodEnough = 100
const goodEnough = 250

nextHashL := hashLen(cv, bestLongTableBits, bestLongLen)
nextHashS := hashLen(cv, bestShortTableBits, bestShortLen)
Expand All @@ -205,7 +205,37 @@ encodeLoop:
panic(fmt.Sprintf("first match mismatch: %v != %v, first: %08x", src[s:s+4], src[offset:offset+4], first))
}
}
cand := match{offset: offset, s: s, length: 4 + e.matchlen(s+4, offset+4, src), rep: rep}
// Try to quick reject if we already have a long match.
if m.length > 16 {
left := len(src) - int(m.s+m.length)
// If we are too close to the end, keep as is.
if left <= 0 {
return
}
if left > 2 {
// Check 4 bytes, 4 bytes from the end of the current match.
a := load3232(src, offset+m.length-8)
b := load3232(src, s+m.length-8)
if a != b {
return
}
}
}
l := 4 + e.matchlen(s+4, offset+4, src)
if rep < 0 {
// Extend candidate match backwards as far as possible.
tMin := s - e.maxMatchOff
if tMin < 0 {
tMin = 0
}
for offset > tMin && s > nextEmit && src[offset-1] == src[s-1] && l < maxMatchLength {
s--
offset--
l++
}
}

cand := match{offset: offset, s: s, length: l, rep: rep}
cand.estBits(bitsPerByte)
if m.est >= highScore || cand.est-m.est+(cand.s-m.s)*bitsPerByte>>10 < 0 {
*m = cand
Expand All @@ -219,17 +249,29 @@ encodeLoop:
improve(&best, candidateS.prev-e.cur, s, uint32(cv), -1)

if canRepeat && best.length < goodEnough {
cv32 := uint32(cv >> 8)
spp := s + 1
improve(&best, spp-offset1, spp, cv32, 1)
improve(&best, spp-offset2, spp, cv32, 2)
improve(&best, spp-offset3, spp, cv32, 3)
if best.length > 0 {
cv32 = uint32(cv >> 24)
spp += 2
if s == nextEmit {
// Check repeats straight after a match.
improve(&best, s-offset2, s, uint32(cv), 1|4)
improve(&best, s-offset3, s, uint32(cv), 2|4)
if offset1 > 1 {
improve(&best, s-(offset1-1), s, uint32(cv), 3|4)
}
}

// If either no match or a non-repeat match, check at + 1
if best.rep <= 0 {
cv32 := uint32(cv >> 8)
spp := s + 1
improve(&best, spp-offset1, spp, cv32, 1)
improve(&best, spp-offset2, spp, cv32, 2)
improve(&best, spp-offset3, spp, cv32, 3)
if best.rep < 0 {
cv32 = uint32(cv >> 24)
spp += 2
improve(&best, spp-offset1, spp, cv32, 1)
improve(&best, spp-offset2, spp, cv32, 2)
improve(&best, spp-offset3, spp, cv32, 3)
}
}
}
// Load next and check...
Expand All @@ -248,7 +290,7 @@ encodeLoop:
continue
}

s++
s := s + 1
candidateS = e.table[hashLen(cv>>8, bestShortTableBits, bestShortLen)]
cv = load6432(src, s)
cv2 := load6432(src, s+1)
Expand Down Expand Up @@ -292,38 +334,22 @@ encodeLoop:

// We have a match, we can store the forward value
if best.rep > 0 {
s = best.s
var seq seq
seq.matchLen = uint32(best.length - zstdMinMatch)

// We might be able to match backwards.
// Extend as long as we can.
start := best.s
// We end the search early, so we don't risk 0 literals
// and have to do special offset treatment.
startLimit := nextEmit + 1

tMin := s - e.maxMatchOff
if tMin < 0 {
tMin = 0
}
repIndex := best.offset
for repIndex > tMin && start > startLimit && src[repIndex-1] == src[start-1] && seq.matchLen < maxMatchLength-zstdMinMatch-1 {
repIndex--
start--
seq.matchLen++
if debugAsserts && s <= nextEmit {
panic("s <= nextEmit")
}
addLiterals(&seq, start)
addLiterals(&seq, best.s)

// rep 0
seq.offset = uint32(best.rep)
// Repeat. If bit 4 is set, this is a non-lit repeat.
seq.offset = uint32(best.rep & 3)
if debugSequences {
println("repeat sequence", seq, "next s:", s)
}
blk.sequences = append(blk.sequences, seq)

// Index match start+1 (long) -> s - 1
index0 := s
// Index old s + 1 -> s - 1
index0 := s + 1
s = best.s + best.length

nextEmit = s
Expand All @@ -336,7 +362,7 @@ encodeLoop:
}
// Index skipped...
off := index0 + e.cur
for index0 < s-1 {
for index0 < s {
cv0 := load6432(src, index0)
h0 := hashLen(cv0, bestLongTableBits, bestLongLen)
h1 := hashLen(cv0, bestShortTableBits, bestShortLen)
Expand All @@ -346,17 +372,20 @@ encodeLoop:
index0++
}
switch best.rep {
case 2:
case 2, 4 | 1:
offset1, offset2 = offset2, offset1
case 3:
case 3, 4 | 2:
offset1, offset2, offset3 = offset3, offset1, offset2
case 4 | 3:
offset1, offset2, offset3 = offset1-1, offset1, offset2
}
cv = load6432(src, s)
continue
}

// A 4-byte match has been found. Update recent offsets.
// We'll later see if more than 4 bytes.
index0 := s + 1
s = best.s
t := best.offset
offset1, offset2, offset3 = s-t, offset1, offset2
Expand All @@ -369,22 +398,9 @@ encodeLoop:
panic("invalid offset")
}

// Extend the n-byte match as long as possible.
l := best.length

// Extend backwards
tMin := s - e.maxMatchOff
if tMin < 0 {
tMin = 0
}
for t > tMin && s > nextEmit && src[t-1] == src[s-1] && l < maxMatchLength {
s--
t--
l++
}

// Write our sequence
var seq seq
l := best.length
seq.litLen = uint32(s - nextEmit)
seq.matchLen = uint32(l - zstdMinMatch)
if seq.litLen > 0 {
Expand All @@ -401,10 +417,8 @@ encodeLoop:
break encodeLoop
}

// Index match start+1 (long) -> s - 1
index0 := s - l + 1
// every entry
for index0 < s-1 {
// Index old s + 1 -> s - 1
for index0 < s {
cv0 := load6432(src, index0)
h0 := hashLen(cv0, bestLongTableBits, bestLongLen)
h1 := hashLen(cv0, bestShortTableBits, bestShortLen)
Expand All @@ -413,50 +427,7 @@ encodeLoop:
e.table[h1] = prevEntry{offset: off, prev: e.table[h1].offset}
index0++
}

cv = load6432(src, s)
if !canRepeat {
continue
}

// Check offset 2
for {
o2 := s - offset2
if load3232(src, o2) != uint32(cv) {
// Do regular search
break
}

// Store this, since we have it.
nextHashS := hashLen(cv, bestShortTableBits, bestShortLen)
nextHashL := hashLen(cv, bestLongTableBits, bestLongLen)

// We have at least 4 byte match.
// No need to check backwards. We come straight from a match
l := 4 + e.matchlen(s+4, o2+4, src)

e.longTable[nextHashL] = prevEntry{offset: s + e.cur, prev: e.longTable[nextHashL].offset}
e.table[nextHashS] = prevEntry{offset: s + e.cur, prev: e.table[nextHashS].offset}
seq.matchLen = uint32(l) - zstdMinMatch
seq.litLen = 0

// Since litlen is always 0, this is offset 1.
seq.offset = 1
s += l
nextEmit = s
if debugSequences {
println("sequence", seq, "next s:", s)
}
blk.sequences = append(blk.sequences, seq)

// Swap offset 1 and 2.
offset1, offset2 = offset2, offset1
if s >= sLimit {
// Finished
break encodeLoop
}
cv = load6432(src, s)
}
}

if int(nextEmit) < len(src) {
Expand Down

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