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new core ldm algorithm

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This commit is contained in:
Quentin Carbonneaux
2021-02-03 22:24:07 +01:00
parent f5b3f64d3f
commit 9f327c02fd
2 changed files with 320 additions and 186 deletions
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lib/compress/zstd_ldm.c
+217 -186
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@@ -11,12 +11,100 @@
#include "zstd_ldm.h"
#include "../common/debug.h"
#include "../common/xxhash.h"
#include "zstd_fast.h" /* ZSTD_fillHashTable() */
#include "zstd_double_fast.h" /* ZSTD_fillDoubleHashTable() */
#include "zstd_ldm_geartab.h"
#define LDM_BUCKET_SIZE_LOG 3
#define LDM_MIN_MATCH_LENGTH 64
#define LDM_HASH_RLOG 7
#define LDM_LOOKAHEAD_SPLITS 64
typedef struct {
U64 rolling;
U64 stopMask;
} ldmRollingHashState_t;
/** ZSTD_ldm_gear_init():
*
* Initializes the rolling hash state such that it will honor the
* settings in params. */
static void ZSTD_ldm_gear_init(ldmRollingHashState_t* state, ldmParams_t const* params)
{
unsigned maxBitsInMask = MIN(params->minMatchLength, 64);
unsigned hashRateLog = params->hashRateLog;
state->rolling = ~(U32)0;
/* The choice of the splitting criterion is subject to two conditions:
* 1. it has to trigger on average every 2^(hashRateLog) bytes;
* 2. ideally, it has to depend on a window of minMatchLength bytes.
*
* In the gear hash algorithm, bit n depends on the last n bytes;
* so in order to obtain a good quality splitting criterion it is
* preferable to use bits with high weight.
*
* To match condition 1 we use a mask with hashRateLog bits set
* and, because of the previous remark, we make sure these bits
* have the highest possible weight while still respecting
* condition 2.
*/
if (hashRateLog > 0 && hashRateLog <= maxBitsInMask) {
state->stopMask = (((U64)1 << hashRateLog) - 1) << (maxBitsInMask - hashRateLog);
} else {
/* In this degenerate case we simply honor the hash rate. */
state->stopMask = ((U64)1 << hashRateLog) - 1;
}
}
/** ZSTD_ldm_gear_feed():
*
* Registers in the splits array all the split points found in the first
* size bytes following the data pointer. This function terminates when
* either all the data has been processed or LDM_LOOKAHEAD_SPLITS splits
* are present in the splits array.
*
* Precondition: The splits array must not be full.
* Returns: The number of bytes processed. */
static size_t ZSTD_ldm_gear_feed(ldmRollingHashState_t* state,
BYTE const* data, size_t size,
size_t* splits, unsigned* numSplits)
{
size_t n;
U64 hash, mask;
hash = state->rolling;
mask = state->stopMask;
n = 0;
#define GEAR_ITER_ONCE() do { \
hash = (hash << 1) + ZSTD_ldm_gearTab[data[n] & 0xff]; \
n += 1; \
if (UNLIKELY((hash & mask) == 0)) { \
splits[*numSplits] = n; \
*numSplits += 1; \
if (*numSplits == LDM_LOOKAHEAD_SPLITS) \
goto done; \
} \
} while (0)
while (n + 3 < size) {
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
}
while (n < size) {
GEAR_ITER_ONCE();
}
#undef GEAR_ITER_ONCE
done:
state->rolling = hash;
return n;
}
void ZSTD_ldm_adjustParameters(ldmParams_t* params,
ZSTD_compressionParameters const* cParams)
@@ -53,38 +141,6 @@ size_t ZSTD_ldm_getMaxNbSeq(ldmParams_t params, size_t maxChunkSize)
return params.enableLdm ? (maxChunkSize / params.minMatchLength) : 0;
}
/** ZSTD_ldm_getSmallHash() :
* numBits should be <= 32
* If numBits==0, returns 0.
* @return : the most significant numBits of hash. */
static U32 ZSTD_ldm_getSmallHash(U64 hash, U32 numBits)
{
assert(numBits <= 32);
return numBits == 0 ? 0 : (U32)(hash >> (64 - numBits));
}
/** ZSTD_ldm_getChecksum() :
* numBitsToDiscard should be <= 32
* @return : the next most significant 32 bits after numBitsToDiscard */
static U32 ZSTD_ldm_getChecksum(U64 hash, U32 numBitsToDiscard)
{
assert(numBitsToDiscard <= 32);
return (hash >> (64 - (32 + numBitsToDiscard))) & 0xFFFFFFFF;
}
/** ZSTD_ldm_getTagMask() :
* Returns the mask against which the rolling hash must be
* checked. */
static U64 ZSTD_ldm_getTagMask(U32 hbits, U32 hashRateLog)
{
assert(hashRateLog < 32 && hbits <= 32);
if (32 - hbits < hashRateLog) {
return (((U64)1 << hashRateLog) - 1);
} else {
return (((U64)1 << hashRateLog) - 1) << (32 - hbits - hashRateLog);
}
}
/** ZSTD_ldm_getBucket() :
* Returns a pointer to the start of the bucket associated with hash. */
static ldmEntry_t* ZSTD_ldm_getBucket(
@@ -107,33 +163,6 @@ static void ZSTD_ldm_insertEntry(ldmState_t* ldmState,
}
/** ZSTD_ldm_makeEntryAndInsertByTag() :
*
* Gets the small hash, checksum, and tag from the rollingHash.
*
* If the tag matches (1 << ldmParams.hashRateLog)-1, then
* creates an ldmEntry from the offset, and inserts it into the hash table.
*
* hBits is the length of the small hash, which is the most significant hBits
* of rollingHash. The checksum is the next 32 most significant bits, followed
* by ldmParams.hashRateLog bits that make up the tag. */
static void ZSTD_ldm_makeEntryAndInsertByTag(ldmState_t* ldmState,
U64 const rollingHash,
U32 const hBits,
U32 const offset,
ldmParams_t const ldmParams)
{
U64 const tagMask = ZSTD_ldm_getTagMask(hBits, ldmParams.hashRateLog);
if ((rollingHash & tagMask) == tagMask) {
U32 const hash = ZSTD_ldm_getSmallHash(rollingHash, hBits);
U32 const checksum = ZSTD_ldm_getChecksum(rollingHash, hBits);
ldmEntry_t entry;
entry.offset = offset;
entry.checksum = checksum;
ZSTD_ldm_insertEntry(ldmState, hash, entry, ldmParams);
}
}
/** ZSTD_ldm_countBackwardsMatch() :
* Returns the number of bytes that match backwards before pIn and pMatch.
*
@@ -209,43 +238,42 @@ static size_t ZSTD_ldm_fillFastTables(ZSTD_matchState_t* ms,
return 0;
}
/** ZSTD_ldm_fillLdmHashTable() :
*
* Fills hashTable from (lastHashed + 1) to iend (non-inclusive).
* lastHash is the rolling hash that corresponds to lastHashed.
*
* Returns the rolling hash corresponding to position iend-1. */
static U64 ZSTD_ldm_fillLdmHashTable(ldmState_t* state,
U64 lastHash, const BYTE* lastHashed,
const BYTE* iend, const BYTE* base,
U32 hBits, ldmParams_t const ldmParams)
{
U64 rollingHash = lastHash;
const BYTE* cur = lastHashed + 1;
while (cur < iend) {
rollingHash = ZSTD_rollingHash_rotate(rollingHash, cur[-1],
cur[ldmParams.minMatchLength-1],
state->hashPower);
ZSTD_ldm_makeEntryAndInsertByTag(state,
rollingHash, hBits,
(U32)(cur - base), ldmParams);
++cur;
}
return rollingHash;
}
void ZSTD_ldm_fillHashTable(
ldmState_t* state, const BYTE* ip,
ldmState_t* ldmState, const BYTE* ip,
const BYTE* iend, ldmParams_t const* params)
{
U32 const minMatchLength = params->minMatchLength;
U32 const hBits = params->hashLog - params->bucketSizeLog;
BYTE const* const base = ldmState->window.base;
BYTE const* const istart = ip;
ldmRollingHashState_t hashState;
size_t splits[LDM_LOOKAHEAD_SPLITS];
unsigned numSplits;
DEBUGLOG(5, "ZSTD_ldm_fillHashTable");
if ((size_t)(iend - ip) >= params->minMatchLength) {
U64 startingHash = ZSTD_rollingHash_compute(ip, params->minMatchLength);
ZSTD_ldm_fillLdmHashTable(
state, startingHash, ip, iend - params->minMatchLength, state->window.base,
params->hashLog - params->bucketSizeLog,
*params);
ZSTD_ldm_gear_init(&hashState, params);
while (ip < iend) {
size_t hashed;
unsigned n;
numSplits = 0;
hashed = ZSTD_ldm_gear_feed(&hashState, ip, iend - ip, splits, &numSplits);
for (n = 0; n < numSplits; n++) {
if (ip + splits[n] >= istart + minMatchLength) {
BYTE const* const split = ip + splits[n] - minMatchLength;
U64 const xxhash = XXH64(split, minMatchLength, 0);
U64 const hash = xxhash & (((U32)1 << hBits) - 1);
ldmEntry_t entry;
entry.offset = (U32)(split - base);
entry.checksum = (U32)(xxhash >> 32);
ZSTD_ldm_insertEntry(ldmState, hash, entry, *params);
}
}
ip += hashed;
}
}
@@ -271,10 +299,8 @@ static size_t ZSTD_ldm_generateSequences_internal(
/* LDM parameters */
int const extDict = ZSTD_window_hasExtDict(ldmState->window);
U32 const minMatchLength = params->minMatchLength;
U64 const hashPower = ldmState->hashPower;
U32 const hBits = params->hashLog - params->bucketSizeLog;
U32 const entsPerBucket = 1U << params->bucketSizeLog;
U64 const tagMask = ZSTD_ldm_getTagMask(hBits, params->hashRateLog);
U32 const hBits = params->hashLog - params->bucketSizeLog;
/* Prefix and extDict parameters */
U32 const dictLimit = ldmState->window.dictLimit;
U32 const lowestIndex = extDict ? ldmState->window.lowLimit : dictLimit;
@@ -286,50 +312,78 @@ static size_t ZSTD_ldm_generateSequences_internal(
/* Input bounds */
BYTE const* const istart = (BYTE const*)src;
BYTE const* const iend = istart + srcSize;
BYTE const* const ilimit = iend - MAX(minMatchLength, HASH_READ_SIZE);
BYTE const* const ilimit = iend - HASH_READ_SIZE;
/* Input positions */
BYTE const* anchor = istart;
BYTE const* ip = istart;
/* Rolling hash */
BYTE const* lastHashed = NULL;
U64 rollingHash = 0;
/* Rolling hash state */
ldmRollingHashState_t hashState;
/* Arrays for staged-processing */
size_t splits[LDM_LOOKAHEAD_SPLITS];
struct {
BYTE const* split;
U32 hash;
U32 checksum;
ldmEntry_t* bucket;
} candidates[LDM_LOOKAHEAD_SPLITS];
unsigned numSplits;
while (ip <= ilimit) {
U32 const currentIndex = (U32)(ip - base);
U32 hash, checksum;
size_t mLength;
size_t forwardMatchLength = 0, backwardMatchLength = 0;
ldmEntry_t const* bestEntry = NULL;
ldmEntry_t newEntry;
/* Initialize the rolling hash state with the first minMatchLength bytes */
ZSTD_ldm_gear_init(&hashState, params);
{
size_t n = 0;
if (ip != istart) {
rollingHash = ZSTD_rollingHash_rotate(rollingHash, lastHashed[0],
lastHashed[minMatchLength],
hashPower);
} else {
rollingHash = ZSTD_rollingHash_compute(ip, minMatchLength);
while (n < minMatchLength) {
numSplits = 0;
n += ZSTD_ldm_gear_feed(&hashState, ip + n, minMatchLength - n,
splits, &numSplits);
}
lastHashed = ip;
ip += minMatchLength;
}
/* Do not insert and do not look for a match */
if ((rollingHash & tagMask) != tagMask) {
ip++;
continue;
while (ip < ilimit) {
size_t hashed;
unsigned n;
numSplits = 0;
hashed = ZSTD_ldm_gear_feed(&hashState, ip, ilimit - ip,
splits, &numSplits);
for (n = 0; n < numSplits; n++) {
BYTE const* const split = ip + splits[n] - minMatchLength;
U64 const xxhash = XXH64(split, minMatchLength, 0);
U64 const hash = xxhash & (((U32)1 << hBits) - 1);
candidates[n].split = split;
candidates[n].hash = hash;
candidates[n].checksum = (U32)(xxhash >> 32);
candidates[n].bucket = ZSTD_ldm_getBucket(ldmState, hash, *params);
PREFETCH_L1(candidates[n].bucket);
}
hash = ZSTD_ldm_getSmallHash(rollingHash, hBits);
checksum = ZSTD_ldm_getChecksum(rollingHash, hBits);
newEntry.offset = currentIndex;
newEntry.checksum = checksum;
/* Get the best entry and compute the match lengths */
{
ldmEntry_t* const bucket = ZSTD_ldm_getBucket(ldmState, hash, *params);
for (n = 0; n < numSplits; n++) {
size_t forwardMatchLength = 0, backwardMatchLength = 0,
bestMatchLength = 0, mLength;
BYTE const* const split = candidates[n].split;
U64 const checksum = candidates[n].checksum;
U64 const hash = candidates[n].hash;
ldmEntry_t* const bucket = candidates[n].bucket;
ldmEntry_t const* cur;
size_t bestMatchLength = 0;
ldmEntry_t const* bestEntry = NULL;
ldmEntry_t newEntry;
for (cur = bucket; cur < bucket + entsPerBucket; ++cur) {
newEntry.offset = (U32)(split - base);
newEntry.checksum = checksum;
/* If a split point would generate a sequence overlapping with
* the previous one, we merely register it in the hash table and
* move on */
if (split < anchor) {
ZSTD_ldm_insertEntry(ldmState, hash, newEntry, *params);
continue;
}
for (cur = bucket; cur < bucket + entsPerBucket; cur++) {
size_t curForwardMatchLength, curBackwardMatchLength,
curTotalMatchLength;
if (cur->checksum != checksum || cur->offset <= lowestIndex) {
@@ -343,31 +397,23 @@ static size_t ZSTD_ldm_generateSequences_internal(
cur->offset < dictLimit ? dictEnd : iend;
BYTE const* const lowMatchPtr =
cur->offset < dictLimit ? dictStart : lowPrefixPtr;
curForwardMatchLength = ZSTD_count_2segments(
ip, pMatch, iend,
matchEnd, lowPrefixPtr);
curForwardMatchLength =
ZSTD_count_2segments(split, pMatch, iend, matchEnd, lowPrefixPtr);
if (curForwardMatchLength < minMatchLength) {
continue;
}
curBackwardMatchLength =
ZSTD_ldm_countBackwardsMatch_2segments(ip, anchor,
pMatch, lowMatchPtr,
dictStart, dictEnd);
curTotalMatchLength = curForwardMatchLength +
curBackwardMatchLength;
curBackwardMatchLength = ZSTD_ldm_countBackwardsMatch_2segments(
split, anchor, pMatch, lowMatchPtr, dictStart, dictEnd);
} else { /* !extDict */
BYTE const* const pMatch = base + cur->offset;
curForwardMatchLength = ZSTD_count(ip, pMatch, iend);
curForwardMatchLength = ZSTD_count(split, pMatch, iend);
if (curForwardMatchLength < minMatchLength) {
continue;
}
curBackwardMatchLength =
ZSTD_ldm_countBackwardsMatch(ip, anchor, pMatch,
lowPrefixPtr);
curTotalMatchLength = curForwardMatchLength +
curBackwardMatchLength;
ZSTD_ldm_countBackwardsMatch(split, anchor, pMatch, lowPrefixPtr);
}
curTotalMatchLength = curForwardMatchLength + curBackwardMatchLength;
if (curTotalMatchLength > bestMatchLength) {
bestMatchLength = curTotalMatchLength;
@@ -376,54 +422,39 @@ static size_t ZSTD_ldm_generateSequences_internal(
bestEntry = cur;
}
}
}
/* No match found -- continue searching */
if (bestEntry == NULL) {
/* No match found -- insert an entry into the hash table
* and process the next candidate match */
if (bestEntry == NULL) {
ZSTD_ldm_insertEntry(ldmState, hash, newEntry, *params);
continue;
}
/* Match found */
mLength = forwardMatchLength + backwardMatchLength;
{
U32 const offset = (U32)(split - base) - bestEntry->offset;
rawSeq* const seq = rawSeqStore->seq + rawSeqStore->size;
/* Out of sequence storage */
if (rawSeqStore->size == rawSeqStore->capacity)
return ERROR(dstSize_tooSmall);
seq->litLength = (U32)(split - backwardMatchLength - anchor);
seq->matchLength = (U32)mLength;
seq->offset = offset;
rawSeqStore->size++;
}
/* Insert the current entry into the hash table --- it must be
* done after the previous block to avoid clobbering bestEntry */
ZSTD_ldm_insertEntry(ldmState, hash, newEntry, *params);
ip++;
continue;
anchor = split + forwardMatchLength;
}
/* Match found */
mLength = forwardMatchLength + backwardMatchLength;
ip -= backwardMatchLength;
{
/* Store the sequence:
* ip = currentIndex - backwardMatchLength
* The match is at (bestEntry->offset - backwardMatchLength)
*/
U32 const matchIndex = bestEntry->offset;
U32 const offset = currentIndex - matchIndex;
rawSeq* const seq = rawSeqStore->seq + rawSeqStore->size;
/* Out of sequence storage */
if (rawSeqStore->size == rawSeqStore->capacity)
return ERROR(dstSize_tooSmall);
seq->litLength = (U32)(ip - anchor);
seq->matchLength = (U32)mLength;
seq->offset = offset;
rawSeqStore->size++;
}
/* Insert the current entry into the hash table --- it must be
* done after the previous block to avoid clobbering bestEntry */
ZSTD_ldm_insertEntry(ldmState, hash, newEntry, *params);
assert(ip + backwardMatchLength == lastHashed);
/* Fill the hash table from lastHashed+1 to ip+mLength*/
/* Heuristic: don't need to fill the entire table at end of block */
if (ip + mLength <= ilimit) {
rollingHash = ZSTD_ldm_fillLdmHashTable(
ldmState, rollingHash, lastHashed,
ip + mLength, base, hBits, *params);
lastHashed = ip + mLength - 1;
}
ip += mLength;
anchor = ip;
ip += hashed;
}
return iend - anchor;
}