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Merge pull request #2602 from terrelln/ldm-opt 

[LDM] Speed optimization on repetitive data
dev
Nick Terrell 2021-05-04 23:13:09 -07:00 committed by GitHub
parent d40f55cd95 32823bc150
commit 8389a5122b
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 47 additions and 12 deletions
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55
lib/compress/zstd_ldm.c
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@ -57,6 +57,33 @@ static void ZSTD_ldm_gear_init(ldmRollingHashState_t* state, ldmParams_t const*
}
}
/** ZSTD_ldm_gear_reset()
* Feeds [data, data + minMatchLength) into the hash without registering any
* splits. This effectively resets the hash state. This is used when skipping
* over data, either at the beginning of a block, or skipping sections.
*/
static void ZSTD_ldm_gear_reset(ldmRollingHashState_t* state,
BYTE const* data, size_t minMatchLength)
{
U64 hash = state->rolling;
size_t n = 0;
#define GEAR_ITER_ONCE() do { \
hash = (hash << 1) + ZSTD_ldm_gearTab[data[n] & 0xff]; \
n += 1; \
} while (0)
while (n + 3 < minMatchLength) {
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
GEAR_ITER_ONCE();
}
while (n < minMatchLength) {
GEAR_ITER_ONCE();
}
#undef GEAR_ITER_ONCE
}
/** ZSTD_ldm_gear_feed():
*
* Registers in the splits array all the split points found in the first
@ -327,16 +354,8 @@ static size_t ZSTD_ldm_generateSequences_internal(
/* Initialize the rolling hash state with the first minMatchLength bytes */
ZSTD_ldm_gear_init(&hashState, params);
{
size_t n = 0;
while (n < minMatchLength) {
numSplits = 0;
n += ZSTD_ldm_gear_feed(&hashState, ip + n, minMatchLength - n,
splits, &numSplits);
}
ZSTD_ldm_gear_reset(&hashState, ip, minMatchLength);
ip += minMatchLength;
}
while (ip < ilimit) {
size_t hashed;
@ -361,6 +380,7 @@ static size_t ZSTD_ldm_generateSequences_internal(
for (n = 0; n < numSplits; n++) {
size_t forwardMatchLength = 0, backwardMatchLength = 0,
bestMatchLength = 0, mLength;
U32 offset;
BYTE const* const split = candidates[n].split;
U32 const checksum = candidates[n].checksum;
U32 const hash = candidates[n].hash;
@ -428,9 +448,9 @@ static size_t ZSTD_ldm_generateSequences_internal(
}
/* Match found */
offset = (U32)(split - base) - bestEntry->offset;
mLength = forwardMatchLength + backwardMatchLength;
{
U32 const offset = (U32)(split - base) - bestEntry->offset;
rawSeq* const seq = rawSeqStore->seq + rawSeqStore->size;
/* Out of sequence storage */
@ -447,6 +467,21 @@ static size_t ZSTD_ldm_generateSequences_internal(
ZSTD_ldm_insertEntry(ldmState, hash, newEntry, *params);
anchor = split + forwardMatchLength;
/* If we find a match that ends after the data that we've hashed
* then we have a repeating, overlapping, pattern. E.g. all zeros.
* If one repetition of the pattern matches our `stopMask` then all
* repetitions will. We don't need to insert them all into out table,
* only the first one. So skip over overlapping matches.
* This is a major speed boost (20x) for compressing a single byte
* repeated, when that byte ends up in the table.
*/
if (anchor > ip + hashed) {
ZSTD_ldm_gear_reset(&hashState, anchor - minMatchLength, minMatchLength);
/* Continue the outter loop at anchor (ip + hashed == anchor). */
ip = anchor - hashed;
break;
}
}
ip += hashed;