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ZSTD_fast_noDict: Avoid Safety Check When Writing ip1 into Table #3129

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Merged
merged 4 commits into from
May 11, 2022
Merged

ZSTD_fast_noDict: Avoid Safety Check When Writing ip1 into Table #3129

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040986a
ZSTD_fast_noDict: Minimize Checks When Writing Hash Table for ip1
felixhandte May 10, 2022
cd1f582
Hoist Hash Table Writes Up into Each Match Found Block
felixhandte May 11, 2022
1dd046a
Fix Comments Slightly
felixhandte May 11, 2022
1bc8019
Update results.csv
felixhandte May 11, 2022
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21 changes: 18 additions & 3 deletions lib/compress/zstd_fast.c
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Original file line number Diff line number Diff line change
Expand Up @@ -224,6 +224,23 @@ ZSTD_compressBlock_fast_noDict_generic(
/* check match at ip[0] */
if (MEM_read32(ip0) == mval) {
/* found a match! */
if (step > 4) {
/* We need to avoid writing an index into the hash table >= the
* position at which we will pick up our searching after we've
* taken this match.
*
* The minimum possible match has length 4, so the earliest ip0
* can be after we take this match will be the current ip0 + 4.
* ip1 is ip0 + step - 1. If ip1 is >= ip0 + 4, we can't safely
* write this position. The expedient thing to do is just to
* write a bad position.
*
* We perform this check here, separate from the write, because
* this is the only match path where this can occur. (In rep-
* code and the first match checks, ip1 == ip0 + 1.)
*/
ip1 = base;
}
goto _offset;
}

Expand Down Expand Up @@ -288,9 +305,7 @@ ZSTD_compressBlock_fast_noDict_generic(
anchor = ip0;

/* write next hash table entry */
if (ip1 < ip0) {
hashTable[hash1] = (U32)(ip1 - base);
}
hashTable[hash1] = (U32)(ip1 - base);

/* Fill table and check for immediate repcode. */
if (ip0 <= ilimit) {
Expand Down