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Use Look-Ahead Hash for Next Long Check after Short Match (+0.5% Speed) 

This costs a little ratio, unfortunately.
dev
W. Felix Handte 2021-09-08 12:41:15 -04:00
parent db4e1b5479
commit 39f2491bfc
1 changed files with 16 additions and 6 deletions
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22
lib/compress/zstd_double_fast.c
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@ -139,6 +139,8 @@ _start:
goto _match_stored;
}
hl1 = ZSTD_hashPtr(ip1, hBitsL, 8);
if (idxl0 > prefixLowestIndex) {
/* check prefix long match */
if (MEM_read64(matchl0) == MEM_read64(ip)) {
@ -149,8 +151,6 @@ _start:
}
}
hl1 = ZSTD_hashPtr(ip1, hBitsL, 8);
if (idxs0 > prefixLowestIndex) {
/* check prefix short match */
if (MEM_read32(matchs0) == MEM_read32(ip)) {
@ -184,13 +184,12 @@ _cleanup:
_search_next_long:
{ idxl1 = hashLong[hl1];
matchl1 = base + idxl1;
hashLong[hl1] = curr + 1;
/* check prefix long +1 match */
if (idxl1 > prefixLowestIndex) {
if (MEM_read64(matchl1) == MEM_read64(ip+1)) {
mLength = ZSTD_count(ip+9, matchl1+8, iend) + 8;
ip++;
if (MEM_read64(matchl1) == MEM_read64(ip1)) {
ip = ip1;
mLength = ZSTD_count(ip+8, matchl1+8, iend) + 8;
offset = (U32)(ip-matchl1);
while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
goto _match_found;
@ -205,6 +204,17 @@ _search_next_long:
while (((ip>anchor) & (matchs0>prefixLowest)) && (ip[-1] == matchs0[-1])) { ip--; matchs0--; mLength++; } /* catch up */
}
if (step < 4) {
/* It is unsafe to write this value back to the hashtable when ip1 is
* greater than or equal to the new ip we will have after we're done
* processing this match. Rather than perform that test directly
* (ip1 >= ip + mLength), which costs speed in practice, we do a simpler
* more predictable test. The minmatch even if we take a short match is
* 4 bytes, so as long as step, the distance between ip and ip1
* (initially) is less than 4, we know ip1 < new ip. */
hashLong[hl1] = (U32)(ip1 - base);
}
/* fall-through */
_match_found: /* requires ip, offset, mLength */