Use Look-Ahead Hash for Next Long Check after Short Match (+0.5% Speed)
This costs a little ratio, unfortunately.dev
parent
db4e1b5479
commit
39f2491bfc
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@ -139,6 +139,8 @@ _start:
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goto _match_stored;
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}
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hl1 = ZSTD_hashPtr(ip1, hBitsL, 8);
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if (idxl0 > prefixLowestIndex) {
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/* check prefix long match */
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if (MEM_read64(matchl0) == MEM_read64(ip)) {
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@ -149,8 +151,6 @@ _start:
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}
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}
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hl1 = ZSTD_hashPtr(ip1, hBitsL, 8);
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if (idxs0 > prefixLowestIndex) {
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/* check prefix short match */
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if (MEM_read32(matchs0) == MEM_read32(ip)) {
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@ -184,13 +184,12 @@ _cleanup:
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_search_next_long:
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{ idxl1 = hashLong[hl1];
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matchl1 = base + idxl1;
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hashLong[hl1] = curr + 1;
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/* check prefix long +1 match */
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if (idxl1 > prefixLowestIndex) {
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if (MEM_read64(matchl1) == MEM_read64(ip+1)) {
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mLength = ZSTD_count(ip+9, matchl1+8, iend) + 8;
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ip++;
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if (MEM_read64(matchl1) == MEM_read64(ip1)) {
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ip = ip1;
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mLength = ZSTD_count(ip+8, matchl1+8, iend) + 8;
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offset = (U32)(ip-matchl1);
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while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
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goto _match_found;
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@ -205,6 +204,17 @@ _search_next_long:
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while (((ip>anchor) & (matchs0>prefixLowest)) && (ip[-1] == matchs0[-1])) { ip--; matchs0--; mLength++; } /* catch up */
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}
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if (step < 4) {
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/* It is unsafe to write this value back to the hashtable when ip1 is
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* greater than or equal to the new ip we will have after we're done
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* processing this match. Rather than perform that test directly
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* (ip1 >= ip + mLength), which costs speed in practice, we do a simpler
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* more predictable test. The minmatch even if we take a short match is
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* 4 bytes, so as long as step, the distance between ip and ip1
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* (initially) is less than 4, we know ip1 < new ip. */
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hashLong[hl1] = (U32)(ip1 - base);
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}
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/* fall-through */
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_match_found: /* requires ip, offset, mLength */
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