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author | Yann Collet <cyan@fb.com> | 2017-12-22 07:07:25 (GMT) |
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committer | Yann Collet <cyan@fb.com> | 2017-12-22 07:07:25 (GMT) |
commit | 9753ac4c91b926114ae636d1d7103dd24e5c0f57 (patch) | |
tree | 84b09d9af6a2d1edc9493ed7fe9e89ac324e3f93 /lib/lz4hc.c | |
parent | 8a9c8e73241672c1db29be454a9b8388bfde5034 (diff) | |
download | lz4-9753ac4c91b926114ae636d1d7103dd24e5c0f57.zip lz4-9753ac4c91b926114ae636d1d7103dd24e5c0f57.tar.gz lz4-9753ac4c91b926114ae636d1d7103dd24e5c0f57.tar.bz2 |
conditional pattern analysis
Pattern analysis (currently limited to long ranges of identical bytes)
is actually detrimental to performance
when `nbSearches` is low.
Reason is : `nbSearches` provides a built-in protection for these cases.
The problem with patterns is that they dramatically increase the number of candidates to visit.
But with a low nbSearches, the match finder just aborts early.
In such cases, pattern analysis adds some complexity without reducing total nb of candidates.
It actually increases compression ratio a little bit, by filtering only "good" candidates,
but at a measurable speed cost, so it's not a good trade-off.
This patch makes pattern analysis optional.
It's enabled for levels 8+ only.
Diffstat (limited to 'lib/lz4hc.c')
-rw-r--r-- | lib/lz4hc.c | 21 |
1 files changed, 14 insertions, 7 deletions
diff --git a/lib/lz4hc.c b/lib/lz4hc.c index 388eb40..2fff29b 100644 --- a/lib/lz4hc.c +++ b/lib/lz4hc.c @@ -191,7 +191,8 @@ LZ4_FORCE_INLINE int LZ4HC_InsertAndGetWiderMatch ( int longest, const BYTE** matchpos, const BYTE** startpos, - const int maxNbAttempts) + const int maxNbAttempts, + const int patternAnalysis) { U16* const chainTable = hc4->chainTable; U32* const HashTable = hc4->hashTable; @@ -264,7 +265,7 @@ LZ4_FORCE_INLINE int LZ4HC_InsertAndGetWiderMatch ( { U32 const nextOffset = DELTANEXTU16(chainTable, matchIndex); matchIndex -= nextOffset; - if (nextOffset==1) { + if (patternAnalysis && nextOffset==1) { /* may be a repeated pattern */ if (repeat == rep_untested) { if ( ((pattern & 0xFFFF) == (pattern >> 16)) @@ -299,13 +300,14 @@ LZ4_FORCE_INLINE int LZ4HC_InsertAndFindBestMatch(LZ4HC_CCtx_internal* const hc4, /* Index table will be updated */ const BYTE* const ip, const BYTE* const iLimit, const BYTE** matchpos, - const int maxNbAttempts) + const int maxNbAttempts, + const int patternAnalysis) { const BYTE* uselessPtr = ip; /* note : LZ4HC_InsertAndGetWiderMatch() is able to modify the starting position of a match (*startpos), * but this won't be the case here, as we define iLowLimit==ip, * so LZ4HC_InsertAndGetWiderMatch() won't be allowed to search past ip */ - return LZ4HC_InsertAndGetWiderMatch(hc4, ip, ip, iLimit, MINMATCH-1, matchpos, &uselessPtr, maxNbAttempts); + return LZ4HC_InsertAndGetWiderMatch(hc4, ip, ip, iLimit, MINMATCH-1, matchpos, &uselessPtr, maxNbAttempts, patternAnalysis); } @@ -403,6 +405,7 @@ static int LZ4HC_compress_hashChain ( ) { const int inputSize = *srcSizePtr; + const int patternAnalysis = (maxNbAttempts > 64); /* levels 8+ */ const BYTE* ip = (const BYTE*) source; const BYTE* anchor = ip; @@ -433,7 +436,7 @@ static int LZ4HC_compress_hashChain ( /* Main Loop */ while (ip < mflimit) { - ml = LZ4HC_InsertAndFindBestMatch (ctx, ip, matchlimit, &ref, maxNbAttempts); + ml = LZ4HC_InsertAndFindBestMatch (ctx, ip, matchlimit, &ref, maxNbAttempts, patternAnalysis); if (ml<MINMATCH) { ip++; continue; } /* saved, in case we would skip too much */ @@ -443,7 +446,9 @@ static int LZ4HC_compress_hashChain ( _Search2: if (ip+ml < mflimit) - ml2 = LZ4HC_InsertAndGetWiderMatch(ctx, ip + ml - 2, ip + 0, matchlimit, ml, &ref2, &start2, maxNbAttempts); + ml2 = LZ4HC_InsertAndGetWiderMatch(ctx, + ip + ml - 2, ip + 0, matchlimit, ml, &ref2, &start2, + maxNbAttempts, patternAnalysis); else ml2 = ml; @@ -488,7 +493,9 @@ _Search3: /* Now, we have start2 = ip+new_ml, with new_ml = min(ml, OPTIMAL_ML=18) */ if (start2 + ml2 < mflimit) - ml3 = LZ4HC_InsertAndGetWiderMatch(ctx, start2 + ml2 - 3, start2, matchlimit, ml2, &ref3, &start3, maxNbAttempts); + ml3 = LZ4HC_InsertAndGetWiderMatch(ctx, + start2 + ml2 - 3, start2, matchlimit, ml2, &ref3, &start3, + maxNbAttempts, patternAnalysis); else ml3 = ml2; |