From: Yann Collet Date: Fri, 19 Aug 2022 23:04:28 +0000 (-0700) Subject: just add some comments to zstd_opt for improved clarity X-Git-Tag: v1.5.4^2~67^2~1 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4a1a79a51229bafbd89964043af236ebd4707b38;p=thirdparty%2Fzstd.git just add some comments to zstd_opt for improved clarity --- diff --git a/lib/compress/zstd_opt.c b/lib/compress/zstd_opt.c index 6e08dc8c7..f0f3791e7 100644 --- a/lib/compress/zstd_opt.c +++ b/lib/compress/zstd_opt.c @@ -37,11 +37,16 @@ # define WEIGHT(stat,opt) (opt ? ZSTD_fracWeight(stat) : ZSTD_bitWeight(stat)) #endif +/* ZSTD_bitWeight() : + * provide estimated "cost" of a stat in full bits only */ MEM_STATIC U32 ZSTD_bitWeight(U32 stat) { return (ZSTD_highbit32(stat+1) * BITCOST_MULTIPLIER); } +/* ZSTD_fracWeight() : + * provide fractional-bit "cost" of a stat, + * using linear interpolation approximation */ MEM_STATIC U32 ZSTD_fracWeight(U32 rawStat) { U32 const stat = rawStat + 1; @@ -50,6 +55,7 @@ MEM_STATIC U32 ZSTD_fracWeight(U32 rawStat) U32 const FWeight = (stat << BITCOST_ACCURACY) >> hb; U32 const weight = BWeight + FWeight; assert(hb + BITCOST_ACCURACY < 31); + assert(FWeight < BITCOST_MULTIPLIER); return weight; } @@ -91,17 +97,19 @@ static U32 sum_u32(const unsigned table[], size_t nbElts) static U32 ZSTD_downscaleStats(unsigned* table, U32 lastEltIndex, U32 shift) { U32 s, sum=0; - DEBUGLOG(5, "ZSTD_downscaleStats (nbElts=%u, shift=%u)", (unsigned)lastEltIndex+1, (unsigned)shift); + DEBUGLOG(5, "ZSTD_downscaleStats (nbElts=%u, shift=%u)", + (unsigned)lastEltIndex+1, (unsigned)shift ); assert(shift < 30); for (s=0; s> shift); - sum += table[s]; + unsigned newStat = 1 + (table[s] >> shift); + sum += newStat; + table[s] = newStat; } return sum; } /* ZSTD_scaleStats() : - * reduce all elements in table is sum too large + * reduce all elt frequencies in table if sum too large * return the resulting sum of elements */ static U32 ZSTD_scaleStats(unsigned* table, U32 lastEltIndex, U32 logTarget) { @@ -129,18 +137,22 @@ ZSTD_rescaleFreqs(optState_t* const optPtr, DEBUGLOG(5, "ZSTD_rescaleFreqs (srcSize=%u)", (unsigned)srcSize); optPtr->priceType = zop_dynamic; - if (optPtr->litLengthSum == 0) { /* first block : init */ - if (srcSize <= ZSTD_PREDEF_THRESHOLD) { /* heuristic */ - DEBUGLOG(5, "(srcSize <= ZSTD_PREDEF_THRESHOLD) => zop_predef"); + if (optPtr->litLengthSum == 0) { /* no literals stats collected -> first block assumed -> init */ + + /* heuristic: use pre-defined stats for too small inputs */ + if (srcSize <= ZSTD_PREDEF_THRESHOLD) { + DEBUGLOG(5, "srcSize <= %i : use predefined stats", ZSTD_PREDEF_THRESHOLD); optPtr->priceType = zop_predef; } assert(optPtr->symbolCosts != NULL); if (optPtr->symbolCosts->huf.repeatMode == HUF_repeat_valid) { - /* huffman table presumed generated by dictionary */ + + /* huffman stats covering the full value set : table presumed generated by dictionary */ optPtr->priceType = zop_dynamic; if (compressedLiterals) { + /* generate literals statistics from huffman table */ unsigned lit; assert(optPtr->litFreq != NULL); optPtr->litSum = 0; @@ -188,10 +200,11 @@ ZSTD_rescaleFreqs(optState_t* const optPtr, optPtr->offCodeSum += optPtr->offCodeFreq[of]; } } - } else { /* not a dictionary */ + } else { /* huf.repeatMode != HUF_repeat_valid => presumed not a dictionary */ assert(optPtr->litFreq != NULL); if (compressedLiterals) { + /* base initial cost of literals on direct frequency within src */ unsigned lit = MaxLit; HIST_count_simple(optPtr->litFreq, &lit, src, srcSize); /* use raw first block to init statistics */ optPtr->litSum = ZSTD_downscaleStats(optPtr->litFreq, MaxLit, 8); @@ -224,10 +237,9 @@ ZSTD_rescaleFreqs(optState_t* const optPtr, optPtr->offCodeSum = sum_u32(baseOFCfreqs, MaxOff+1); } - } - } else { /* new block : re-use previous statistics, scaled down */ + } else { /* new block : scale down accumulated statistics */ if (compressedLiterals) optPtr->litSum = ZSTD_scaleStats(optPtr->litFreq, MaxLit, 12); @@ -275,10 +287,11 @@ static U32 ZSTD_litLengthPrice(U32 const litLength, const optState_t* const optP assert(litLength <= ZSTD_BLOCKSIZE_MAX); if (optPtr->priceType == zop_predef) return WEIGHT(litLength, optLevel); - /* We can't compute the litLength price for sizes >= ZSTD_BLOCKSIZE_MAX - * because it isn't representable in the zstd format. So instead just - * call it 1 bit more than ZSTD_BLOCKSIZE_MAX - 1. In this case the block - * would be all literals. + + /* ZSTD_LLcode() can't compute litLength price for sizes >= ZSTD_BLOCKSIZE_MAX + * because it isn't representable in the zstd format. + * So instead just pretend it would cost 1 bit more than ZSTD_BLOCKSIZE_MAX - 1. + * In such a case, the block would be all literals. */ if (litLength == ZSTD_BLOCKSIZE_MAX) return BITCOST_MULTIPLIER + ZSTD_litLengthPrice(ZSTD_BLOCKSIZE_MAX - 1, optPtr, optLevel); @@ -292,7 +305,7 @@ static U32 ZSTD_litLengthPrice(U32 const litLength, const optState_t* const optP } /* ZSTD_getMatchPrice() : - * Provides the cost of the match part (offset + matchLength) of a sequence + * Provides the cost of the match part (offset + matchLength) of a sequence. * Must be combined with ZSTD_fullLiteralsCost() to get the full cost of a sequence. * @offBase : sumtype, representing an offset or a repcode, and using numeric representation of ZSTD_storeSeq() * @optLevel: when <2, favors small offset for decompression speed (improved cache efficiency) @@ -308,8 +321,9 @@ ZSTD_getMatchPrice(U32 const offBase, U32 const mlBase = matchLength - MINMATCH; assert(matchLength >= MINMATCH); - if (optPtr->priceType == zop_predef) /* fixed scheme, do not use statistics */ - return WEIGHT(mlBase, optLevel) + ((16 + offCode) * BITCOST_MULTIPLIER); + if (optPtr->priceType == zop_predef) /* fixed scheme, does not use statistics */ + return WEIGHT(mlBase, optLevel) + + ((16 + offCode) * BITCOST_MULTIPLIER); /* emulated offset cost */ /* dynamic statistics */ price = (offCode * BITCOST_MULTIPLIER) + (optPtr->offCodeSumBasePrice - WEIGHT(optPtr->offCodeFreq[offCode], optLevel)); @@ -347,7 +361,7 @@ static void ZSTD_updateStats(optState_t* const optPtr, optPtr->litLengthSum++; } - /* offset code : expected to follow storeSeq() numeric representation */ + /* offset code : follows storeSeq() numeric representation */ { U32 const offCode = ZSTD_highbit32(offBase); assert(offCode <= MaxOff); optPtr->offCodeFreq[offCode]++; @@ -1352,7 +1366,7 @@ size_t ZSTD_compressBlock_btopt( /* ZSTD_initStats_ultra(): * make a first compression pass, just to seed stats with more accurate starting values. * only works on first block, with no dictionary and no ldm. - * this function cannot error, hence its contract must be respected. + * this function cannot error out, its narrow contract must be respected. */ static void ZSTD_initStats_ultra(ZSTD_matchState_t* ms, @@ -1371,7 +1385,7 @@ ZSTD_initStats_ultra(ZSTD_matchState_t* ms, ZSTD_compressBlock_opt2(ms, seqStore, tmpRep, src, srcSize, ZSTD_noDict); /* generate stats into ms->opt*/ - /* invalidate first scan from history */ + /* invalidate first scan from history, only keep entropy stats */ ZSTD_resetSeqStore(seqStore); ms->window.base -= srcSize; ms->window.dictLimit += (U32)srcSize; @@ -1395,20 +1409,20 @@ size_t ZSTD_compressBlock_btultra2( U32 const curr = (U32)((const BYTE*)src - ms->window.base); DEBUGLOG(5, "ZSTD_compressBlock_btultra2 (srcSize=%zu)", srcSize); - /* 2-pass strategy: + /* 2-passes strategy: * this strategy makes a first pass over first block to collect statistics - * and seed next round's statistics with it. - * After 1st pass, function forgets everything, and starts a new block. + * in order to seed next round's statistics with it. + * After 1st pass, function forgets history, and starts a new block. * Consequently, this can only work if no data has been previously loaded in tables, * aka, no dictionary, no prefix, no ldm preprocessing. * The compression ratio gain is generally small (~0.5% on first block), - * the cost is 2x cpu time on first block. */ + ** the cost is 2x cpu time on first block. */ assert(srcSize <= ZSTD_BLOCKSIZE_MAX); if ( (ms->opt.litLengthSum==0) /* first block */ && (seqStore->sequences == seqStore->sequencesStart) /* no ldm */ && (ms->window.dictLimit == ms->window.lowLimit) /* no dictionary */ - && (curr == ms->window.dictLimit) /* start of frame, nothing already loaded nor skipped */ - && (srcSize > ZSTD_PREDEF_THRESHOLD) + && (curr == ms->window.dictLimit) /* start of frame, nothing already loaded nor skipped */ + && (srcSize > ZSTD_PREDEF_THRESHOLD) /* input large enough to not employ default stats */ ) { ZSTD_initStats_ultra(ms, seqStore, rep, src, srcSize); }