diff --git a/lib/compress/zstd_compress_internal.h b/lib/compress/zstd_compress_internal.h index 6c76b5848..622903398 100644 --- a/lib/compress/zstd_compress_internal.h +++ b/lib/compress/zstd_compress_internal.h @@ -557,6 +557,25 @@ MEM_STATIC int ZSTD_cParam_withinBounds(ZSTD_cParameter cParam, int value) return 1; } +/* ZSTD_selectAddr: + * @return index >= lowLimit ? candidate : backup, + * tries to force branchless codegen. */ +MEM_STATIC const BYTE* +ZSTD_selectAddr(U32 index, U32 lowLimit, const BYTE* candidate, const BYTE* backup) +{ +#if defined(__GNUC__) && defined(__x86_64__) + __asm__ ( + "cmp %1, %2\n" + "cmova %3, %0\n" + : "+r"(candidate) + : "r"(index), "r"(lowLimit), "r"(backup) + ); + return candidate; +#else + return index >= lowLimit ? candidate : backup; +#endif +} + /* ZSTD_noCompressBlock() : * Writes uncompressed block to dst buffer from given src. * Returns the size of the block */ diff --git a/lib/compress/zstd_double_fast.c b/lib/compress/zstd_double_fast.c index 1b5f64f20..c04010b11 100644 --- a/lib/compress/zstd_double_fast.c +++ b/lib/compress/zstd_double_fast.c @@ -140,11 +140,17 @@ size_t ZSTD_compressBlock_doubleFast_noDict_generic( U32 idxl1; /* the long match index for ip1 */ const BYTE* matchl0; /* the long match for ip */ + const BYTE* matchl0_safe; /* matchl0 or safe address */ const BYTE* matchs0; /* the short match for ip */ const BYTE* matchl1; /* the long match for ip1 */ + const BYTE* matchs0_safe; /* matchs0 or safe address */ const BYTE* ip = istart; /* the current position */ const BYTE* ip1; /* the next position */ + /* Array of ~random data, should have low probability of matching data + * we load from here instead of from tables, if matchl0/matchl1 are + * invalid indices. Used to avoid unpredictable branches. */ + const BYTE dummy[] = {0x12,0x34,0x56,0x78,0x9a,0xbc,0xde,0xf0,0xe2,0xb4}; DEBUGLOG(5, "ZSTD_compressBlock_doubleFast_noDict_generic"); @@ -191,24 +197,29 @@ size_t ZSTD_compressBlock_doubleFast_noDict_generic( hl1 = ZSTD_hashPtr(ip1, hBitsL, 8); - if (idxl0 > prefixLowestIndex) { - /* check prefix long match */ - if (MEM_read64(matchl0) == MEM_read64(ip)) { - mLength = ZSTD_count(ip+8, matchl0+8, iend) + 8; - offset = (U32)(ip-matchl0); - while (((ip>anchor) & (matchl0>prefixLowest)) && (ip[-1] == matchl0[-1])) { ip--; matchl0--; mLength++; } /* catch up */ - goto _match_found; - } + /* idxl0 > prefixLowestIndex is a (somewhat) unpredictable branch. + * However expression below complies into conditional move. Since + * match is unlikely and we only *branch* on idxl0 > prefixLowestIndex + * if there is a match, all branches become predictable. */ + matchl0_safe = ZSTD_selectAddr(idxl0, prefixLowestIndex, matchl0, &dummy[0]); + + /* check prefix long match */ + if (MEM_read64(matchl0_safe) == MEM_read64(ip) && matchl0_safe == matchl0) { + mLength = ZSTD_count(ip+8, matchl0+8, iend) + 8; + offset = (U32)(ip-matchl0); + while (((ip>anchor) & (matchl0>prefixLowest)) && (ip[-1] == matchl0[-1])) { ip--; matchl0--; mLength++; } /* catch up */ + goto _match_found; } idxl1 = hashLong[hl1]; matchl1 = base + idxl1; - if (idxs0 > prefixLowestIndex) { - /* check prefix short match */ - if (MEM_read32(matchs0) == MEM_read32(ip)) { - goto _search_next_long; - } + /* Same optimization as matchl0 above */ + matchs0_safe = ZSTD_selectAddr(idxs0, prefixLowestIndex, matchs0, &dummy[0]); + + /* check prefix short match */ + if(MEM_read32(matchs0_safe) == MEM_read32(ip) && matchs0_safe == matchs0) { + goto _search_next_long; } if (ip1 >= nextStep) { @@ -651,7 +662,7 @@ size_t ZSTD_compressBlock_doubleFast_extDict_generic( size_t mLength; hashSmall[hSmall] = hashLong[hLong] = curr; /* update hash table */ - if (((ZSTD_index_overlap_check(prefixStartIndex, repIndex)) + if (((ZSTD_index_overlap_check(prefixStartIndex, repIndex)) & (offset_1 <= curr+1 - dictStartIndex)) /* note: we are searching at curr+1 */ && (MEM_read32(repMatch) == MEM_read32(ip+1)) ) { const BYTE* repMatchEnd = repIndex < prefixStartIndex ? dictEnd : iend; diff --git a/lib/compress/zstd_fast.c b/lib/compress/zstd_fast.c index c6baa49d4..e7edf9490 100644 --- a/lib/compress/zstd_fast.c +++ b/lib/compress/zstd_fast.c @@ -45,7 +45,7 @@ void ZSTD_fillHashTableForCDict(ZSTD_matchState_t* ms, size_t const hashAndTag = ZSTD_hashPtr(ip + p, hBits, mls); if (hashTable[hashAndTag >> ZSTD_SHORT_CACHE_TAG_BITS] == 0) { /* not yet filled */ ZSTD_writeTaggedIndex(hashTable, hashAndTag, curr + p); - } } } } + } } } } } static @@ -97,6 +97,50 @@ void ZSTD_fillHashTable(ZSTD_matchState_t* ms, } +typedef int (*ZSTD_match4Found) (const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit); + +static int +ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit) +{ + /* Array of ~random data, should have low probability of matching data. + * Load from here if the index is invalid. + * Used to avoid unpredictable branches. */ + static const BYTE dummy[] = {0x12,0x34,0x56,0x78}; + + /* currentIdx >= lowLimit is a (somewhat) unpredictable branch. + * However expression below compiles into conditional move. + */ + const BYTE* mvalAddr = ZSTD_selectAddr(matchIdx, idxLowLimit, matchAddress, dummy); + /* Note: this used to be written as : return test1 && test2; + * Unfortunately, once inlined, these tests become branches, + * in which case it becomes critical that they are executed in the right order (test1 then test2). + * So we have to write these tests in a specific manner to ensure their ordering. + */ + if (MEM_read32(currentPtr) != MEM_read32(mvalAddr)) return 0; + /* force ordering of these tests, which matters once the function is inlined, as they become branches */ +#if defined(__GNUC__) + __asm__(""); +#endif + return matchIdx >= idxLowLimit; +} + +static int +ZSTD_match4Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 matchIdx, U32 idxLowLimit) +{ + /* using a branch instead of a cmov, + * because it's faster in scenarios where matchIdx >= idxLowLimit is generally true, + * aka almost all candidates are within range */ + U32 mval; + if (matchIdx >= idxLowLimit) { + mval = MEM_read32(matchAddress); + } else { + mval = MEM_read32(currentPtr) ^ 1; /* guaranteed to not match. */ + } + + return (MEM_read32(currentPtr) == mval); +} + + /** * If you squint hard enough (and ignore repcodes), the search operation at any * given position is broken into 4 stages: @@ -148,13 +192,12 @@ ZSTD_ALLOW_POINTER_OVERFLOW_ATTR size_t ZSTD_compressBlock_fast_noDict_generic( ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM], void const* src, size_t srcSize, - U32 const mls, U32 const hasStep) + U32 const mls, int useCmov) { const ZSTD_compressionParameters* const cParams = &ms->cParams; U32* const hashTable = ms->hashTable; U32 const hlog = cParams->hashLog; - /* support stepSize of 0 */ - size_t const stepSize = hasStep ? (cParams->targetLength + !(cParams->targetLength) + 1) : 2; + size_t const stepSize = cParams->targetLength + !(cParams->targetLength) + 1; /* min 2 */ const BYTE* const base = ms->window.base; const BYTE* const istart = (const BYTE*)src; const U32 endIndex = (U32)((size_t)(istart - base) + srcSize); @@ -176,8 +219,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic( size_t hash0; /* hash for ip0 */ size_t hash1; /* hash for ip1 */ - U32 idx; /* match idx for ip0 */ - U32 mval; /* src value at match idx */ + U32 matchIdx; /* match idx for ip0 */ U32 offcode; const BYTE* match0; @@ -190,6 +232,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic( size_t step; const BYTE* nextStep; const size_t kStepIncr = (1 << (kSearchStrength - 1)); + const ZSTD_match4Found matchFound = useCmov ? ZSTD_match4Found_cmov : ZSTD_match4Found_branch; DEBUGLOG(5, "ZSTD_compressBlock_fast_generic"); ip0 += (ip0 == prefixStart); @@ -218,7 +261,7 @@ _start: /* Requires: ip0 */ hash0 = ZSTD_hashPtr(ip0, hlog, mls); hash1 = ZSTD_hashPtr(ip1, hlog, mls); - idx = hashTable[hash0]; + matchIdx = hashTable[hash0]; do { /* load repcode match for ip[2]*/ @@ -238,35 +281,25 @@ _start: /* Requires: ip0 */ offcode = REPCODE1_TO_OFFBASE; mLength += 4; - /* First write next hash table entry; we've already calculated it. - * This write is known to be safe because the ip1 is before the + /* Write next hash table entry: it's already calculated. + * This write is known to be safe because ip1 is before the * repcode (ip2). */ hashTable[hash1] = (U32)(ip1 - base); goto _match; } - /* load match for ip[0] */ - if (idx >= prefixStartIndex) { - mval = MEM_read32(base + idx); - } else { - mval = MEM_read32(ip0) ^ 1; /* guaranteed to not match. */ - } - - /* check match at ip[0] */ - if (MEM_read32(ip0) == mval) { - /* found a match! */ - - /* First write next hash table entry; we've already calculated it. - * This write is known to be safe because the ip1 == ip0 + 1, so - * we know we will resume searching after ip1 */ + if (matchFound(ip0, base + matchIdx, matchIdx, prefixStartIndex)) { + /* Write next hash table entry (it's already calculated). + * This write is known to be safe because the ip1 == ip0 + 1, + * so searching will resume after ip1 */ hashTable[hash1] = (U32)(ip1 - base); goto _offset; } /* lookup ip[1] */ - idx = hashTable[hash1]; + matchIdx = hashTable[hash1]; /* hash ip[2] */ hash0 = hash1; @@ -281,36 +314,19 @@ _start: /* Requires: ip0 */ current0 = (U32)(ip0 - base); hashTable[hash0] = current0; - /* load match for ip[0] */ - if (idx >= prefixStartIndex) { - mval = MEM_read32(base + idx); - } else { - mval = MEM_read32(ip0) ^ 1; /* guaranteed to not match. */ - } - - /* check match at ip[0] */ - if (MEM_read32(ip0) == mval) { - /* found a match! */ - - /* first write next hash table entry; we've already calculated it */ + if (matchFound(ip0, base + matchIdx, matchIdx, prefixStartIndex)) { + /* Write next hash table entry, since it's already calculated */ 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. - */ + /* Avoid writing an index if it's >= position where search will resume. + * The minimum possible match has length 4, so search can resume at ip0 + 4. + */ hashTable[hash1] = (U32)(ip1 - base); } - goto _offset; } /* lookup ip[1] */ - idx = hashTable[hash1]; + matchIdx = hashTable[hash1]; /* hash ip[2] */ hash0 = hash1; @@ -332,7 +348,7 @@ _start: /* Requires: ip0 */ } while (ip3 < ilimit); _cleanup: - /* Note that there are probably still a couple positions we could search. + /* Note that there are probably still a couple positions one could search. * However, it seems to be a meaningful performance hit to try to search * them. So let's not. */ @@ -361,7 +377,7 @@ _cleanup: _offset: /* Requires: ip0, idx */ /* Compute the offset code. */ - match0 = base + idx; + match0 = base + matchIdx; rep_offset2 = rep_offset1; rep_offset1 = (U32)(ip0-match0); offcode = OFFSET_TO_OFFBASE(rep_offset1); @@ -406,12 +422,12 @@ _match: /* Requires: ip0, match0, offcode */ goto _start; } -#define ZSTD_GEN_FAST_FN(dictMode, mls, step) \ - static size_t ZSTD_compressBlock_fast_##dictMode##_##mls##_##step( \ +#define ZSTD_GEN_FAST_FN(dictMode, mml, cmov) \ + static size_t ZSTD_compressBlock_fast_##dictMode##_##mml##_##cmov( \ ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM], \ void const* src, size_t srcSize) \ { \ - return ZSTD_compressBlock_fast_##dictMode##_generic(ms, seqStore, rep, src, srcSize, mls, step); \ + return ZSTD_compressBlock_fast_##dictMode##_generic(ms, seqStore, rep, src, srcSize, mml, cmov); \ } ZSTD_GEN_FAST_FN(noDict, 4, 1) @@ -428,10 +444,12 @@ size_t ZSTD_compressBlock_fast( ZSTD_matchState_t* ms, seqStore_t* seqStore, U32 rep[ZSTD_REP_NUM], void const* src, size_t srcSize) { - U32 const mls = ms->cParams.minMatch; + U32 const mml = ms->cParams.minMatch; + /* use cmov when "candidate in range" branch is likely unpredictable */ + int const useCmov = ms->cParams.windowLog < 19; assert(ms->dictMatchState == NULL); - if (ms->cParams.targetLength > 1) { - switch(mls) + if (useCmov) { + switch(mml) { default: /* includes case 3 */ case 4 : @@ -444,7 +462,8 @@ size_t ZSTD_compressBlock_fast( return ZSTD_compressBlock_fast_noDict_7_1(ms, seqStore, rep, src, srcSize); } } else { - switch(mls) + /* use a branch instead */ + switch(mml) { default: /* includes case 3 */ case 4 : @@ -456,7 +475,6 @@ size_t ZSTD_compressBlock_fast( case 7 : return ZSTD_compressBlock_fast_noDict_7_0(ms, seqStore, rep, src, srcSize); } - } } @@ -546,7 +564,7 @@ size_t ZSTD_compressBlock_fast_dictMatchState_generic( size_t const dictHashAndTag1 = ZSTD_hashPtr(ip1, dictHBits, mls); hashTable[hash0] = curr; /* update hash table */ - if ((ZSTD_index_overlap_check(prefixStartIndex, repIndex)) + if ((ZSTD_index_overlap_check(prefixStartIndex, repIndex)) && (MEM_read32(repMatch) == MEM_read32(ip0 + 1))) { const BYTE* const repMatchEnd = repIndex < prefixStartIndex ? dictEnd : iend; mLength = ZSTD_count_2segments(ip0 + 1 + 4, repMatch + 4, iend, repMatchEnd, prefixStart) + 4;