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refactor search into an inline function

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for easier swapping with a parameter
This commit is contained in:
Yann Collet
2024-10-08 11:10:48 -07:00
parent 1e7fa242f4
commit 2cc600bab2
2 changed files with 47 additions and 48 deletions
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lib/compress/zstd_compress_internal.h
+2 -2
View File
@@ -561,14 +561,14 @@ MEM_STATIC int ZSTD_cParam_withinBounds(ZSTD_cParameter cParam, int value)
* @return a >= b ? trueAddr : falseAddr,
* tries to force branchless codegen. */
MEM_STATIC const BYTE*
ZSTD_selectAddr(U32 a, U32 b, const BYTE* trueAddr, const BYTE* falseAddr)
ZSTD_selectAddr(U32 index, U32 lowLimit, const BYTE* trueAddr, const BYTE* falseAddr)
{
#if defined(__GNUC__) && defined(__x86_64__)
__asm__ (
"cmp %1, %2\n"
"cmova %3, %0\n"
: "+r"(trueAddr)
: "r"(a), "r"(b), "r"(falseAddr)
: "r"(index), "r"(lowLimit), "r"(falseAddr)
);
return trueAddr;
#else
lib/compress/zstd_fast.c
+45 -46
View File
@@ -97,6 +97,18 @@ void ZSTD_fillHashTable(ZSTD_matchState_t* ms,
}
static int
ZSTD_findMatch_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit, const BYTE* fakeAddress)
{
/* idx >= prefixStartIndex 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. */
const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, fakeAddress);
return ((MEM_read32(currentPtr) == MEM_read32(mvalAddr)) & (currentIdx >= lowLimit));
}
/**
* If you squint hard enough (and ignore repcodes), the search operation at any
* given position is broken into 4 stages:
@@ -148,13 +160,12 @@ 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, U32 const unpredictable)
{
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);
@@ -193,6 +204,7 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
size_t step;
const BYTE* nextStep;
const size_t kStepIncr = (1 << (kSearchStrength - 1));
(void)unpredictable;
DEBUGLOG(5, "ZSTD_compressBlock_fast_generic");
ip0 += (ip0 == prefixStart);
@@ -249,25 +261,15 @@ _start: /* Requires: ip0 */
goto _match;
}
/* idx >= prefixStartIndex 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. */
{ const BYTE* mvalAddr = ZSTD_selectAddr(idx, prefixStartIndex, base + idx, &dummy[0]);
/* load match for ip[0] */
U32 const mval = MEM_read32(mvalAddr);
if (ZSTD_findMatch_cmov(ip0, base + idx, idx, prefixStartIndex, dummy)) {
/* found a match! */
/* check match at ip[0] */
if (MEM_read32(ip0) == mval && idx >= prefixStartIndex) {
/* found a match! */
/* 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);
/* 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;
}
goto _offset;
}
/* lookup ip[1] */
@@ -286,30 +288,24 @@ _start: /* Requires: ip0 */
current0 = (U32)(ip0 - base);
hashTable[hash0] = current0;
{ const BYTE* mvalAddr = ZSTD_selectAddr(idx, prefixStartIndex, base + idx, &dummy[0]);
/* load match for ip[0] */
U32 const mval = MEM_read32(mvalAddr);
if (ZSTD_findMatch_cmov(ip0, base + idx, idx, prefixStartIndex, dummy)) {
/* found a match! */
/* check match at ip[0] */
if (MEM_read32(ip0) == mval && idx >= prefixStartIndex) {
/* found a match! */
/* first write next hash table entry; we've already calculated it */
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.
*/
hashTable[hash1] = (U32)(ip1 - base);
}
goto _offset;
/* first write next hash table entry; 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.
*/
hashTable[hash1] = (U32)(ip1 - base);
}
goto _offset;
}
/* lookup ip[1] */
@@ -409,12 +405,12 @@ _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, mls, cmov) \
static size_t ZSTD_compressBlock_fast_##dictMode##_##mls##_##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, mls, cmov); \
}
ZSTD_GEN_FAST_FN(noDict, 4, 1)
@@ -432,8 +428,10 @@ size_t ZSTD_compressBlock_fast(
void const* src, size_t srcSize)
{
U32 const mls = ms->cParams.minMatch;
/* use cmov instead of branch when the branch is likely unpredictable */
int const useCmov = 1;
assert(ms->dictMatchState == NULL);
if (ms->cParams.targetLength > 1) {
if (useCmov) {
switch(mls)
{
default: /* includes case 3 */
@@ -447,6 +445,7 @@ size_t ZSTD_compressBlock_fast(
return ZSTD_compressBlock_fast_noDict_7_1(ms, seqStore, rep, src, srcSize);
}
} else {
/* use a branch instead */
switch(mls)
{
default: /* includes case 3 */