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store dummy bytes within ZSTD_match4Found_cmov()

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feels more logical, better contained
This commit is contained in:
Yann Collet 2024-10-08 16:02:54 -07:00
parent 197c258a79
commit 741b860fc1
1 changed files with 12 additions and 11 deletions
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23
lib/compress/zstd_fast.c
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@ -97,15 +97,20 @@ void ZSTD_fillHashTable(ZSTD_matchState_t* ms,
} }
typedef int (*ZSTD_match4Found) (const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit, const BYTE* fakeAddress); typedef int (*ZSTD_match4Found) (const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit);
static int static int
ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit, const BYTE* fakeAddress) ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit)
{ {
/* 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. /* currentIdx >= lowLimit is a (somewhat) unpredictable branch.
* However expression below compiles into conditional move. * However expression below compiles into conditional move.
*/ */
const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, fakeAddress); const BYTE* mvalAddr = ZSTD_selectAddr(currentIdx, lowLimit, matchAddress, dummy);
/* Note: this used to be written as : return test1 && test2; /* Note: this used to be written as : return test1 && test2;
* Unfortunately, once inlined, these tests become branches, * Unfortunately, once inlined, these tests become branches,
* in which case it becomes critical that they are executed in the right order (test1 then test2). * in which case it becomes critical that they are executed in the right order (test1 then test2).
@ -118,12 +123,12 @@ ZSTD_match4Found_cmov(const BYTE* currentPtr, const BYTE* matchAddress, U32 curr
} }
static int static int
ZSTD_match4Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit, const BYTE* fakeAddress) ZSTD_match4Found_branch(const BYTE* currentPtr, const BYTE* matchAddress, U32 currentIdx, U32 lowLimit)
{ {
/* using a branch instead of a cmov, /* using a branch instead of a cmov,
* because it's faster in scenarios where currentIdx >= lowLimit is generally true, * because it's faster in scenarios where currentIdx >= lowLimit is generally true,
* aka almost all candidates are within range */ * aka almost all candidates are within range */
U32 mval; (void)fakeAddress; U32 mval;
if (currentIdx >= lowLimit) { if (currentIdx >= lowLimit) {
mval = MEM_read32(matchAddress); mval = MEM_read32(matchAddress);
} else { } else {
@ -198,10 +203,6 @@ size_t ZSTD_compressBlock_fast_noDict_generic(
const BYTE* const prefixStart = base + prefixStartIndex; const BYTE* const prefixStart = base + prefixStartIndex;
const BYTE* const iend = istart + srcSize; const BYTE* const iend = istart + srcSize;
const BYTE* const ilimit = iend - HASH_READ_SIZE; const BYTE* const ilimit = iend - HASH_READ_SIZE;
/* Array of ~random data, should have low probability of matching data
* we load from here instead of from tables, if the index is invalid.
* Used to avoid unpredictable branches. */
const BYTE dummy[] = {0x12,0x34,0x56,0x78,0x9a,0xbc,0xde,0xf0,0xe2,0xb4};
const BYTE* anchor = istart; const BYTE* anchor = istart;
const BYTE* ip0 = istart; const BYTE* ip0 = istart;
@ -286,7 +287,7 @@ _start: /* Requires: ip0 */
goto _match; goto _match;
} }
if (findMatch(ip0, base + idx, idx, prefixStartIndex, dummy)) { if (findMatch(ip0, base + idx, idx, prefixStartIndex)) {
/* found a match! */ /* found a match! */
/* Write next hash table entry (it's already calculated). /* Write next hash table entry (it's already calculated).
@ -313,7 +314,7 @@ _start: /* Requires: ip0 */
current0 = (U32)(ip0 - base); current0 = (U32)(ip0 - base);
hashTable[hash0] = current0; hashTable[hash0] = current0;
if (findMatch(ip0, base + idx, idx, prefixStartIndex, dummy)) { if (findMatch(ip0, base + idx, idx, prefixStartIndex)) {
/* found a match! */ /* found a match! */
/* Write next hash table entry; it's already calculated */ /* Write next hash table entry; it's already calculated */