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Rewrite step to Track Increment Between Pairs of Positions

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The position updates are rewritten from `ip[N] = ip[N-1] + step` to be
`ip[N] = ip[N-2] + step`. This lets us only deal with the asymmetric spacing
of gaps at setup and then we only have to keep a single `step` variable.

This seems to work quite well on GCC and Clang!
This commit is contained in:
W. Felix Handte
2021-12-13 14:48:26 -05:00
parent b8434cb754
commit 6ca5f42402
1 changed files with 13 additions and 12 deletions
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lib/compress/zstd_fast.c
+13 -12
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@@ -99,7 +99,7 @@ ZSTD_compressBlock_fast_noDict_generic(
U32* const hashTable = ms->hashTable;
U32 const hlog = cParams->hashLog;
/* support stepSize of 0 */
size_t const stepSize = cParams->targetLength + !(cParams->targetLength);
size_t const stepSize = hasStep ? (cParams->targetLength + !(cParams->targetLength) + 1) : 2;
const BYTE* const base = ms->window.base;
const BYTE* const istart = (const BYTE*)src;
const U32 endIndex = (U32)((size_t)(istart - base) + srcSize);
@@ -128,8 +128,11 @@ ZSTD_compressBlock_fast_noDict_generic(
const BYTE* match0;
size_t mLength;
size_t step0;
size_t step1;
/* ip0 and ip1 are always adjacent. The targetLength skipping and
* uncompressibility acceleration is applied to every other position,
* matching the behavior of #1562. step therefore represents the gap
* between pairs of positions, from ip0 to ip2 or ip1 to ip3. */
size_t step;
const BYTE* nextStep;
const size_t kStepIncr = (1 << (kSearchStrength - 1));
@@ -145,14 +148,13 @@ ZSTD_compressBlock_fast_noDict_generic(
/* start each op */
_start: /* Requires: ip0 */
step0 = 1;
step1 = stepSize;
step = stepSize;
nextStep = ip0 + kStepIncr;
/* calculate positions, ip0 - anchor == 0, so we skip step calc */
ip1 = ip0 + step0;
ip2 = ip1 + step1;
ip3 = ip2 + step0;
ip1 = ip0 + 1;
ip2 = ip0 + step;
ip3 = ip2 + 1;
if (ip3 >= ilimit) {
goto _cleanup;
@@ -236,16 +238,15 @@ _start: /* Requires: ip0 */
if (ip2 >= nextStep) {
PREFETCH_L1(ip1 + 64);
PREFETCH_L1(ip1 + 128);
step0++;
step1++;
step += 2;
nextStep += kStepIncr;
}
/* advance to next positions */
ip0 = ip1;
ip1 = ip2;
ip2 = ip2 + step1;
ip3 = ip2 + step0;
ip2 = ip0 + step;
ip3 = ip1 + step;
} while (ip3 < ilimit);
_cleanup: