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[bug-fix] Fix a determinism bug with the DUBT #2726

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Jul 21, 2021
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[bug-fix] Fix a determinism bug with the DUBT #2726

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19 changes: 11 additions & 8 deletions lib/compress/zstd_compress_internal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -199,6 +199,8 @@ typedef struct {
*/
} ZSTD_window_t;

#define ZSTD_WINDOW_START_INDEX 2

typedef struct ZSTD_matchState_t ZSTD_matchState_t;

#define ZSTD_ROW_HASH_CACHE_SIZE 8 /* Size of prefetching hash cache for row-based matchfinder */
Expand Down Expand Up @@ -884,9 +886,9 @@ MEM_STATIC void ZSTD_window_clear(ZSTD_window_t* window)

MEM_STATIC U32 ZSTD_window_isEmpty(ZSTD_window_t const window)
{
return window.dictLimit == 1 &&
window.lowLimit == 1 &&
(window.nextSrc - window.base) == 1;
return window.dictLimit == ZSTD_WINDOW_START_INDEX &&
window.lowLimit == ZSTD_WINDOW_START_INDEX &&
(window.nextSrc - window.base) == ZSTD_WINDOW_START_INDEX;
}

/**
Expand Down Expand Up @@ -1149,11 +1151,12 @@ ZSTD_checkDictValidity(const ZSTD_window_t* window,

MEM_STATIC void ZSTD_window_init(ZSTD_window_t* window) {
ZSTD_memset(window, 0, sizeof(*window));
window->base = (BYTE const*)"";
window->dictBase = (BYTE const*)"";
window->dictLimit = 1; /* start from 1, so that 1st position is valid */
window->lowLimit = 1; /* it ensures first and later CCtx usages compress the same */
window->nextSrc = window->base + 1; /* see issue #1241 */
window->base = (BYTE const*)" ";
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for my understanding : why " " ? avoiding UB pointer arithmetic ?

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@terrelln terrelln Jul 15, 2021 •
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Yeah, CI compiler warnings were complaining about nextSrc = window->base + 2 being beyond the end of the index.

window->dictBase = (BYTE const*)" ";
ZSTD_STATIC_ASSERT(ZSTD_DUBT_UNSORTED_MARK < ZSTD_WINDOW_START_INDEX); /* Start above ZSTD_DUBT_UNSORTED_MARK */
window->dictLimit = ZSTD_WINDOW_START_INDEX; /* start from >0, so that 1st position is valid */
window->lowLimit = ZSTD_WINDOW_START_INDEX; /* it ensures first and later CCtx usages compress the same */
window->nextSrc = window->base + ZSTD_WINDOW_START_INDEX; /* see issue #1241 */
window->nbOverflowCorrections = 0;
}

Expand Down