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/* slide_hash.c -- slide hash table C implementation
*
* Copyright (C) 1995-2024 Jean-loup Gailly and Mark Adler
* For conditions of distribution and use, see copyright notice in zlib.h
*/
#include "zbuild.h"
#include "deflate.h"
/* ===========================================================================
* Slide the hash table when sliding the window down (could be avoided with 32
* bit values at the expense of memory usage). We slide even when level == 0 to
* keep the hash table consistent if we switch back to level > 0 later.
*/
static inline void slide_hash_c_chain(Pos *table, uint32_t entries, uint16_t wsize) {
#ifdef NOT_TWEAK_COMPILER
table += entries;
do {
unsigned m;
m = *--table;
*table = (Pos)(m >= wsize ? m-wsize : 0);
/* If entries is not on any hash chain, prev[entries] is garbage but
* its value will never be used.
*/
} while (--entries);
#else
{
/* As of I make this change, gcc (4.8.*) isn't able to vectorize
* this hot loop using saturated-subtraction on x86-64 architecture.
* To avoid this defect, we can change the loop such that
* o. the pointer advance forward, and
* o. demote the variable 'm' to be local to the loop, and
* choose type "Pos" (instead of 'unsigned int') for the
* variable to avoid unnecessary zero-extension.
*/
unsigned int i;
Pos *q = table;
for (i = 0; i < entries; i++) {
Pos m = *q;
Pos t = (Pos)wsize;
*q++ = (Pos)(m >= t ? m-t: 0);
}
}
#endif /* NOT_TWEAK_COMPILER */
}
Z_INTERNAL void slide_hash_c(deflate_state *s) {
uint16_t wsize = (uint16_t)s->w_size;
slide_hash_c_chain(s->head, HASH_SIZE, wsize);
slide_hash_c_chain(s->prev, wsize, wsize);
}
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