1#ifndef JEMALLOC_INTERNAL_DIV_H
2#define JEMALLOC_INTERNAL_DIV_H
3
4#include "jemalloc/internal/assert.h"
5
6/*
7 * This module does the division that computes the index of a region in a slab,
8 * given its offset relative to the base.
9 * That is, given a divisor d, an n = i * d (all integers), we'll return i.
10 * We do some pre-computation to do this more quickly than a CPU division
11 * instruction.
12 * We bound n < 2^32, and don't support dividing by one.
13 */
14
15typedef struct div_info_s div_info_t;
16struct div_info_s {
17	uint32_t magic;
18#ifdef JEMALLOC_DEBUG
19	size_t d;
20#endif
21};
22
23void div_init(div_info_t *div_info, size_t divisor);
24
25static inline size_t
26div_compute(div_info_t *div_info, size_t n) {
27	assert(n <= (uint32_t)-1);
28	/*
29	 * This generates, e.g. mov; imul; shr on x86-64. On a 32-bit machine,
30	 * the compilers I tried were all smart enough to turn this into the
31	 * appropriate "get the high 32 bits of the result of a multiply" (e.g.
32	 * mul; mov edx eax; on x86, umull on arm, etc.).
33	 */
34	size_t i = ((uint64_t)n * (uint64_t)div_info->magic) >> 32;
35#ifdef JEMALLOC_DEBUG
36	assert(i * div_info->d == n);
37#endif
38	return i;
39}
40
41#endif /* JEMALLOC_INTERNAL_DIV_H */
42