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 
15 typedef struct div_info_s div_info_t;
16 struct div_info_s {
17 	uint32_t magic;
18 #ifdef JEMALLOC_DEBUG
19 	size_t d;
20 #endif
21 };
22 
23 void div_init(div_info_t *div_info, size_t divisor);
24 
25 static inline size_t
div_compute(div_info_t * div_info,size_t n)26 div_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