1 #include "jemalloc/internal/jemalloc_preamble.h"
3 #include "jemalloc/internal/div.h"
5 #include "jemalloc/internal/assert.h"
8 * Suppose we have n = q * d, all integers. We know n and d, and want q = n / d.
10 * For any k, we have (here, all division is exact; not C-style rounding):
11 * floor(ceil(2^k / d) * n / 2^k) = floor((2^k + r) / d * n / 2^k), where
15 * ... = floor(2^k / d * n / 2^k + r / d * n / 2^k)
16 * = floor(n / d + (r / d) * (n / 2^k)).
18 * The fractional part of n / d is 0 (because of the assumption that d divides n
19 * exactly), so we have:
20 * ... = n / d + floor((r / d) * (n / 2^k))
22 * So that our initial expression is equal to the quantity we seek, so long as
23 * (r / d) * (n / 2^k) < 1.
25 * r is a remainder mod d, so r < d and r / d < 1 always. We can make
26 * n / 2 ^ k < 1 by setting k = 32. This gets us a value of magic that works.
30 div_init(div_info_t *div_info, size_t d) {
34 * This would make the value of magic too high to fit into a uint32_t
35 * (we would want magic = 2^32 exactly). This would mess with code gen
40 uint64_t two_to_k = ((uint64_t)1 << 32);
41 uint32_t magic = (uint32_t)(two_to_k / d);
44 * We want magic = ceil(2^k / d), but C gives us floor. We have to
45 * increment it unless the result was exact (i.e. unless d is a power of
48 if (two_to_k % d != 0) {
51 div_info->magic = magic;