1 | #include "jemalloc/internal/jemalloc_preamble.h" |
2 | #include "jemalloc/internal/jemalloc_internal_includes.h" |
3 | |
4 | /* |
5 | * To avoid using floating point math down core paths (still necessary because |
6 | * versions of the glibc dynamic loader that did not preserve xmm registers are |
7 | * still somewhat common, requiring us to be compilable with -mno-sse), and also |
8 | * to avoid generally expensive library calls, we use a precomputed table of |
9 | * values. We want to sample U uniformly on [0, 1], and then compute |
10 | * ceil(log(u)/log(1-1/nticks)). We're mostly interested in the case where |
11 | * nticks is reasonably big, so 1/log(1-1/nticks) is well-approximated by |
12 | * -nticks. |
13 | * |
14 | * To compute log(u), we sample an integer in [1, 64] and divide, then just look |
15 | * up results in a table. As a space-compression mechanism, we store these as |
16 | * uint8_t by dividing the range (255) by the highest-magnitude value the log |
17 | * can take on, and using that as a multiplier. We then have to divide by that |
18 | * multiplier at the end of the computation. |
19 | * |
20 | * The values here are computed in src/ticker.py |
21 | */ |
22 | |
23 | const uint8_t ticker_geom_table[1 << TICKER_GEOM_NBITS] = { |
24 | 254, 211, 187, 169, 156, 144, 135, 127, |
25 | 120, 113, 107, 102, 97, 93, 89, 85, |
26 | 81, 77, 74, 71, 68, 65, 62, 60, |
27 | 57, 55, 53, 50, 48, 46, 44, 42, |
28 | 40, 39, 37, 35, 33, 32, 30, 29, |
29 | 27, 26, 24, 23, 21, 20, 19, 18, |
30 | 16, 15, 14, 13, 12, 10, 9, 8, |
31 | 7, 6, 5, 4, 3, 2, 1, 0 |
32 | }; |
33 | |