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author | David Goldblatt <davidgoldblatt@fb.com> | 2017-12-19 17:30:50 -0800 |
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committer | David Goldblatt <davidtgoldblatt@gmail.com> | 2017-12-21 14:25:43 -0800 |
commit | 21f7c13d0b172dac6ea76236bbe0a2f3ee4bcb7b (patch) | |
tree | 0b57b8ba7a9946d599adc21070f58983ce59d1f3 /include | |
parent | 7f1b02e3fa9de7e0bb5e2562994b5ab3b82c0ec3 (diff) | |
download | platform_external_jemalloc_new-21f7c13d0b172dac6ea76236bbe0a2f3ee4bcb7b.tar.gz platform_external_jemalloc_new-21f7c13d0b172dac6ea76236bbe0a2f3ee4bcb7b.tar.bz2 platform_external_jemalloc_new-21f7c13d0b172dac6ea76236bbe0a2f3ee4bcb7b.zip |
Add the div module, which allows fast division by dynamic values.
Diffstat (limited to 'include')
-rw-r--r-- | include/jemalloc/internal/div.h | 41 |
1 files changed, 41 insertions, 0 deletions
diff --git a/include/jemalloc/internal/div.h b/include/jemalloc/internal/div.h new file mode 100644 index 00000000..aebae939 --- /dev/null +++ b/include/jemalloc/internal/div.h @@ -0,0 +1,41 @@ +#ifndef JEMALLOC_INTERNAL_DIV_H +#define JEMALLOC_INTERNAL_DIV_H + +#include "jemalloc/internal/assert.h" + +/* + * This module does the division that computes the index of a region in a slab, + * given its offset relative to the base. + * That is, given a divisor d, an n = i * d (all integers), we'll return i. + * We do some pre-computation to do this more quickly than a CPU division + * instruction. + * We bound n < 2^32, and don't support dividing by one. + */ + +typedef struct div_info_s div_info_t; +struct div_info_s { + uint32_t magic; +#ifdef JEMALLOC_DEBUG + size_t d; +#endif +}; + +void div_init(div_info_t *div_info, size_t divisor); + +static inline size_t +div_compute(div_info_t *div_info, size_t n) { + assert(n <= (uint32_t)-1); + /* + * This generates, e.g. mov; imul; shr on x86-64. On a 32-bit machine, + * the compilers I tried were all smart enough to turn this into the + * appropriate "get the high 32 bits of the result of a multiply" (e.g. + * mul; mov edx eax; on x86, umull on arm, etc.). + */ + size_t i = ((uint64_t)n * (uint64_t)div_info->magic) >> 32; +#ifdef JEMALLOC_DEBUG + assert(i * div_info->d == n); +#endif + return i; +} + +#endif /* JEMALLOC_INTERNAL_DIV_H */ |