42 lines
1.1 KiB
C
42 lines
1.1 KiB
C
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#ifndef JEMALLOC_INTERNAL_DIV_H
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#define JEMALLOC_INTERNAL_DIV_H
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#include "jemalloc/internal/assert.h"
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/*
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* This module does the division that computes the index of a region in a slab,
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* given its offset relative to the base.
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* That is, given a divisor d, an n = i * d (all integers), we'll return i.
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* We do some pre-computation to do this more quickly than a CPU division
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* instruction.
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* We bound n < 2^32, and don't support dividing by one.
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*/
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typedef struct div_info_s div_info_t;
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struct div_info_s {
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uint32_t magic;
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#ifdef JEMALLOC_DEBUG
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size_t d;
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#endif
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};
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void div_init(div_info_t *div_info, size_t divisor);
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static inline size_t
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div_compute(div_info_t *div_info, size_t n) {
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assert(n <= (uint32_t)-1);
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/*
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* This generates, e.g. mov; imul; shr on x86-64. On a 32-bit machine,
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* the compilers I tried were all smart enough to turn this into the
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* appropriate "get the high 32 bits of the result of a multiply" (e.g.
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* mul; mov edx eax; on x86, umull on arm, etc.).
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*/
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size_t i = ((uint64_t)n * (uint64_t)div_info->magic) >> 32;
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#ifdef JEMALLOC_DEBUG
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assert(i * div_info->d == n);
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#endif
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return i;
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}
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#endif /* JEMALLOC_INTERNAL_DIV_H */
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