56 lines
1.5 KiB
C
56 lines
1.5 KiB
C
#include "jemalloc/internal/jemalloc_preamble.h"
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#include "jemalloc/internal/div.h"
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#include "jemalloc/internal/assert.h"
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/*
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* Suppose we have n = q * d, all integers. We know n and d, and want q = n / d.
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*
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* For any k, we have (here, all division is exact; not C-style rounding):
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* floor(ceil(2^k / d) * n / 2^k) = floor((2^k + r) / d * n / 2^k), where
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* r = (-2^k) mod d.
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*
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* Expanding this out:
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* ... = floor(2^k / d * n / 2^k + r / d * n / 2^k)
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* = floor(n / d + (r / d) * (n / 2^k)).
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*
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* The fractional part of n / d is 0 (because of the assumption that d divides n
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* exactly), so we have:
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* ... = n / d + floor((r / d) * (n / 2^k))
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*
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* So that our initial expression is equal to the quantity we seek, so long as
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* (r / d) * (n / 2^k) < 1.
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*
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* r is a remainder mod d, so r < d and r / d < 1 always. We can make
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* n / 2 ^ k < 1 by setting k = 32. This gets us a value of magic that works.
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*/
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void
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div_init(div_info_t *div_info, size_t d) {
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/* Nonsensical. */
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assert(d != 0);
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/*
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* This would make the value of magic too high to fit into a uint32_t
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* (we would want magic = 2^32 exactly). This would mess with code gen
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* on 32-bit machines.
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*/
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assert(d != 1);
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uint64_t two_to_k = ((uint64_t)1 << 32);
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uint32_t magic = (uint32_t)(two_to_k / d);
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/*
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* We want magic = ceil(2^k / d), but C gives us floor. We have to
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* increment it unless the result was exact (i.e. unless d is a power of
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* two).
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*/
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if (two_to_k % d != 0) {
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magic++;
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}
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div_info->magic = magic;
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#ifdef JEMALLOC_DEBUG
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div_info->d = d;
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#endif
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}
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