80 lines
2.2 KiB
C
80 lines
2.2 KiB
C
/******************************************************************************/
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#ifdef JEMALLOC_H_TYPES
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/*
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* Simple linear congruential pseudo-random number generator:
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*
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* prng(y) = (a*x + c) % m
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*
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* where the following constants ensure maximal period:
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*
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* a == Odd number (relatively prime to 2^n), and (a-1) is a multiple of 4.
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* c == Odd number (relatively prime to 2^n).
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* m == 2^32
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*
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* See Knuth's TAOCP 3rd Ed., Vol. 2, pg. 17 for details on these constraints.
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*
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* This choice of m has the disadvantage that the quality of the bits is
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* proportional to bit position. For example, the lowest bit has a cycle of 2,
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* the next has a cycle of 4, etc. For this reason, we prefer to use the upper
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* bits.
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*/
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#define PRNG_A UINT64_C(6364136223846793005)
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#define PRNG_C UINT64_C(1442695040888963407)
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#endif /* JEMALLOC_H_TYPES */
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/******************************************************************************/
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#ifdef JEMALLOC_H_STRUCTS
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#endif /* JEMALLOC_H_STRUCTS */
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/******************************************************************************/
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#ifdef JEMALLOC_H_EXTERNS
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#endif /* JEMALLOC_H_EXTERNS */
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/******************************************************************************/
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#ifdef JEMALLOC_H_INLINES
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#ifndef JEMALLOC_ENABLE_INLINE
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uint64_t prng_lg_range(uint64_t *state, unsigned lg_range);
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uint64_t prng_range(uint64_t *state, uint64_t range);
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#endif
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#if (defined(JEMALLOC_ENABLE_INLINE) || defined(JEMALLOC_PRNG_C_))
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JEMALLOC_ALWAYS_INLINE uint64_t
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prng_lg_range(uint64_t *state, unsigned lg_range)
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{
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uint64_t ret;
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assert(lg_range > 0);
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assert(lg_range <= 64);
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ret = (*state * PRNG_A) + PRNG_C;
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*state = ret;
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ret >>= (64 - lg_range);
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return (ret);
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}
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JEMALLOC_ALWAYS_INLINE uint64_t
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prng_range(uint64_t *state, uint64_t range)
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{
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uint64_t ret;
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unsigned lg_range;
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assert(range > 1);
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/* Compute the ceiling of lg(range). */
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lg_range = ffs_u64(pow2_ceil_u64(range)) - 1;
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/* Generate a result in [0..range) via repeated trial. */
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do {
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ret = prng_lg_range(state, lg_range);
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} while (ret >= range);
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return (ret);
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}
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#endif
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#endif /* JEMALLOC_H_INLINES */
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/******************************************************************************/
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