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#ifndef JEMALLOC_INTERNAL_PRNG_H
#define JEMALLOC_INTERNAL_PRNG_H
#include "jemalloc/internal/atomic.h"
#include "jemalloc/internal/bit_util.h"
/*
* Simple linear congruential pseudo-random number generator:
*
* prng(y) = (a*x + c) % m
*
* where the following constants ensure maximal period:
*
* a == Odd number (relatively prime to 2^n), and (a-1) is a multiple of 4.
* c == Odd number (relatively prime to 2^n).
* m == 2^32
*
* See Knuth's TAOCP 3rd Ed., Vol. 2, pg. 17 for details on these constraints.
*
* This choice of m has the disadvantage that the quality of the bits is
* proportional to bit position. For example, the lowest bit has a cycle of 2,
* the next has a cycle of 4, etc. For this reason, we prefer to use the upper
* bits.
*/
/******************************************************************************/
/* INTERNAL DEFINITIONS -- IGNORE */
/******************************************************************************/
#define PRNG_A_32 UINT32_C(1103515241)
#define PRNG_C_32 UINT32_C(12347)
#define PRNG_A_64 UINT64_C(6364136223846793005)
#define PRNG_C_64 UINT64_C(1442695040888963407)
JEMALLOC_ALWAYS_INLINE uint32_t
prng_state_next_u32(uint32_t state) {
return (state * PRNG_A_32) + PRNG_C_32;
}
JEMALLOC_ALWAYS_INLINE uint64_t
prng_state_next_u64(uint64_t state) {
return (state * PRNG_A_64) + PRNG_C_64;
}
JEMALLOC_ALWAYS_INLINE size_t
prng_state_next_zu(size_t state) {
#if LG_SIZEOF_PTR == 2
return (state * PRNG_A_32) + PRNG_C_32;
#elif LG_SIZEOF_PTR == 3
return (state * PRNG_A_64) + PRNG_C_64;
#else
#error Unsupported pointer size
#endif
}
/******************************************************************************/
/* BEGIN PUBLIC API */
/******************************************************************************/
/*
* The prng_lg_range functions give a uniform int in the half-open range [0,
* 2**lg_range). If atomic is true, they do so safely from multiple threads.
* Multithreaded 64-bit prngs aren't supported.
*/
JEMALLOC_ALWAYS_INLINE uint32_t
prng_lg_range_u32(atomic_u32_t *state, unsigned lg_range, bool atomic) {
uint32_t ret, state0, state1;
assert(lg_range > 0);
assert(lg_range <= 32);
state0 = atomic_load_u32(state, ATOMIC_RELAXED);
if (atomic) {
do {
state1 = prng_state_next_u32(state0);
} while (!atomic_compare_exchange_weak_u32(state, &state0,
state1, ATOMIC_RELAXED, ATOMIC_RELAXED));
} else {
state1 = prng_state_next_u32(state0);
atomic_store_u32(state, state1, ATOMIC_RELAXED);
}
ret = state1 >> (32 - lg_range);
return ret;
}
JEMALLOC_ALWAYS_INLINE uint64_t
prng_lg_range_u64(uint64_t *state, unsigned lg_range) {
uint64_t ret, state1;
assert(lg_range > 0);
assert(lg_range <= 64);
state1 = prng_state_next_u64(*state);
*state = state1;
ret = state1 >> (64 - lg_range);
return ret;
}
JEMALLOC_ALWAYS_INLINE size_t
prng_lg_range_zu(atomic_zu_t *state, unsigned lg_range, bool atomic) {
size_t ret, state0, state1;
assert(lg_range > 0);
assert(lg_range <= ZU(1) << (3 + LG_SIZEOF_PTR));
state0 = atomic_load_zu(state, ATOMIC_RELAXED);
if (atomic) {
do {
state1 = prng_state_next_zu(state0);
} while (atomic_compare_exchange_weak_zu(state, &state0,
state1, ATOMIC_RELAXED, ATOMIC_RELAXED));
} else {
state1 = prng_state_next_zu(state0);
atomic_store_zu(state, state1, ATOMIC_RELAXED);
}
ret = state1 >> ((ZU(1) << (3 + LG_SIZEOF_PTR)) - lg_range);
return ret;
}
/*
* The prng_range functions behave like the prng_lg_range, but return a result
* in [0, range) instead of [0, 2**lg_range).
*/
JEMALLOC_ALWAYS_INLINE uint32_t
prng_range_u32(atomic_u32_t *state, uint32_t range, bool atomic) {
uint32_t ret;
unsigned lg_range;
assert(range > 1);
/* Compute the ceiling of lg(range). */
lg_range = ffs_u32(pow2_ceil_u32(range)) - 1;
/* Generate a result in [0..range) via repeated trial. */
do {
ret = prng_lg_range_u32(state, lg_range, atomic);
} while (ret >= range);
return ret;
}
JEMALLOC_ALWAYS_INLINE uint64_t
prng_range_u64(uint64_t *state, uint64_t range) {
uint64_t ret;
unsigned lg_range;
assert(range > 1);
/* Compute the ceiling of lg(range). */
lg_range = ffs_u64(pow2_ceil_u64(range)) - 1;
/* Generate a result in [0..range) via repeated trial. */
do {
ret = prng_lg_range_u64(state, lg_range);
} while (ret >= range);
return ret;
}
JEMALLOC_ALWAYS_INLINE size_t
prng_range_zu(atomic_zu_t *state, size_t range, bool atomic) {
size_t ret;
unsigned lg_range;
assert(range > 1);
/* Compute the ceiling of lg(range). */
lg_range = ffs_u64(pow2_ceil_u64(range)) - 1;
/* Generate a result in [0..range) via repeated trial. */
do {
ret = prng_lg_range_zu(state, lg_range, atomic);
} while (ret >= range);
return ret;
}
#endif /* JEMALLOC_INTERNAL_PRNG_H */
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