include/jemalloc/internal/prng.h

/******************************************************************************/
#ifdef JEMALLOC_H_TYPES

/*
 * 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.
 */

#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)

#endif /* JEMALLOC_H_TYPES */
/******************************************************************************/
#ifdef JEMALLOC_H_STRUCTS

#endif /* JEMALLOC_H_STRUCTS */
/******************************************************************************/
#ifdef JEMALLOC_H_EXTERNS

#endif /* JEMALLOC_H_EXTERNS */
/******************************************************************************/
#ifdef JEMALLOC_H_INLINES

#ifndef JEMALLOC_ENABLE_INLINE
uint32_t	prng_state_next_u32(uint32_t state);
uint64_t	prng_state_next_u64(uint64_t state);
size_t	prng_state_next_zu(size_t state);

uint32_t	prng_lg_range_u32(uint32_t *state, unsigned lg_range,
    bool atomic);
uint64_t	prng_lg_range_u64(uint64_t *state, unsigned lg_range);
size_t	prng_lg_range_zu(size_t *state, unsigned lg_range, bool atomic);

uint32_t	prng_range_u32(uint32_t *state, uint32_t range, bool atomic);
uint64_t	prng_range_u64(uint64_t *state, uint64_t range);
size_t	prng_range_zu(size_t *state, size_t range, bool atomic);
#endif

#if (defined(JEMALLOC_ENABLE_INLINE) || defined(JEMALLOC_PRNG_C_))
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
}

JEMALLOC_ALWAYS_INLINE uint32_t
prng_lg_range_u32(uint32_t *state, unsigned lg_range, bool atomic)
{
	uint32_t ret, state1;

	assert(lg_range > 0);
	assert(lg_range <= 32);

	if (atomic) {
		uint32_t state0;

		do {
			state0 = atomic_read_u32(state);
			state1 = prng_state_next_u32(state0);
		} while (atomic_cas_u32(state, state0, state1));
	} else {
		state1 = prng_state_next_u32(*state);
		*state = state1;
	}
	ret = state1 >> (32 - lg_range);

	return (ret);
}

/* 64-bit atomic operations cannot be supported on all relevant platforms. */
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(size_t *state, unsigned lg_range, bool atomic)
{
	size_t ret, state1;

	assert(lg_range > 0);
	assert(lg_range <= ZU(1) << (3 + LG_SIZEOF_PTR));

	if (atomic) {
		size_t state0;

		do {
			state0 = atomic_read_zu(state);
			state1 = prng_state_next_zu(state0);
		} while (atomic_cas_zu(state, state0, state1));
	} else {
		state1 = prng_state_next_zu(*state);
		*state = state1;
	}
	ret = state1 >> ((ZU(1) << (3 + LG_SIZEOF_PTR)) - lg_range);

	return (ret);
}

JEMALLOC_ALWAYS_INLINE uint32_t
prng_range_u32(uint32_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(size_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

#endif /* JEMALLOC_H_INLINES */
/******************************************************************************/