xref: /aosp_15_r20/external/jemalloc_new/include/jemalloc/internal/prng.h (revision 1208bc7e437ced7eb82efac44ba17e3beba411da)

#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 */