cosmopolitan/libc/rand/rand64.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=8 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/dce.h"
#include "libc/nexgen32e/x86feature.h"
#include "libc/rand/rand.h"
#include "libc/rand/xorshift.h"

hidden extern uint64_t g_rando64;

/**
 * Returns nondeterministic random number.
 *
 * This function uses a good random source if it's available, which
 * takes ~400 cycles (~99ns). Otherwise it's seeded at program start
 * with the system provided random value and may perform a few
 * microseconds worth of system calls to get a good value.
 *
 * @see rngset()
 */
nodebuginfo uint64_t rand64(void) {
  uint64_t res;
  if (X86_HAVE(RDRND)) {
    res = rdrand();
  } else {
    if (IsWindows()) {
      res = winrandish();
    } else {
      devrand(&res, sizeof(res));
    }
    res ^= MarsagliaXorshift64(&g_rando64);
  }
  return res;
}