53 lines · cpp
1//===-- Implementation of rand --------------------------------------------===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8 9#include "src/stdlib/rand.h"10#include "src/__support/common.h"11#include "src/__support/macros/config.h"12#include "src/__support/threads/sleep.h"13#include "src/stdlib/rand_util.h"14 15namespace LIBC_NAMESPACE_DECL {16 17LLVM_LIBC_FUNCTION(int, rand, (void)) {18 unsigned long orig = rand_next.load(cpp::MemoryOrder::RELAXED);19 20 // An implementation of the xorshift64star pseudo random number generator.21 // This is a good general purpose generator for most non-cryptographics22 // applications.23 if constexpr (sizeof(void *) == sizeof(uint64_t)) {24 for (;;) {25 unsigned long x = orig;26 x ^= x >> 12;27 x ^= x << 25;28 x ^= x >> 27;29 if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,30 cpp::MemoryOrder::RELAXED))31 return static_cast<int>((x * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;32 sleep_briefly();33 }34 } else {35 // This is the xorshift32 pseudo random number generator, slightly different36 // from the 64-bit star version above, as the previous version fails to37 // generate uniform enough LSB in 32-bit systems.38 for (;;) {39 unsigned long x = orig;40 x ^= x >> 13;41 x ^= x << 27;42 x ^= x >> 5;43 if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,44 cpp::MemoryOrder::RELAXED))45 return static_cast<int>(x * 1597334677ul) & RAND_MAX;46 sleep_briefly();47 }48 }49 __builtin_unreachable();50}51 52} // namespace LIBC_NAMESPACE_DECL53