5d42d69d93
This patch makes rand select different algorithms depending on the arch. This is needed to avoid a test failure in 32-bit systems where the LSB of rand was not uniform enough when the 64-bit constants are used in 32-bit systems.
53 lines
1.9 KiB
C++
53 lines
1.9 KiB
C++
//===-- Implementation of rand --------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "src/stdlib/rand.h"
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#include "src/__support/common.h"
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#include "src/__support/macros/config.h"
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#include "src/__support/threads/sleep.h"
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#include "src/stdlib/rand_util.h"
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namespace LIBC_NAMESPACE_DECL {
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LLVM_LIBC_FUNCTION(int, rand, (void)) {
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unsigned long orig = rand_next.load(cpp::MemoryOrder::RELAXED);
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// An implementation of the xorshift64star pseudo random number generator.
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// This is a good general purpose generator for most non-cryptographics
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// applications.
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if constexpr (sizeof(void *) == sizeof(uint64_t)) {
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for (;;) {
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unsigned long x = orig;
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x ^= x >> 12;
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x ^= x << 25;
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x ^= x >> 27;
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if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,
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cpp::MemoryOrder::RELAXED))
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return static_cast<int>((x * 0x2545F4914F6CDD1Dul) >> 32) & RAND_MAX;
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sleep_briefly();
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}
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} else {
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// This is the xorshift32 pseudo random number generator, slightly different
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// from the 64-bit star version above, as the previous version fails to
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// generate uniform enough LSB in 32-bit systems.
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for (;;) {
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unsigned long x = orig;
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x ^= x >> 13;
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x ^= x << 27;
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x ^= x >> 5;
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if (rand_next.compare_exchange_strong(orig, x, cpp::MemoryOrder::ACQUIRE,
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cpp::MemoryOrder::RELAXED))
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return static_cast<int>(x * 1597334677ul) & RAND_MAX;
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sleep_briefly();
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}
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}
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__builtin_unreachable();
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}
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} // namespace LIBC_NAMESPACE_DECL
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