Daniel Andersson
17a6b944d1
Thread barrier with:
* fixed (at construction) number n of participating threads {T1,T2,T3,...,Tn}
* unknown number of rounds.
Requirements:
* there is some R such that each participating thread makes
R calls to Sync() followed by its one and only call to Exit().
Guarantees:
* for any two threads Ti and Tj and round number r <= R,
everything done by Ti before its r'th call to Sync() happens before
everything done by Tj after its r'th call to Sync().
Note:
* it's not required that the thread that constructs the barrier participates.
BUG=
Review URL: https://codereview.chromium.org//1337943004 .
72 lines
1.7 KiB
C++
72 lines
1.7 KiB
C++
// Copyright (c) 2013, the Dart project authors. Please see the AUTHORS file
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// for details. All rights reserved. Use of this source code is governed by a
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// BSD-style license that can be found in the LICENSE file.
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#include "vm/random.h"
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#include "vm/flags.h"
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#include "vm/isolate.h"
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namespace dart {
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DEFINE_FLAG(int, random_seed, 0, "Override the random seed for debugging.");
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Random::Random() {
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uint64_t seed = FLAG_random_seed;
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if (seed == 0) {
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Dart_EntropySource callback = Isolate::entropy_source_callback();
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if (callback != NULL) {
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if (!callback(reinterpret_cast<uint8_t*>(&seed), sizeof(seed))) {
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// Callback failed. Reset the seed to 0.
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seed = 0;
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}
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}
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}
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if (seed == 0) {
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// We did not get a seed so far. As a fallback we do use the current time.
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seed = OS::GetCurrentTimeMicros();
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}
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Initialize(seed);
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}
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void Random::Initialize(uint64_t seed) {
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ASSERT(seed != 0);
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// Crank the next state a couple of times.
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_state = seed;
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NextState();
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NextState();
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NextState();
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NextState();
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}
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Random::Random(uint64_t seed) {
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Initialize(seed);
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}
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Random::~Random() {
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// Nothing to be done here.
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}
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// The algorithm used here is Multiply with Carry (MWC) with a Base b = 2^32.
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// http://en.wikipedia.org/wiki/Multiply-with-carry
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// The constant A is selected from "Numerical Recipes 3rd Edition" p.348 B1.
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void Random::NextState() {
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const uint64_t MASK_32 = 0xffffffff;
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const uint64_t A = 0xffffda61;
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uint64_t state_lo = _state & MASK_32;
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uint64_t state_hi = (_state >> 32) & MASK_32;
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_state = (A * state_lo) + state_hi;
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}
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uint32_t Random::NextUInt32() {
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const uint64_t MASK_32 = 0xffffffff;
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NextState();
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return static_cast<uint32_t>(_state & MASK_32);
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}
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} // namespace dart
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