Abseil Common Libraries (C++) (grcp 依赖)
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79 lines
2.8 KiB
79 lines
2.8 KiB
// Copyright 2017 The Abseil Authors. |
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// |
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// Licensed under the Apache License, Version 2.0 (the "License"); |
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// you may not use this file except in compliance with the License. |
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// You may obtain a copy of the License at |
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// |
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// http://www.apache.org/licenses/LICENSE-2.0 |
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// |
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// Unless required by applicable law or agreed to in writing, software |
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// distributed under the License is distributed on an "AS IS" BASIS, |
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
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// See the License for the specific language governing permissions and |
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// limitations under the License. |
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// The OS-specific header included below must provide two calls: |
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// base::subtle::SpinLockDelay() and base::subtle::SpinLockWake(). |
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// See spinlock_wait.h for the specs. |
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#include <atomic> |
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#include <cstdint> |
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#include "absl/base/internal/spinlock_wait.h" |
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#if defined(_WIN32) |
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#include "absl/base/internal/spinlock_win32.inc" |
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#elif defined(__akaros__) |
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#include "absl/base/internal/spinlock_akaros.inc" |
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#else |
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#include "absl/base/internal/spinlock_posix.inc" |
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#endif |
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namespace absl { |
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namespace base_internal { |
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// See spinlock_wait.h for spec. |
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uint32_t SpinLockWait(std::atomic<uint32_t> *w, int n, |
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const SpinLockWaitTransition trans[], |
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base_internal::SchedulingMode scheduling_mode) { |
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for (int loop = 0; ; loop++) { |
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uint32_t v = w->load(std::memory_order_acquire); |
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int i; |
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for (i = 0; i != n && v != trans[i].from; i++) { |
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} |
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if (i == n) { |
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SpinLockDelay(w, v, loop, scheduling_mode); // no matching transition |
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} else if (trans[i].to == v || // null transition |
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w->compare_exchange_strong(v, trans[i].to, |
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std::memory_order_acquire, |
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std::memory_order_relaxed)) { |
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if (trans[i].done) return v; |
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} |
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} |
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} |
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static std::atomic<uint64_t> delay_rand; |
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// Return a suggested delay in nanoseconds for iteration number "loop" |
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int SpinLockSuggestedDelayNS(int loop) { |
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// Weak pseudo-random number generator to get some spread between threads |
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// when many are spinning. |
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uint64_t r = delay_rand.load(std::memory_order_relaxed); |
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r = 0x5deece66dLL * r + 0xb; // numbers from nrand48() |
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delay_rand.store(r, std::memory_order_relaxed); |
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r <<= 16; // 48-bit random number now in top 48-bits. |
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if (loop < 0 || loop > 32) { // limit loop to 0..32 |
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loop = 32; |
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} |
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// loop>>3 cannot exceed 4 because loop cannot exceed 32. |
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// Select top 20..24 bits of lower 48 bits, |
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// giving approximately 0ms to 16ms. |
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// Mean is exponential in loop for first 32 iterations, then 8ms. |
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// The futex path multiplies this by 16, since we expect explicit wakeups |
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// almost always on that path. |
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return r >> (44 - (loop >> 3)); |
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} |
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} // namespace base_internal |
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} // namespace absl
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