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71 lines
1.7 KiB
71 lines
1.7 KiB
#pragma once
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#include "compiler.h"
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#include <atomic>
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#include <cstdint>
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// A reusable spinning barrier.
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class spin_barrier
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{
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uint32_t init_ __mpalign__;
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// Bit 63 is 0 for enter phase, 1 for exit phase. Remaining bits
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// are the reached count.
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std::atomic<uint64_t> entered_;
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__padout__;
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static constexpr uint64_t phase_mask = 1ull << 63;
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static constexpr uint64_t reached_mask = ~phase_mask;
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public:
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spin_barrier() : init_(0), entered_(0) { }
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spin_barrier(unsigned val) : init_(val), entered_(0) { }
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spin_barrier(const spin_barrier&) = delete;
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~spin_barrier()
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{
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// Wait if the barrier is in the exit phase, since it's not safe
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// to delete until all completed waiters have exited
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while (entered_ & phase_mask)
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/* spin */;
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}
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void init(unsigned val)
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{
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init_ = val;
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entered_ = 0;
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}
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void join()
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{
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// Wait if the barrier is in the exit phase
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while (entered_ & phase_mask)
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#if defined(__x86_64__)
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asm volatile("pause":::);
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#elif defined(__aarch64__)
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asm volatile("yield":::);
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#endif
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// Enter the barrier
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auto v = ++entered_;
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if (v == init_) {
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// This is the last thread to reach the barrier. Switch to the
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// exit phase and exit the barrier.
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entered_ = (v | phase_mask) - 1;
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return;
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}
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// Wait until the barrier switches to the exit phase
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while (!(entered_.load(std::memory_order_relaxed) & phase_mask))
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#if defined(__x86_64__)
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asm volatile("pause":::);
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#elif defined(__aarch64__)
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asm volatile("yield":::);
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#endif
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// Exit the batter
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if ((v = --entered_) == phase_mask)
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// Reached count is now zero. Switch to the enter phase.
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entered_ = 0;
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}
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};
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