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 //          Copyright Oliver Kowalke 2016.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          http://www.boost.org/LICENSE_1_0.txt)
 
 #ifndef BOOST_FIBERS_spinlock_ttas_futex_FUTEX_H
 #define BOOST_FIBERS_spinlock_ttas_futex_FUTEX_H
 
 #include <algorithm>
 #include <atomic>
 #include <cmath>
 #include <random>
 #include <thread>
 
 #include <boost/fiber/detail/config.hpp>
 #include <boost/fiber/detail/cpu_relax.hpp>
 #include <boost/fiber/detail/futex.hpp>
 
 // based on informations from:
 // https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops
 // https://software.intel.com/en-us/articles/long-duration-spin-wait-loops-on-hyper-threading-technology-enabled-intel-processors
 
 namespace boost {
 namespace fibers {
 namespace detail {
 
 class spinlock_ttas_futex {
 private:
     template< typename FBSplk >
     friend class spinlock_rtm;
 
     std::atomic< std::int32_t >                 value_{ 0 };
 
 public:
     spinlock_ttas_futex() = default;
 
     spinlock_ttas_futex( spinlock_ttas_futex const&) = delete;
     spinlock_ttas_futex & operator=( spinlock_ttas_futex const&) = delete;
 
     void lock() noexcept {
         static thread_local std::minstd_rand generator{ std::random_device{}() };
         std::int32_t collisions = 0, retries = 0, expected = 0;
         // after max. spins or collisions suspend via futex
         while ( retries++ < BOOST_FIBERS_RETRY_THRESHOLD) {
             // avoid using multiple pause instructions for a delay of a specific cycle count
             // the delay of cpu_relax() (pause on Intel) depends on the processor family
             // the cycle count can not guaranteed from one system to the next
             // -> check the shared variable 'value_' in between each cpu_relax() to prevent
             //    unnecessarily long delays on some systems
             // test shared variable 'status_'
             // first access to 'value_' -> chache miss
             // sucessive acccess to 'value_' -> cache hit
             // if 'value_' was released by other fiber
             // cached 'value_' is invalidated -> cache miss
             if ( 0 != ( expected = value_.load( std::memory_order_relaxed) ) ) {
 #if !defined(BOOST_FIBERS_SPIN_SINGLE_CORE)
                 if ( BOOST_FIBERS_SPIN_BEFORE_SLEEP0 > retries) {
                     // give CPU a hint that this thread is in a "spin-wait" loop
                     // delays the next instruction's execution for a finite period of time (depends on processor family)
                     // the CPU is not under demand, parts of the pipeline are no longer being used
                     // -> reduces the power consumed by the CPU
                     // -> prevent pipeline stalls
                     cpu_relax();
                 } else if ( BOOST_FIBERS_SPIN_BEFORE_YIELD > retries) {
                     // std::this_thread::sleep_for( 0us) has a fairly long instruction path length,
                     // combined with an expensive ring3 to ring 0 transition costing about 1000 cycles
                     // std::this_thread::sleep_for( 0us) lets give up this_thread the remaining part of its time slice
                     // if and only if a thread of equal or greater priority is ready to run
                     static constexpr std::chrono::microseconds us0{ 0 };
                     std::this_thread::sleep_for( us0);
                 } else {
                     // std::this_thread::yield() allows this_thread to give up the remaining part of its time slice,
                     // but only to another thread on the same processor
                     // instead of constant checking, a thread only checks if no other useful work is pending
                     std::this_thread::yield();
                 }
 #else
                 // std::this_thread::yield() allows this_thread to give up the remaining part of its time slice,
                 // but only to another thread on the same processor
                 // instead of constant checking, a thread only checks if no other useful work is pending
                 std::this_thread::yield();
 #endif
             } else if ( ! value_.compare_exchange_strong( expected, 1, std::memory_order_acquire) ) {
                 // spinlock now contended
                 // utilize 'Binary Exponential Backoff' algorithm
                 // linear_congruential_engine is a random number engine based on Linear congruential generator (LCG)
                 std::uniform_int_distribution< std::int32_t > distribution{
                     0, static_cast< std::int32_t >( 1) << (std::min)(collisions, static_cast< std::int32_t >( BOOST_FIBERS_CONTENTION_WINDOW_THRESHOLD)) };
                 const std::int32_t z = distribution( generator);
                 ++collisions;
                 for ( std::int32_t i = 0; i < z; ++i) {
                     // -> reduces the power consumed by the CPU
                     // -> prevent pipeline stalls
                     cpu_relax();
                 }
             } else {
                 // success, lock acquired
                 return;
             }
         }
         // failure, lock not acquired
         // pause via futex
         if ( 2 != expected) {
             expected = value_.exchange( 2, std::memory_order_acquire);
         }
         while ( 0 != expected) {
             futex_wait( & value_, 2);
             expected = value_.exchange( 2, std::memory_order_acquire);
         }
     }
 
     bool try_lock() noexcept {
         std::int32_t expected = 0;
         return value_.compare_exchange_strong( expected, 1, std::memory_order_acquire);
     }
 
     void unlock() noexcept {
         if ( 1 != value_.fetch_sub( 1, std::memory_order_acquire) ) {
             value_.store( 0, std::memory_order_release);
             futex_wake( & value_);
         }
     }
 };
 
 }}}
 
 #endif // BOOST_FIBERS_spinlock_ttas_futex_FUTEX_H

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