RdtscClock.h

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* (c) Copyright 1998-2026 CERN for the benefit of the LHCb and ATLAS collaborations *
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* This software is distributed under the terms of the Apache version 2 licence,     *
* copied verbatim in the file "LICENSE".                                            *
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#pragma once
 
#ifndef __x86_64__
#  error "<Gaudi/Timers/RdtscClock.h> is only supported on x86"
#else // not __x86_64__
 
#  include <chrono>
#  include <functional>
#  include <ratio>
#  include <thread>
#  include <x86intrin.h>
 
namespace Gaudi {
  namespace Timers {
    template <typename Precision = std::chrono::microseconds>
    class RdtscClock {
    public:
      // to meet requirements of TrivialClock:
      using rep    = typename Precision::rep;
      using period = typename Precision::period;
      typedef std::chrono::duration<rep, period>  duration;
      typedef std::chrono::time_point<RdtscClock> time_point;
 
      static constexpr bool is_steady{ true };
 
      // some sanity checks:
      static_assert( period::num == 1, "The Precision of RdtscClock must be reducible to 1/N" );
      static_assert( std::ratio_less_equal<period, std::milli>(),
                     "The Precision of RdtscClock must be at least std::chrono::milliseconds" );

      static rep calibrate() noexcept { return ticks_per_unit(); }
 
      static time_point now() noexcept { return time_point{ duration( __rdtsc() / ticks_per_unit() ) }; }
 
    private:
      static rep ticks_per_unit() noexcept {
        static rep ticks_per_unit = do_calibrate(); // local static guarantees thread-safety
        return ticks_per_unit;
      }
 
      static rep do_calibrate() noexcept {
        // Calibration time and conversion factor to unit of Precision
        static constexpr auto calibTime = std::chrono::milliseconds( 100 );
        static constexpr auto toPrec    = std::ratio_divide<std::milli, period>::num;
 
        // Calibrate against wall clock
        auto t1_ref = std::chrono::high_resolution_clock::now();
        auto t1     = __rdtsc();
        std::this_thread::sleep_for( calibTime );
        auto t2     = __rdtsc();
        auto t2_ref = std::chrono::high_resolution_clock::now();
 
        // Calculate ticks per unit of Precision
        auto dt_ref         = std::chrono::duration_cast<std::chrono::milliseconds>( t2_ref - t1_ref ).count();
        rep  ticks_per_unit = ( t2 - t1 ) / ( dt_ref * toPrec );
 
        return ticks_per_unit;
      }
    };
  } // namespace Timers
} // namespace Gaudi
#endif