brick-timer

/*
 * Copyright (C) 2020 Petr Rockai <code@fixp.eu>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#pragma once
#include <x86intrin.h>
#include <atomic>
#include <array>

namespace brq
{
    /* This class provides a low-overhead, thread-safe profiling timer &
     * counter. On recent hardware, the typical single-threaded overhead (for
     * entry + exit) is 100-200 cycles (or about 40-80ns), exact numbers
     * depending on the CPU model. Each timer is global and distinct timers can
     * be created by instantiating the 'timer' template with different tag
     * types. E.g. `struct foo_timer_tag; using foo_timer = brq::timer<
     * foo_timer_tag >;` Instances of such a type work as RAII interval timers
     * that sum up over the lifetime of the program. Nesting is *not* guarded
     * and will result in double-counting. */

    template< typename >
    struct timer
    {
        struct alignas( 64 ) counter_t /* 64 = expected cache line size */
        {
            std::atomic< long > time, hits;
            void reset() { time.store( 0 ); hits.store( 0 ); }
        };

        using counters_t = std::array< counter_t, 32 >;
        counter_t *_ctr;

        [[gnu::always_inline]] timer( bool measure = true )
        {
            static counters_t ctr;
            if ( measure )
            {
                auto t = __rdtsc();
                _ctr = &ctr[ t & 0x1f ];
                _ctr->time -= t;
                _ctr->hits ++;
            }
            else
                _ctr = ctr.begin();
        }

        static std::pair< long, long > read()
        {
            timer x( false );
            long time = 0, hits = 0;
            for ( int i = 0; i < 32; ++i )
            {
                time += x._ctr[ i ].time;
                hits += x._ctr[ i ].hits;
            }
            x._ctr = nullptr;
            return { time, hits };
        }

        static void reset()
        {
            timer x( false );
            for ( int i = 0; i < 32; ++i )
                x._ctr[ i ].reset();
            x._ctr = nullptr;
        }

        void stop()
        {
            _ctr->time += __rdtsc();
            _ctr = nullptr;
        }

        [[gnu::always_inline]] ~timer()
        {
            if ( _ctr )
                _ctr->time += __rdtsc();
        }
    };
}