source: libcfa/src/concurrency/barrier.hfa

//
// Cforall Version 1.0.0 Copyright (C) 2022 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// barrier.hfa -- simple barrier implemented from monitors
//
// Author           : Thierry Delisle
// Created On       : Thu Mar 31 16:51:35 2022
// Last Modified By :
// Last Modified On :
// Update Count     :
//

#pragma once

#include <monitor.hfa>

// Simple barrier based on a monitor
monitor barrier {
        // Number of threads blocking needed to unblock the barrier
        // Unsigned should be enough, I don't expect use cases with 2^32 thread barriers.
        unsigned width;

        // Current count (counting backwards)
        unsigned count;

        // Barrier uses internal scheduling
        condition c;
};

// Constructor
void ?{}( barrier & this, unsigned width ) {
        this.width = width;
        this.count = width; // Count backwards so initialize at width
}

// block until the number of threads needed have blocked
// returns an value indicating the reverse order the threads arrived in
// i.e. last thread will return 0 (and not block)
//      second last thread returns 1
//      etc.
// last is an optional hook that will be called by the last thread
// before unblocking the others
static inline unsigned block(barrier & mutex this, fptr_t last = (fptr_t)0 ) {
        this.count -= 1; // prefix decrement so we the last is 0 and not 1
        unsigned arrival = this.count; // Note arrival order
        if(arrival == 0) {
                if(last) last();
                // If arrived last unblock everyone and reset
                signal_all(this.c);
                this.count = this.width;
        } else {
                // Otherwise block
                wait(this.c);
        }
        return arrival; // return arrival order
}