source: doc/theses/thierry_delisle_PhD/code/snzi.hpp@ 95cb63b

#pragma once

#include "utils.hpp"


class snzi_t {
        class node;
public:
        const unsigned mask;
        const int root;
        std::unique_ptr<snzi_t::node[]> nodes;

        snzi_t(unsigned depth);

        void arrive(int idx) {
                idx &= mask;
                nodes[idx].arrive();
        }

        void depart(int idx) {
                idx &= mask;
                nodes[idx].depart();
        }

        bool query() const {
                return nodes[root].query();
        }


private:
        class __attribute__((aligned(64))) node {
                friend class snzi_t;
        private:

                union val_t {
                        static constexpr char Half = -1;

                        uint64_t _all;
                        struct __attribute__((packed)) {
                                char cnt;
                                uint64_t ver:56;
                        };

                        bool cas(val_t & exp, char _cnt, uint64_t _ver) volatile {
                                val_t t;
                                t.ver = _ver;
                                t.cnt = _cnt;
                                /* paranoid */ assert(t._all == ((_ver << 8) | ((unsigned char)_cnt)));
                                return __atomic_compare_exchange_n(&this->_all, &exp._all, t._all, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
                        }

                        bool cas(val_t & exp, const val_t & tar) volatile {
                                return __atomic_compare_exchange_n(&this->_all, &exp._all, tar._all, false, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
                        }

                        val_t() : _all(0) {}
                        val_t(const volatile val_t & o) : _all(o._all) {}
                };

                //--------------------------------------------------
                // Hierarchical node
                void arrive_h() {
                        int undoArr = 0;
                        bool success = false;
                        while(!success) {
                                auto x{ value };
                                /* paranoid */ assert(x.cnt <= 120);
                                if( x.cnt >= 1 ) {
                                        if( value.cas(x, x.cnt + 1, x.ver ) ) {
                                                success = true;
                                        }
                                }
                                /* paranoid */ assert(x.cnt <= 120);
                                if( x.cnt == 0 ) {
                                        if( value.cas(x, val_t::Half, x.ver + 1) ) {
                                                success = true;
                                                x.cnt = val_t::Half;
                                                x.ver = x.ver + 1;
                                        }
                                }
                                /* paranoid */ assert(x.cnt <= 120);
                                if( x.cnt == val_t::Half ) {
                                        /* paranoid */ assert(parent);
                                        parent->arrive();
                                        if( !value.cas(x, 1, x.ver) ) {
                                                undoArr = undoArr + 1;
                                        }
                                }
                        }

                        for(int i = 0; i < undoArr; i++) {
                                /* paranoid */ assert(parent);
                                parent->depart();
                        }
                }

                void depart_h() {
                        while(true) {
                                auto x = (const val_t)value;
                                /* paranoid */ assertf(x.cnt >= 1, "%d", x.cnt);
                                if( value.cas( x, x.cnt - 1, x.ver ) ) {
                                        if( x.cnt == 1 ) {
                                                /* paranoid */ assert(parent);
                                                parent->depart();
                                        }
                                        return;
                                }
                        }
                }

                //--------------------------------------------------
                // Root node
                void arrive_r() {
                        __atomic_fetch_add(&value._all, 1, __ATOMIC_SEQ_CST);
                }

                void depart_r() {
                        __atomic_fetch_sub(&value._all, 1, __ATOMIC_SEQ_CST);
                }

        private:
                volatile val_t value;
                class node * parent = nullptr;

                bool is_root() {
                        return parent == nullptr;
                }

        public:
                void arrive() {
                        if(is_root()) arrive_r();
                        else arrive_h();
                }

                void depart() {
                        if(is_root()) depart_r();
                        else depart_h();
                }

                bool query() {
                        /* paranoid */ assert(is_root());
                        return value._all > 0;
                }
        };
};

snzi_t::snzi_t(unsigned depth)
        : mask((1 << depth) - 1)
        , root( ((1 << depth) * 2) - 2)
        , nodes(new node[ root + 1 ]())
{
        int width = (1 << depth);
        for(int i = 0; i < root; i++) {
                nodes[i].parent = &nodes[(i / 2) + width ];
        }
}