stockfish/src/tt.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2024 The Stockfish developers (see AUTHORS file)

  Stockfish is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.

  Stockfish is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include "tt.h"

#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <iostream>

#include "memory.h"
#include "syzygy/tbprobe.h"
#include "thread.h"

namespace Stockfish {

// DEPTH_ENTRY_OFFSET exists because 1) we use `bool(depth8)` as the occupancy check, but
// 2) we need to store negative depths for QS. (`depth8` is the only field with "spare bits":
// we sacrifice the ability to store depths greater than 1<<8 less the offset, as asserted below.)

// Populates the TTEntry with a new node's data, possibly
// overwriting an old position. The update is not atomic and can be racy.
void TTEntry::save(
  Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8) {

    // Preserve the old ttmove if we don't have a new one
    if (m || uint16_t(k) != key16)
        move16 = m;

    // Overwrite less valuable entries (cheapest checks first)
    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_ENTRY_OFFSET + 2 * pv > depth8 - 4
        || relative_age(generation8))
    {
        assert(d > DEPTH_ENTRY_OFFSET);
        assert(d < 256 + DEPTH_ENTRY_OFFSET);

        key16     = uint16_t(k);
        depth8    = uint8_t(d - DEPTH_ENTRY_OFFSET);
        genBound8 = uint8_t(generation8 | uint8_t(pv) << 2 | b);
        value16   = int16_t(v);
        eval16    = int16_t(ev);
    }
}


uint8_t TTEntry::relative_age(const uint8_t generation8) const {
    // Due to our packed storage format for generation and its cyclic
    // nature we add GENERATION_CYCLE (256 is the modulus, plus what
    // is needed to keep the unrelated lowest n bits from affecting
    // the result) to calculate the entry age correctly even after
    // generation8 overflows into the next cycle.

    return (TranspositionTable::GENERATION_CYCLE + generation8 - genBound8)
         & TranspositionTable::GENERATION_MASK;
}


// Sets the size of the transposition table,
// measured in megabytes. Transposition table consists
// of clusters and each cluster consists of ClusterSize number of TTEntry.
void TranspositionTable::resize(size_t mbSize, ThreadPool& threads) {
    clusterCount = mbSize * 1024 * 1024 / sizeof(Cluster);

    table = make_unique_large_page<Cluster[]>(clusterCount);

    if (!table)
    {
        std::cerr << "Failed to allocate " << mbSize << "MB for transposition table." << std::endl;
        exit(EXIT_FAILURE);
    }

    clear(threads);
}


// Initializes the entire transposition table to zero,
// in a multi-threaded way.
void TranspositionTable::clear(ThreadPool& threads) {
    const size_t threadCount = threads.num_threads();

    for (size_t i = 0; i < threadCount; ++i)
    {
        threads.run_on_thread(i, [this, i, threadCount]() {
            // Each thread will zero its part of the hash table
            const size_t stride = clusterCount / threadCount;
            const size_t start  = stride * i;
            const size_t len    = i + 1 != threadCount ? stride : clusterCount - start;

            std::memset(&table[start], 0, len * sizeof(Cluster));
        });
    }

    for (size_t i = 0; i < threadCount; ++i)
        threads.wait_on_thread(i);
}


// Looks up the current position in the transposition
// table. It returns true and a pointer to the TTEntry if the position is found.
// Otherwise, it returns false and a pointer to an empty or least valuable TTEntry
// to be replaced later. The replace value of an entry is calculated as its depth
// minus 8 times its relative age. TTEntry t1 is considered more valuable than
// TTEntry t2 if its replace value is greater than that of t2.
TTEntry* TranspositionTable::probe(const Key key, bool& found) const {

    TTEntry* const tte   = first_entry(key);
    const uint16_t key16 = uint16_t(key);  // Use the low 16 bits as key inside the cluster

    for (int i = 0; i < ClusterSize; ++i)
        if (tte[i].key16 == key16)
            return found = bool(tte[i].depth8), &tte[i];

    // Find an entry to be replaced according to the replacement strategy
    TTEntry* replace = tte;
    for (int i = 1; i < ClusterSize; ++i)
        if (replace->depth8 - replace->relative_age(generation8) * 2
            > tte[i].depth8 - tte[i].relative_age(generation8) * 2)
            replace = &tte[i];

    return found = false, replace;
}


// Returns an approximation of the hashtable
// occupation during a search. The hash is x permill full, as per UCI protocol.
// Only counts entries which match the current generation.
int TranspositionTable::hashfull() const {

    int cnt = 0;
    for (int i = 0; i < 1000; ++i)
        for (int j = 0; j < ClusterSize; ++j)
            cnt += table[i].entry[j].depth8
                && (table[i].entry[j].genBound8 & GENERATION_MASK) == generation8;

    return cnt / ClusterSize;
}

}  // namespace Stockfish