/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
- Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
- Copyright (C) 2008-2014 Marco Costalba, Joona Kiiski, Tord Romstad
+ 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
#ifndef TT_H_INCLUDED
#define TT_H_INCLUDED
-#include "misc.h"
+#include <cstddef>
+#include <cstdint>
+#include <tuple>
+
+#include "memory.h"
#include "types.h"
-/// The TTEntry is the 10 bytes transposition table entry, defined as below:
-///
-/// key 16 bit
-/// move 16 bit
-/// value 16 bit
-/// eval value 16 bit
-/// generation 6 bit
-/// bound type 2 bit
-/// depth 8 bit
-
-struct TTEntry {
-
- Move move() const { return (Move )move16; }
- Value value() const { return (Value)value16; }
- Value eval_value() const { return (Value)evalValue; }
- Depth depth() const { return (Depth)(depth8) + DEPTH_NONE; }
- Bound bound() const { return (Bound)(genBound8 & 0x3); }
-
-private:
- friend class TranspositionTable;
-
- void save(uint16_t k, Value v, Bound b, Depth d, Move m, uint8_t g, Value ev) {
-
- key16 = (uint16_t)k;
- move16 = (uint16_t)m;
- value16 = (int16_t)v;
- evalValue = (int16_t)ev;
- depth8 = (uint8_t)(d - DEPTH_NONE);
- genBound8 = g | (uint8_t)b;
- }
-
- uint16_t key16;
- uint16_t move16;
- int16_t value16;
- int16_t evalValue;
- uint8_t genBound8;
- uint8_t depth8;
+namespace Stockfish {
+
+class ThreadPool;
+struct TTEntry;
+struct Cluster;
+
+// There is only one global hash table for the engine and all its threads. For chess in particular, we even allow racy
+// updates between threads to and from the TT, as taking the time to synchronize access would cost thinking time and
+// thus elo. As a hash table, collisions are possible and may cause chess playing issues (bizarre blunders, faulty mate
+// reports, etc). Fixing these also loses elo; however such risk decreases quickly with larger TT size.
+//
+// `probe` is the primary method: given a board position, we lookup its entry in the table, and return a tuple of:
+// 1) whether the entry already has this position
+// 2) a copy of the prior data (if any) (may be inconsistent due to read races)
+// 3) a writer object to this entry
+// The copied data and the writer are separated to maintain clear boundaries between local vs global objects.
+
+
+// A copy of the data already in the entry (possibly collided). `probe` may be racy, resulting in inconsistent data.
+struct TTData {
+ Move move;
+ Value value, eval;
+ Depth depth;
+ Bound bound;
+ bool is_pv;
};
-/// TTCluster is a 32 bytes cluster of TT entries consisting of:
-///
-/// 3 x TTEntry (3 x 10 bytes)
-/// padding (2 bytes)
-
-const unsigned TTClusterSize = 3;
-struct TTCluster {
+// This is used to make racy writes to the global TT.
+struct TTWriter {
+ public:
+ void write(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8);
- TTEntry entry[TTClusterSize];
- char padding[2];
+ private:
+ friend class TranspositionTable;
+ TTEntry* entry;
+ TTWriter(TTEntry* tte);
};
-/// A TranspositionTable consists of a power of 2 number of clusters and each
-/// cluster consists of TTClusterSize number of TTEntry. Each non-empty entry
-/// contains information of exactly one position. The size of a cluster should
-/// not be bigger than a cache line size. In case it is less, it should be padded
-/// to guarantee always aligned accesses.
class TranspositionTable {
-public:
- ~TranspositionTable() { free(mem); }
- void new_search() { generation += 4; } // Lower 2 bits are used by Bound
-
- const TTEntry* probe(const Key key) const;
- TTEntry* first_entry(const Key key) const;
- void resize(uint64_t mbSize);
- void clear();
- void store(const Key key, Value v, Bound type, Depth d, Move m, Value statV);
-
-private:
- uint32_t clusterCount;
- TTCluster* table;
- void* mem;
- uint8_t generation; // Size must be not bigger than TTEntry::genBound8
-};
+ public:
+ ~TranspositionTable() { aligned_large_pages_free(table); }
-extern TranspositionTable TT;
+ void resize(size_t mbSize, ThreadPool& threads); // Set TT size
+ void clear(ThreadPool& threads); // Re-initialize memory, multithreaded
+ int hashfull()
+ const; // Approximate what fraction of entries (permille) have been written to during this root search
+ void
+ new_search(); // This must be called at the beginning of each root search to track entry aging
+ uint8_t generation() const; // The current age, used when writing new data to the TT
+ std::tuple<bool, TTData, TTWriter>
+ probe(const Key key) const; // The main method, whose retvals separate local vs global objects
+ TTEntry* first_entry(const Key key)
+ const; // This is the hash function; its only external use is memory prefetching.
-/// TranspositionTable::first_entry() returns a pointer to the first entry of
-/// a cluster given a position. The lowest order bits of the key are used to
-/// get the index of the cluster inside the table.
+ private:
+ friend struct TTEntry;
-inline TTEntry* TranspositionTable::first_entry(const Key key) const {
+ size_t clusterCount;
+ Cluster* table = nullptr;
+
+ uint8_t generation8 = 0; // Size must be not bigger than TTEntry::genBound8
+};
- return &table[(uint32_t)key & (clusterCount - 1)].entry[0];
-}
+} // namespace Stockfish
-#endif // #ifndef TT_H_INCLUDED
+#endif // #ifndef TT_H_INCLUDED
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