/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
- Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
- Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
- Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, 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"
-/// TTEntry struct 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() const { return (Value)eval16; }
- Depth depth() const { return (Depth)(depth8 * int(ONE_PLY)); }
- Bound bound() const { return (Bound)(genBound8 & 0x3); }
-
- void save(Key k, Value v, Bound b, Depth d, Move m, Value ev, uint8_t g) {
-
- assert(d / ONE_PLY * ONE_PLY == d);
-
- // Preserve any existing move for the same position
- if (m || (k >> 48) != key16)
- move16 = (uint16_t)m;
-
- // Don't overwrite more valuable entries
- if ( (k >> 48) != key16
- || d / ONE_PLY > depth8 - 4
- /* || g != (genBound8 & 0xFC) // Matching non-zero keys are already refreshed by probe() */
- || b == BOUND_EXACT)
- {
- key16 = (uint16_t)(k >> 48);
- value16 = (int16_t)v;
- eval16 = (int16_t)ev;
- genBound8 = (uint8_t)(g | b);
- depth8 = (int8_t)(d / ONE_PLY);
- }
- }
-
-private:
- friend class TranspositionTable;
-
- uint16_t key16;
- uint16_t move16;
- int16_t value16;
- int16_t eval16;
- uint8_t genBound8;
- int8_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;
};
-/// A TranspositionTable consists of a power of 2 number of clusters and each
-/// cluster consists of ClusterSize number of TTEntry. Each non-empty entry
-/// contains information of exactly one position. The size of a cluster should
-/// divide the size of a cache line size, to ensure that clusters never cross
-/// cache lines. This ensures best cache performance, as the cacheline is
-/// prefetched, as soon as possible.
+// 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);
+
+ private:
+ friend class TranspositionTable;
+ TTEntry* entry;
+ TTWriter(TTEntry* tte);
+};
+
class TranspositionTable {
- static const int CacheLineSize = 64;
- static const int ClusterSize = 3;
-
- struct Cluster {
- TTEntry entry[ClusterSize];
- char padding[2]; // Align to a divisor of the cache line size
- };
-
- static_assert(CacheLineSize % sizeof(Cluster) == 0, "Cluster size incorrect");
-
-public:
- ~TranspositionTable() { free(mem); }
- void new_search() { generation8 += 4; } // Lower 2 bits are used by Bound
- uint8_t generation() const { return generation8; }
- TTEntry* probe(const Key key, bool& found) const;
- int hashfull() const;
- void resize(size_t mbSize);
- void clear();
-
- // The lowest order bits of the key are used to get the index of the cluster
- TTEntry* first_entry(const Key key) const {
- return &table[(size_t)key & (clusterCount - 1)].entry[0];
- }
-
-private:
- size_t clusterCount;
- Cluster* table;
- void* mem;
- uint8_t generation8; // Size must be not bigger than TTEntry::genBound8
+ public:
+ ~TranspositionTable() { aligned_large_pages_free(table); }
+
+ 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.
+
+ private:
+ friend struct TTEntry;
+
+ size_t clusterCount;
+ Cluster* table = nullptr;
+
+ uint8_t generation8 = 0; // Size must be not bigger than TTEntry::genBound8
};
-extern TranspositionTable TT;
+} // namespace Stockfish
-#endif // #ifndef TT_H_INCLUDED
+#endif // #ifndef TT_H_INCLUDED