/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
- Copyright (C) 2004-2023 The Stockfish developers (see AUTHORS file)
+ 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
#include <cstddef>
#include <cstdint>
+#include <tuple>
-#include "misc.h"
+#include "memory.h"
#include "types.h"
namespace Stockfish {
-// TTEntry struct is the 10 bytes transposition table entry, defined as below:
+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.
//
-// key 16 bit
-// depth 8 bit
-// generation 5 bit
-// pv node 1 bit
-// bound type 2 bit
-// move 16 bit
-// value 16 bit
-// eval value 16 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 + DEPTH_OFFSET); }
- bool is_pv() const { return bool(genBound8 & 0x4); }
- Bound bound() const { return Bound(genBound8 & 0x3); }
- void save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev);
+// `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;
+};
+
+
+// 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;
-
- uint16_t key16;
- uint8_t depth8;
- uint8_t genBound8;
- uint16_t move16;
- int16_t value16;
- int16_t eval16;
+ TTEntry* entry;
+ TTWriter(TTEntry* tte);
};
-// A TranspositionTable is an array of Cluster, of size clusterCount. Each
-// cluster consists of ClusterSize number of TTEntry. Each non-empty TTEntry
-// contains information on exactly one position. The size of a Cluster should
-// divide the size of a cache line for best performance, as the cacheline is
-// prefetched when possible.
class TranspositionTable {
- static constexpr int ClusterSize = 3;
-
- struct Cluster {
- TTEntry entry[ClusterSize];
- char padding[2]; // Pad to 32 bytes
- };
-
- static_assert(sizeof(Cluster) == 32, "Unexpected Cluster size");
-
- // Constants used to refresh the hash table periodically
- static constexpr unsigned GENERATION_BITS = 3; // nb of bits reserved for other things
- static constexpr int GENERATION_DELTA =
- (1 << GENERATION_BITS); // increment for generation field
- static constexpr int GENERATION_CYCLE = 255 + (1 << GENERATION_BITS); // cycle length
- static constexpr int GENERATION_MASK =
- (0xFF << GENERATION_BITS) & 0xFF; // mask to pull out generation number
-
public:
~TranspositionTable() { aligned_large_pages_free(table); }
- void new_search() { generation8 += GENERATION_DELTA; } // Lower bits are used for other things
- TTEntry* probe(const Key key, bool& found) const;
- int hashfull() const;
- void resize(size_t mbSize);
- void clear();
- TTEntry* first_entry(const Key key) const {
- return &table[mul_hi64(key, clusterCount)].entry[0];
- }
+ 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;
- uint8_t generation8; // Size must be not bigger than TTEntry::genBound8
-};
+ Cluster* table = nullptr;
-extern TranspositionTable TT;
+ uint8_t generation8 = 0; // Size must be not bigger than TTEntry::genBound8
+};
} // namespace Stockfish
projects / stockfish / blobdiff