Better code for hash table generation

[stockfish] / src / tt.h

diff --git a/src/tt.h b/src/tt.h
index 76db03dabafd744cb813b82654e3da4a3c129219..a750b6c465706c1daa22947b082b42a8decbf641 100644 (file)
--- a/src/tt.h
+++ b/src/tt.h
@@ -1,8 +1,6 @@
 /*
   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) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
+  Copyright (C) 2004-2021 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
@@ -27,13 +25,13 @@
 /// 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
+/// depth       8 bit
 /// generation  5 bit
 /// pv node     1 bit
 /// bound type  2 bit
-/// depth       8 bit
+/// move       16 bit
+/// value      16 bit
+/// eval value 16 bit
 
 struct TTEntry {
 
@@ -49,24 +47,23 @@ private:
   friend class TranspositionTable;
 
   uint16_t key16;
+  uint8_t  depth8;
+  uint8_t  genBound8;
   uint16_t move16;
   int16_t  value16;
   int16_t  eval16;
-  uint8_t  genBound8;
-  uint8_t  depth8;
 };
 
 
 /// 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.
+/// divide the size of a cache line for best performance, as the cacheline is
+/// prefetched when possible.
 
 class TranspositionTable {
 
   static constexpr int ClusterSize = 3;
-  static constexpr int ClustersPerSuperCluster = 256;
 
   struct Cluster {
     TTEntry entry[ClusterSize];
@@ -75,31 +72,29 @@ class TranspositionTable {
 
   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_ttmem_free(mem); }
-  void new_search() { generation8 += 8; } // Lower 3 bits are used by PV flag and Bound
+ ~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 {
-
-    // The index is computed from
-    // Idx = (K48 * SCC) / 2^40, with K48 the 48 lowest bits swizzled.
-
-    const uint64_t firstTerm =  uint32_t(key) * uint64_t(superClusterCount);
-    const uint64_t secondTerm = (uint16_t(key >> 32) * uint64_t(superClusterCount)) >> 16;
-
-    return &table[(firstTerm + secondTerm) >> 24].entry[0];
+    return &table[mul_hi64(key, clusterCount)].entry[0];
   }
 
 private:
   friend struct TTEntry;
 
-  size_t superClusterCount;
+  size_t clusterCount;
   Cluster* table;
-  void* mem;
   uint8_t generation8; // Size must be not bigger than TTEntry::genBound8
 };