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Allow for higher depths. (official-stockfish#2147)

High rootDepths, selDepths and generally searches are increasingly
common with long time control games, analysis, and improving hardware.
In this case, depths of MAX_DEPTH/MAX_PLY (128) can be reached,
and the search tree is truncated.

In principle MAX_PLY can be easily increased, except for a technicality
of storing depths in a signed 8 bit int in the TT. This patch increases
MAX_PLY by storing the depth in an unsigned 8 bit, after shifting by the
most negative depth stored in TT (DEPTH_NONE).

No regression at STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 42235 W: 9565 L: 9484 D: 23186
http://tests.stockfishchess.org/tests/view/5cdb35360ebc5925cf0595e1

Verified to reach high depths on
k1b5/1p1p4/pP1Pp3/K2pPp2/1P1p1P2/3P1P2/5P2/8 w - -
info depth 142 seldepth 154 multipv 1 score cp 537 nodes 26740713110 ...

No bench change.
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vondele authored and mcostalba committed May 15, 2019
1 parent 7df832f commit 893a08a8c2eac3c2e7d2216b51811cc7378e2239
Showing with 6 additions and 5 deletions.
  1. +3 −2 src/tt.cpp
  2. +2 −2 src/tt.h
  3. +1 −1 src/types.h
@@ -43,14 +43,15 @@ void TTEntry::save(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev)

// Overwrite less valuable entries
if ( (k >> 48) != key16
|| d / ONE_PLY > depth8 - 4
|| d / ONE_PLY + 10 > depth8
|| b == BOUND_EXACT)
{
key16 = (uint16_t)(k >> 48);
value16 = (int16_t)v;
eval16 = (int16_t)ev;
genBound8 = (uint8_t)(TT.generation8 | uint8_t(pv) << 2 | b);
depth8 = (int8_t)(d / ONE_PLY);
assert((d - DEPTH_NONE) / ONE_PLY >= 0);
depth8 = (uint8_t)((d - DEPTH_NONE) / ONE_PLY);
}
}

@@ -40,7 +40,7 @@ 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)); }
Depth depth() const { return (Depth)(depth8 * int(ONE_PLY)) + DEPTH_NONE; }
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);
@@ -53,7 +53,7 @@ struct TTEntry {
int16_t value16;
int16_t eval16;
uint8_t genBound8;
int8_t depth8;
uint8_t depth8;
};


@@ -101,7 +101,7 @@ typedef uint64_t Key;
typedef uint64_t Bitboard;

constexpr int MAX_MOVES = 256;
constexpr int MAX_PLY = 128;
constexpr int MAX_PLY = 246;

/// A move needs 16 bits to be stored
///

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