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Better code for hash table generation

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This patch removes some magic numbers in TT bit management and introduce proper
constants in the code, to improve documentation and ease further modifications.

No function change
This commit is contained in:
mattginsberg 2021-02-11 22:29:28 +01:00 • committed by Stéphane Nicolet
parent 550fed3343
commit 573f0e364f
3 changed files with 16 additions and 8 deletions
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1
AUTHORS
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@ -113,6 +113,7 @@ Maciej Żenczykowski (zenczykowski)
Malcolm Campbell (xoto10)
Mark Tenzer (31m059)
marotear
Matt Ginsberg (mattginsberg)
Matthew Lai (matthewlai)
Matthew Sullivan (Matt14916)
Maxim Molchanov (Maxim)

15
src/tt.cpp
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@ -123,7 +123,7 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
for (int i = 0; i < ClusterSize; ++i)
if (tte[i].key16 == key16 || !tte[i].depth8)
{
tte[i].genBound8 = uint8_t(generation8 | (tte[i].genBound8 & 0x7)); // Refresh
tte[i].genBound8 = uint8_t(generation8 | (tte[i].genBound8 & (GENERATION_DELTA - 1))); // Refresh
return found = (bool)tte[i].depth8, &tte[i];
}
@ -132,11 +132,12 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
TTEntry* replace = tte;
for (int i = 1; i < ClusterSize; ++i)
// Due to our packed storage format for generation and its cyclic
// nature we add 263 (256 is the modulus plus 7 to keep the unrelated
// lowest three bits from affecting the result) to calculate the entry
// age correctly even after generation8 overflows into the next cycle.
if ( replace->depth8 - ((263 + generation8 - replace->genBound8) & 0xF8)
> tte[i].depth8 - ((263 + generation8 - tte[i].genBound8) & 0xF8))
// nature we add GENERATION_CYCLE (256 is the modulus, plus what
// is needed to keep the unrelated lowest n bits from affecting
// the result) to calculate the entry age correctly even after
// generation8 overflows into the next cycle.
if ( replace->depth8 - ((GENERATION_CYCLE + generation8 - replace->genBound8) & GENERATION_MASK)
> tte[i].depth8 - ((GENERATION_CYCLE + generation8 - tte[i].genBound8) & GENERATION_MASK))
replace = &tte[i];
return found = false, replace;
@ -151,7 +152,7 @@ int TranspositionTable::hashfull() const {
int cnt = 0;
for (int i = 0; i < 1000; ++i)
for (int j = 0; j < ClusterSize; ++j)
cnt += table[i].entry[j].depth8 && (table[i].entry[j].genBound8 & 0xF8) == generation8;
cnt += table[i].entry[j].depth8 && (table[i].entry[j].genBound8 & GENERATION_MASK) == generation8;
return cnt / ClusterSize;
}

8
src/tt.h
View file

@ -72,9 +72,15 @@ 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_large_pages_free(table); }
void new_search() { generation8 += 8; } // Lower 3 bits are used by PV flag and Bound
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);