Tweak TT aging and replacement strategies

We change the definition of "age" from "age of this position" to "age of this TT entry".
In this way, despite being on the same position, when we save into TT, we always prefer the new entry as compared to the old one.

Passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 152256 W: 39597 L: 39110 D: 73549
Ptnml(0-2): 556, 17562, 39398, 18063, 549
https://tests.stockfishchess.org/tests/view/6620faee3fe04ce4cefbf215

Passed LTC:
LLR: 2.95 (-2.94,2.94) <0.50,2.50>
Total: 51564 W: 13242 L: 12895 D: 25427
Ptnml(0-2): 24, 5464, 14463, 5803, 28
https://tests.stockfishchess.org/tests/view/66231ab53fe04ce4cefc153e

closes #5184

Bench 1479416

diff --git a/src/tt.cpp b/src/tt.cpp
index 41ed4591f392e342a9475d1b8eafba75f9d6a0d6..4885a781a5e817ec139e8b4576888f4da356a9bd 100644 (file)
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -40,7 +40,8 @@ void TTEntry::save(
         move16 = m;
 
     // Overwrite less valuable entries (cheapest checks first)
-    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_OFFSET + 2 * pv > depth8 - 4)
+    if (b == BOUND_EXACT || uint16_t(k) != key16 || d - DEPTH_OFFSET + 2 * pv > depth8 - 4
+        || relative_age(generation8))
     {
         assert(d > DEPTH_OFFSET);
         assert(d < 256 + DEPTH_OFFSET);
@@ -123,13 +124,7 @@ TTEntry* TranspositionTable::probe(const Key key, bool& found) const {
 
     for (int i = 0; i < ClusterSize; ++i)
         if (tte[i].key16 == key16 || !tte[i].depth8)
-        {
-            constexpr uint8_t lowerBits = GENERATION_DELTA - 1;
-
-            // Refresh with new generation, keeping the lower bits the same.
-            tte[i].genBound8 = uint8_t(generation8 | (tte[i].genBound8 & lowerBits));
-            return found     = bool(tte[i].depth8), &tte[i];
-        }
+            return found = bool(tte[i].depth8), &tte[i];
 
     // Find an entry to be replaced according to the replacement strategy
     TTEntry* replace = tte;