From: Marco Costalba <mcostalba@gmail.com>
Date: Tue, 11 Aug 2009 07:30:19 +0000 (+0100)
Subject: Reuse 5 slots instead of 4
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=166c09a7a0eafb706a961d6533c73cc248f6df94

Reuse 5 slots instead of 4

But this time with the guarantee of an always aligned
access so that prefetching is not adversely impacted.

On Joona PC
1+0, 64Mb hash:

Orig - Mod: 174 - 237 - 359

Instead after 1000 games at 1+0 with 128MB hash size
we are at + 1 ELO (just 4 games of difference).

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
---

diff --git a/src/tt.cpp b/src/tt.cpp
index ffb8b012..5ea6a808 100644
--- a/src/tt.cpp
+++ b/src/tt.cpp
@@ -33,9 +33,6 @@
 #include <xmmintrin.h>
 #endif
 
-// This is the number of TTEntry slots for each position
-static const int ClusterSize = 4;
-
 // The main transposition table
 TranspositionTable TT;
 
@@ -67,14 +64,14 @@ void TranspositionTable::set_size(unsigned mbSize) {
 
   // We store a cluster of ClusterSize number of TTEntry for each position
   // and newSize is the maximum number of storable positions.
-  while ((2 * newSize) * ClusterSize * (sizeof(TTEntry)) <= (mbSize << 20))
+  while ((2 * newSize) * sizeof(TTCluster) <= (mbSize << 20))
       newSize *= 2;
 
   if (newSize != size)
   {
       size = newSize;
       delete [] entries;
-      entries = new TTEntry[size * ClusterSize];
+      entries = new TTCluster[size];
       if (!entries)
       {
           std::cerr << "Failed to allocate " << mbSize
@@ -93,7 +90,7 @@ void TranspositionTable::set_size(unsigned mbSize) {
 
 void TranspositionTable::clear() {
 
-  memset(entries, 0, size * ClusterSize * sizeof(TTEntry));
+  memset(entries, 0, size * sizeof(TTCluster));
 }
 
 
@@ -103,7 +100,7 @@ void TranspositionTable::clear() {
 
 inline TTEntry* TranspositionTable::first_entry(const Key posKey) const {
 
-  return entries + ((uint32_t(posKey) & (size - 1)) * ClusterSize);
+  return entries[uint32_t(posKey) & (size - 1)].data;
 }
 
 
diff --git a/src/tt.h b/src/tt.h
index e838b090..2c989dac 100644
--- a/src/tt.h
+++ b/src/tt.h
@@ -70,10 +70,23 @@ private:
   uint32_t data;
   int16_t value_;
   int16_t depth_;
-  uint32_t pad_to_16_bytes;
 };
 
-/// The transposition table class.  This is basically just a huge array
+
+/// This is the number of TTEntry slots for each position
+const int ClusterSize = 5;
+
+/// Each group of ClusterSize number of TTEntry form a TTCluster
+/// that is indexed by a single position key. Cluster is padded
+/// to a cache line size so to guarantee always aligned accesses.
+
+struct TTCluster {
+  TTEntry data[ClusterSize];
+  char cache_line_padding[64 - sizeof(TTEntry[ClusterSize])];
+};
+
+
+/// The transposition table class. This is basically just a huge array
 /// containing TTEntry objects, and a few methods for writing new entries
 /// and reading new ones.
 
@@ -95,14 +108,14 @@ public:
 private:
   inline TTEntry* first_entry(const Key posKey) const;
 
-  // Be sure 'writes' is at least one cacheline away
+  // Be sure 'writes' is at least one cache line away
   // from read only variables.
   unsigned char pad_before[64 - sizeof(unsigned)];
   unsigned writes; // heavy SMP read/write access here
   unsigned char pad_after[64];
 
   unsigned size;
-  TTEntry* entries;
+  TTCluster* entries;
   uint8_t generation;
 };