Reduce SMP contention on TT

Move TT object away from heavy write accessed NodesSincePoll
and also, inside TT isolate the heavy accessed writes variable.

No functional change.

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

src/search.cpp

@@ -189,6 +189,9 @@ namespace {
   // Remaining depth:                 1 ply         1.5 ply       2 ply         2.5 ply       3 ply         3.5 ply
   const Value RazorApprMargins[6] = { Value(0x520), Value(0x300), Value(0x300), Value(0x300), Value(0x300), Value(0x300) };
 
+  // The main transposition table
+  TranspositionTable TT;
+
 
   /// Variables initialized by UCI options
 
@@ -268,9 +271,6 @@ namespace {
   int NodesSincePoll;
   int NodesBetweenPolls = 30000;
 
-   // The main transposition table
-   TranspositionTable TT;
-
 
   /// Functions
 

src/tt.h

@@ -92,7 +92,13 @@ public:
 private:
   inline TTEntry* first_entry(const Key posKey) const;
 
-  unsigned size, writes;
+  // Be sure 'writes' is at least one cacheline 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;
   uint8_t generation;
 };