prefetch: 增加置换表的预读取特性但暂不启用

对于自对弈，因为局面模拟走下一步，入栈出栈花了7s时间，
而 prefetch 预读置换表，节5s时间，总时间还是增加2s。
故暂不启用。

include/config.h

@@ -95,6 +95,8 @@
 //#define TRANSPOSITION_TABLE_DEBUG
 #endif
 
+//#define PREFETCH_SUPPORT
+
 //#define USE_STD_STACK
 
 //#define RAPID_GAME

src/ai/search.cpp

@@ -863,6 +863,17 @@ value_t AIAlgorithm::search(depth_t depth, value_t alpha, value_t beta, Node *no
 
     int nchild = node->childrenSize;
     for (int i = 0; i < nchild; i++) {
+#ifdef PREFETCH_SUPPORT
+        if (i + 1 < nchild)
+        {
+            stashPosition();
+            doMove(node->children[i + 1]->move);
+            hash_t nextHash = st->getHash();
+            TT::prefetchHash(nextHash);
+            undoMove();
+        }
+#endif // PREFETCH_SUPPORT
+
         // 棋局入栈保存，以便后续撤销着法
         stashPosition();
 

src/ai/tt.cpp

@@ -112,6 +112,11 @@ bool TT::findHash(const hash_t &hash, TT::HashValue &hashValue)
 #endif
 }
 
+void TT::prefetchHash(const hash_t &hash)
+{
+    transpositionTable.prefetch(hash);
+}
+
 int TT::recordHash(const value_t &value,
                    const depth_t &depth,
                    const TT::HashType &type,

src/ai/tt.h

@@ -82,6 +82,9 @@ public:
 
     // 清空置换表
     static void clear();
+
+    // 预读取
+    static void prefetchHash(const hash_t &hash);
 };
 
 using TT = TranspositionTable;

src/base/hashmap.h

@@ -76,6 +76,24 @@ namespace CTSL //Concurrent Thread Safe Library
 #endif
             }
 
+            void prefetch(const K &key)
+            {
+                K hashValue = hashFn(key) & (hashSize - 1);
+                const V *addr = &(hashTable[hashValue].getValue());
+
+#  if defined(__INTEL_COMPILER)
+                // This hack prevents prefetches from being optimized away by
+                // Intel compiler. Both MSVC and gcc seem not be affected by this.
+                __asm__("");
+#  endif
+
+#  if defined(__INTEL_COMPILER) || defined(_MSC_VER)
+                _mm_prefetch((const char *)addr, _MM_HINT_T0);
+#  else
+                __builtin_prefetch(addr);
+#  endif
+            }
+
             //Function to insert into the hash map.
             //If key already exists, update the value, else insert a new node in the bucket with the <key, value> pair.
             K insert(const K &key, const V &value)