From e4ec6e22eed1129dd96997ef9f5017a1b34357eb Mon Sep 17 00:00:00 2001
From: Calcitem <calcitem@outlook.com>
Date: Sun, 19 Apr 2020 02:53:04 +0800
Subject: [PATCH] =?UTF-8?q?prefetch:=20=E5=A2=9E=E5=8A=A0=E7=BD=AE?=
 =?UTF-8?q?=E6=8D=A2=E8=A1=A8=E7=9A=84=E9=A2=84=E8=AF=BB=E5=8F=96=E7=89=B9?=
 =?UTF-8?q?=E6=80=A7=E4=BD=86=E6=9A=82=E4=B8=8D=E5=90=AF=E7=94=A8?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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   |  2 ++
 src/ai/search.cpp  | 11 +++++++++++
 src/ai/tt.cpp      |  5 +++++
 src/ai/tt.h        |  3 +++
 src/base/hashmap.h | 18 ++++++++++++++++++
 5 files changed, 39 insertions(+)

diff --git a/include/config.h b/include/config.h
index 382fd48d..c1ad8ac0 100644
--- a/include/config.h
+++ b/include/config.h
@@ -95,6 +95,8 @@
 //#define TRANSPOSITION_TABLE_DEBUG
 #endif
 
+//#define PREFETCH_SUPPORT
+
 //#define USE_STD_STACK
 
 //#define RAPID_GAME
diff --git a/src/ai/search.cpp b/src/ai/search.cpp
index 862d57b0..1e359f54 100644
--- a/src/ai/search.cpp
+++ b/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();
 
diff --git a/src/ai/tt.cpp b/src/ai/tt.cpp
index e17df4ba..9e537217 100644
--- a/src/ai/tt.cpp
+++ b/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,
diff --git a/src/ai/tt.h b/src/ai/tt.h
index 5356628c..6c7aee7c 100644
--- a/src/ai/tt.h
+++ b/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;
diff --git a/src/base/hashmap.h b/src/base/hashmap.h
index 7ad59b62..e31f5b7b 100644
--- a/src/base/hashmap.h
+++ b/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)