From a091ae4cc8ec12fc43a5e6b26d7e82125f7eff3e Mon Sep 17 00:00:00 2001
From: Marco Costalba <mcostalba@gmail.com>
Date: Sat, 15 Mar 2014 23:43:35 +0100
Subject: [PATCH] Split also if no slaves are found

Because we test for available slaves before
entering split(), we almost always allocate a
slave, only in the rare case of a race (less
then 2% of cases) this is not true, but to
special case this occurrence is not worth
the added complexity.

bench: 7451319
---
 src/thread.cpp | 54 ++++++++++++++++++++++----------------------------
 1 file changed, 24 insertions(+), 30 deletions(-)

diff --git a/src/thread.cpp b/src/thread.cpp
index 60c0ce71..cdc94ee0 100644
--- a/src/thread.cpp
+++ b/src/thread.cpp
@@ -296,41 +296,35 @@ void Thread::split(Position& pos, const Stack* ss, Value alpha, Value beta, Valu
   activeSplitPoint = &sp;
   activePosition = NULL;
 
-  int slavesCnt = 1; // This thread is always included
-  Thread* slave;
-
-  while (!Fake && (slave = Threads.available_slave(this)) != NULL)
-  {
-      ++slavesCnt;
-      sp.slavesMask |= 1ULL << slave->idx;
-      slave->activeSplitPoint = &sp;
-      slave->searching = true; // Slave leaves idle_loop()
-      slave->notify_one(); // Could be sleeping
-  }
+  if (!Fake)
+      for (Thread* slave; (slave = Threads.available_slave(this)) != NULL; )
+      {
+          sp.slavesMask |= 1ULL << slave->idx;
+          slave->activeSplitPoint = &sp;
+          slave->searching = true; // Slave leaves idle_loop()
+          slave->notify_one(); // Could be sleeping
+      }
 
   // Everything is set up. The master thread enters the idle loop, from which
   // it will instantly launch a search, because its 'searching' flag is set.
   // The thread will return from the idle loop when all slaves have finished
   // their work at this split point.
-  if (slavesCnt > 1 || Fake)
-  {
-      sp.mutex.unlock();
-      Threads.mutex.unlock();
-
-      Thread::idle_loop(); // Force a call to base class idle_loop()
-
-      // In the helpful master concept, a master can help only a sub-tree of its
-      // split point and because everything is finished here, it's not possible
-      // for the master to be booked.
-      assert(!searching);
-      assert(!activePosition);
-
-      // We have returned from the idle loop, which means that all threads are
-      // finished. Note that setting 'searching' and decreasing splitPointsSize is
-      // done under lock protection to avoid a race with Thread::available_to().
-      Threads.mutex.lock();
-      sp.mutex.lock();
-  }
+  sp.mutex.unlock();
+  Threads.mutex.unlock();
+
+  Thread::idle_loop(); // Force a call to base class idle_loop()
+
+  // In the helpful master concept, a master can help only a sub-tree of its
+  // split point and because everything is finished here, it's not possible
+  // for the master to be booked.
+  assert(!searching);
+  assert(!activePosition);
+
+  // We have returned from the idle loop, which means that all threads are
+  // finished. Note that setting 'searching' and decreasing splitPointsSize is
+  // done under lock protection to avoid a race with Thread::available_to().
+  Threads.mutex.lock();
+  sp.mutex.lock();
 
   searching = true;
   --splitPointsSize;
-- 
2.39.2