From 088e37c77aafaec5ead8fbe7ebf918265e6b86f2 Mon Sep 17 00:00:00 2001
From: Howard Hinnant <hhinnant@apple.com>
Date: Mon, 30 Jul 2012 17:13:21 +0000
Subject: [PATCH] Despite my pathological distrust of spin locks, the number
 just don't lie.  I've put a small spin in __sp_mut::lock() on
 std::mutex::try_lock(), which is testing quite well.  In my experience,
 putting in a yield for every failed iteration is also a major performance
 booster.  This change makes one of the performance tests I was using (a
 highly contended one) run about 20 times faster.

llvm-svn: 160967
---
 libcxx/src/memory.cpp | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/libcxx/src/memory.cpp b/libcxx/src/memory.cpp
index 7caab26c42eb..1c108b8c352a 100644
--- a/libcxx/src/memory.cpp
+++ b/libcxx/src/memory.cpp
@@ -10,6 +10,7 @@
 #define _LIBCPP_BUILDING_MEMORY
 #include "memory"
 #include "mutex"
+#include "thread"
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
@@ -129,13 +130,23 @@ _LIBCPP_CONSTEXPR __sp_mut::__sp_mut(void* p) _NOEXCEPT
 void
 __sp_mut::lock() _NOEXCEPT
 {
-    reinterpret_cast<mutex*>(_)->lock();
+    mutex& m = *static_cast<mutex*>(_);
+    unsigned count = 0;
+    while (!m.try_lock())
+    {
+        if (++count > 16)
+        {
+            m.lock();
+            break;
+        }
+        this_thread::yield();
+    }
 }
 
 void
 __sp_mut::unlock() _NOEXCEPT
 {
-    reinterpret_cast<mutex*>(_)->unlock();
+    static_cast<mutex*>(_)->unlock();
 }
 
 __sp_mut&