From 8afe162910d08a29712582f2b4f13a051a4d8599 Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Sun, 7 Jun 2020 21:07:35 +0300
Subject: [PATCH] Use a relaxed load before XCHG to not lock cache line on
 contention (AMD spinlock recommendation per
 <https://probablydance.com/2019/12/30/measuring-mutexes-spinlocks-and-how-bad-the-linux-scheduler-really-is/>)

---
 include/boost/smart_ptr/detail/spinlock_w32.hpp | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/boost/smart_ptr/detail/spinlock_w32.hpp b/include/boost/smart_ptr/detail/spinlock_w32.hpp
index 39092ce..ddaf4ff 100644
--- a/include/boost/smart_ptr/detail/spinlock_w32.hpp
+++ b/include/boost/smart_ptr/detail/spinlock_w32.hpp
@@ -66,6 +66,11 @@ public:
 
     bool try_lock()
     {
+        if( *const_cast< long const volatile* >( &v_ ) != 0 )
+        {
+            return false;
+        }
+
         long r = BOOST_SP_INTERLOCKED_EXCHANGE( &v_, 1 );
 
         BOOST_COMPILER_FENCE