Use std::call_once instead of a mutex

libethcore/EthashAux.cpp

@@ -43,13 +43,19 @@ using namespace eth;
 
 const char* DAGChannel::name() { return EthGreen "DAG"; }
 
-Mutex dev::eth::EthashAux::x_this;
 EthashAux* dev::eth::EthashAux::s_this = nullptr;
 
 EthashAux::~EthashAux()
 {
 }
 
+EthashAux* EthashAux::get()
+{
+    static std::once_flag flag;
+    std::call_once(flag, []{s_this = new EthashAux();});
+    return s_this;
+}
+
 uint64_t EthashAux::cacheSize(BlockInfo const& _header)
 {
 	return ethash_get_cachesize((uint64_t)_header.number);

libethcore/EthashAux.h

@@ -38,7 +38,7 @@ class EthashAux
 public:
 	~EthashAux();
 
-	static EthashAux* get() {Guard l(x_this);if (!s_this) s_this = new EthashAux(); return s_this; }
+	static EthashAux* get();
 
 	struct LightAllocation
 	{
@@ -89,7 +89,6 @@ private:
 
 	void killCache(h256 const& _s);
 
-	static Mutex x_this;
 	static EthashAux* s_this;
 
 	SharedMutex x_lights;