[dxvk] Make DxvkEvent lock-free

Reduces locking overhead and potential stuttering
issues when an app is spinning on the event.

src/dxvk/dxvk_event.cpp

@@ -2,35 +2,55 @@
 
 namespace dxvk {
   
-  DxvkEvent:: DxvkEvent() { }
-  DxvkEvent::~DxvkEvent() { }
+  DxvkEvent::DxvkEvent()
+  : m_packed(pack({ DxvkEventStatus::Signaled, 0u })) { }
+
+
+  DxvkEvent::~DxvkEvent() {
+
+  }
   
   
   uint32_t DxvkEvent::reset() {
-    std::unique_lock<std::mutex> lock(m_mutex);
-    
-    m_status = DxvkEventStatus::Reset;
-    return ++m_revision;
+    Status info;
+
+    uint64_t packed = m_packed.load();
+
+    do {
+      info.status   = DxvkEventStatus::Reset;
+      info.revision = unpack(packed).revision + 1;
+    } while (!m_packed.compare_exchange_strong(packed, pack(info)));
+
+    return info.revision;
   }
   
   
   void DxvkEvent::signal(uint32_t revision) {
-    std::unique_lock<std::mutex> lock(m_mutex);
-    
-    if (m_revision == revision)
-      m_status = DxvkEventStatus::Signaled;
+    uint64_t expected = pack({ DxvkEventStatus::Reset,    revision });
+    uint64_t desired  = pack({ DxvkEventStatus::Signaled, revision });
+    m_packed.compare_exchange_strong(expected, desired);
   }
   
   
-  DxvkEventStatus DxvkEvent::getStatus() {
-    std::unique_lock<std::mutex> lock(m_mutex);
-    return m_status;
+  DxvkEventStatus DxvkEvent::getStatus() const {
+    return unpack(m_packed.load()).status;
   }
 
 
-  void DxvkEvent::wait() {
+  void DxvkEvent::wait() const {
     while (this->getStatus() != DxvkEventStatus::Signaled)
       dxvk::this_thread::yield();
   }
+
+
+  uint64_t DxvkEvent::pack(Status info) {
+    return (uint64_t(info.revision))
+         | (uint64_t(info.status) << 32);
+  }
+
+
+  DxvkEvent::Status DxvkEvent::unpack(uint64_t packed) {
+    return { DxvkEventStatus(packed >> 32), uint32_t(packed) };
+  }
   
 }

src/dxvk/dxvk_event.h

@@ -44,7 +44,7 @@ namespace dxvk {
      * \brief Queries event status
      * \returns Current event status
      */
-    DxvkEventStatus getStatus();
+    DxvkEventStatus getStatus() const;
     
     /**
      * \brief Waits for event to get signaled
@@ -53,14 +53,20 @@ namespace dxvk {
      * thread calls \ref signal for the current
      * revision of the event.
      */
-    void wait();
+    void wait() const;
 
   private:
     
-    std::mutex m_mutex;
-    
-    DxvkEventStatus         m_status   = DxvkEventStatus::Signaled;
-    uint32_t                m_revision = 0;
+    struct Status {
+      DxvkEventStatus status   = DxvkEventStatus::Signaled;
+      uint32_t        revision = 0;
+    };
+
+    // Packed status and revision
+    std::atomic<uint64_t> m_packed;
+
+    static uint64_t pack(Status info);
+    static Status unpack(uint64_t packed);
     
   };