From 794c81626e548311e31ff28871c3482a873ff566 Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Mon, 18 Mar 2019 20:52:23 +0100
Subject: [PATCH] WindowServer: Cache the last two backing stores to make
 flipping faster.

Instead of reattaching to the shared buffer on every flip, keep a reference
to the last seen backing store GraphicsBitmap. This way we can simply
swap the two last buffers when a flip happens.

This does increase memory consumption in apps that disable double-buffering
but I'll address that issue separately.
---
 WindowServer/WSClientConnection.cpp | 20 ++++++++++++--------
 WindowServer/WSWindow.h             | 13 ++++++++++++-
 2 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/WindowServer/WSClientConnection.cpp b/WindowServer/WSClientConnection.cpp
index 0ad739b82d..f8f00db758 100644
--- a/WindowServer/WSClientConnection.cpp
+++ b/WindowServer/WSClientConnection.cpp
@@ -445,14 +445,18 @@ void WSClientConnection::handle_request(WSAPISetWindowBackingStoreRequest& reque
         return;
     }
     auto& window = *(*it).value;
-    auto shared_buffer = SharedBuffer::create_from_shared_buffer_id(request.shared_buffer_id());
-    if (!shared_buffer)
-        return;
-    auto backing_store = GraphicsBitmap::create_with_shared_buffer(
-        request.has_alpha_channel() ? GraphicsBitmap::Format::RGBA32 : GraphicsBitmap::Format::RGB32,
-        *shared_buffer,
-        request.size());
-    window.set_backing_store(move(backing_store));
+    if (window.last_backing_store() && window.last_backing_store()->shared_buffer_id() == request.shared_buffer_id()) {
+        window.swap_backing_stores();
+    } else {
+        auto shared_buffer = SharedBuffer::create_from_shared_buffer_id(request.shared_buffer_id());
+        if (!shared_buffer)
+            return;
+        auto backing_store = GraphicsBitmap::create_with_shared_buffer(
+            request.has_alpha_channel() ? GraphicsBitmap::Format::RGBA32 : GraphicsBitmap::Format::RGB32,
+            *shared_buffer,
+            request.size());
+        window.set_backing_store(move(backing_store));
+    }
 
     if (request.flush_immediately())
         window.invalidate();
diff --git a/WindowServer/WSWindow.h b/WindowServer/WSWindow.h
index 7d0273a742..dc116ccdf7 100644
--- a/WindowServer/WSWindow.h
+++ b/WindowServer/WSWindow.h
@@ -58,7 +58,17 @@ public:
     virtual void on_message(WSMessage&) override;
 
     GraphicsBitmap* backing_store() { return m_backing_store.ptr(); }
-    void set_backing_store(RetainPtr<GraphicsBitmap>&& backing_store) { m_backing_store = move(backing_store); }
+    void set_backing_store(RetainPtr<GraphicsBitmap>&& backing_store)
+    {
+        m_last_backing_store = move(m_backing_store);
+        m_backing_store = move(backing_store);
+    }
+    void swap_backing_stores()
+    {
+        swap(m_backing_store, m_last_backing_store);
+    }
+
+    GraphicsBitmap* last_backing_store() { return m_last_backing_store.ptr(); }
 
     void set_global_cursor_tracking_enabled(bool);
     bool global_cursor_tracking() const { return m_global_cursor_tracking_enabled; }
@@ -97,6 +107,7 @@ private:
     bool m_has_alpha_channel { false };
     bool m_has_painted_since_last_resize { false };
     RetainPtr<GraphicsBitmap> m_backing_store;
+    RetainPtr<GraphicsBitmap> m_last_backing_store;
     int m_window_id { -1 };
     float m_opacity { 1 };
     Rect m_last_lazy_resize_rect;