auto merge of #729 : eschweic/servo/manage-buffers, r=metajack

Prevents unnecessary copying of buffers, and implements buffer recycling.

src/components/gfx/buffer_map.rs

@@ -0,0 +1,135 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::hashmap::HashMap;
+use std::to_bytes::Cb;
+use geom::size::Size2D;
+use servo_msg::compositor_msg::Tile;
+
+/// This is a struct used to store buffers when they are not in use.
+/// The render task can quickly query for a particular size of buffer when it
+/// needs it.
+pub struct BufferMap<T> {
+    /// A HashMap that stores the Buffers.
+    map: HashMap<BufferKey, BufferValue<T>>,
+    /// The current amount of memory stored by the BufferMap's buffers.
+    mem: uint,
+    /// The maximum allowed memory. Unused buffers willl be deleted
+    /// when this threshold is exceeded.
+    max_mem: uint,
+    /// A monotonically increasing counter to track how recently tile sizes were used.
+    counter: uint,
+}
+
+/// A key with which to store buffers. It is based on the size of the buffer.
+struct BufferKey([uint, ..2]);
+
+impl IterBytes for BufferKey {
+    fn iter_bytes(&self, lsb0: bool, f: Cb) -> bool {
+        let i = if lsb0 {0} else {1};
+        self[i].iter_bytes(lsb0, |x| f(x)) && self[1 - i].iter_bytes(lsb0, |x| f(x))
+    }
+}
+
+impl Eq for BufferKey {
+    fn eq(&self, other: &BufferKey) -> bool {
+        self[0] == other[0] && self[1] == other[1]
+    }
+}
+
+/// Create a key from a given size
+impl BufferKey {
+    fn get(input: Size2D<uint>) -> BufferKey {
+        BufferKey([input.width, input.height])
+    }
+}
+
+/// A helper struct to keep track of buffers in the HashMap
+struct BufferValue<T> {
+    /// An array of buffers, all the same size
+    buffers: ~[T],
+    /// The counter when this size was last requested
+    last_action: uint,
+}
+
+impl<T: Tile> BufferMap<T> {
+    // Creates a new BufferMap with a given buffer limit.
+    pub fn new(max_mem: uint) -> BufferMap<T> {
+        BufferMap {
+            map: HashMap::new(),
+            mem: 0u,
+            max_mem: max_mem,
+            counter: 0u,
+        }
+    }
+    
+    // Insert a new buffer into the map.
+    pub fn insert(&mut self, new_buffer: T) {
+        let new_key = BufferKey::get(new_buffer.get_size_2d());
+
+        // If all our buffers are the same size and we're already at our
+        // memory limit, no need to store this new buffer; just let it drop.
+        if self.mem + new_buffer.get_mem() > self.max_mem && self.map.len() == 1 &&
+            self.map.contains_key(&new_key) {
+            return;
+        }
+
+        self.mem += new_buffer.get_mem();
+        // use lazy insertion function to prevent unnecessary allocation
+        self.map.find_or_insert_with(new_key, |_| BufferValue {
+            buffers: ~[],
+            last_action: self.counter
+        }).buffers.push(new_buffer);
+
+        let mut opt_key: Option<BufferKey> = None;
+        while self.mem > self.max_mem {
+            let old_key = match opt_key {
+                Some(key) => key,
+                None => {
+                    match self.map.iter().min_by(|&(_, x)| x.last_action) {
+                        Some((k, _)) => *k,
+                        None => fail!("BufferMap: tried to delete with no elements in map"),
+                    }
+                }
+            };
+            if {
+                let list = &mut self.map.get_mut(&old_key).buffers;
+                self.mem -= list.pop().get_mem();
+                list.is_empty()
+            }
+            { // then
+                self.map.pop(&old_key); // Don't store empty vectors!
+                opt_key = None;
+            } else {
+                opt_key = Some(old_key);
+            }
+        }
+    }
+
+    // Try to find a buffer for the given size.
+    pub fn find(&mut self, size: Size2D<uint>) -> Option<T> {
+        let mut flag = false; // True if key needs to be popped after retrieval.
+        let key = BufferKey::get(size);
+        let ret = match self.map.find_mut(&key) {
+            Some(ref mut buffer_val) => {
+                buffer_val.last_action = self.counter;
+                self.counter += 1;
+                
+                let buffer = buffer_val.buffers.pop();
+                self.mem -= buffer.get_mem();
+                if buffer_val.buffers.is_empty() {
+                    flag = true;
+                }
+                Some(buffer)
+            }
+            None => None,
+        };
+        
+        if flag {
+            self.map.pop(&key); // Don't store empty vectors!
+        }
+
+        ret
+    }
+}

src/components/gfx/gfx.rc

@@ -51,6 +51,7 @@ pub mod font_list;
 
 // Misc.
 pub mod opts;
+mod buffer_map;
 
 // Platform-specific implementations.
 #[path="platform/mod.rs"]

src/components/gfx/render_context.rs

@@ -19,7 +19,7 @@ use servo_net::image::base::Image;
 use extra::arc::Arc;
 
 pub struct RenderContext<'self> {
-    canvas: &'self LayerBuffer,
+    canvas: &'self ~LayerBuffer,
     font_ctx: @mut FontContext,
     opts: &'self Opts
 }

src/components/gfx/render_task.rs

@@ -24,8 +24,7 @@ use extra::arc::Arc;
 use servo_util::time::{ProfilerChan, profile};
 use servo_util::time;
 
-use extra::arc;
-
+use buffer_map::BufferMap;
 
 
 pub struct RenderLayer<T> {
@@ -35,7 +34,8 @@ pub struct RenderLayer<T> {
 
 pub enum Msg<T> {
     RenderMsg(RenderLayer<T>),
-    ReRenderMsg(~[BufferRequest], f32, PipelineId, Epoch),
+    ReRenderMsg(~[BufferRequest], f32, Epoch),
+    UnusedBufferMsg(~[~LayerBuffer]),
     PaintPermissionGranted,
     PaintPermissionRevoked,
     ExitMsg(Chan<()>),
@@ -91,9 +91,11 @@ struct RenderTask<C,T> {
     /// Permission to send paint messages to the compositor
     paint_permission: bool,
     /// Cached copy of last layers rendered
-    last_paint_msg: Option<(arc::Arc<LayerBufferSet>, Size2D<uint>)>,
+    last_paint_msg: Option<~LayerBufferSet>,
     /// A counter for epoch messages
     epoch: Epoch,
+    /// A data structure to store unused LayerBuffers
+    buffer_map: BufferMap<~LayerBuffer>,
 }
 
 impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
@@ -129,6 +131,7 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
                 paint_permission: false,
                 last_paint_msg: None,
                 epoch: Epoch(0),
+                buffer_map: BufferMap::new(10000000),
             };
 
             render_task.start();
@@ -147,13 +150,19 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
                     }
                     self.render_layer = Some(render_layer);
                 }
-                ReRenderMsg(tiles, scale, id, epoch) => {
+                ReRenderMsg(tiles, scale, epoch) => {
                     if self.epoch == epoch {
-                        self.render(tiles, scale, id);
+                        self.render(tiles, scale);
                     } else {
                         debug!("renderer epoch mismatch: %? != %?", self.epoch, epoch);
                     }
                 }
+                UnusedBufferMsg(unused_buffers) => {
+                    // move_rev_iter is more efficient
+                    for buffer in unused_buffers.move_rev_iter() {
+                        self.buffer_map.insert(buffer);
+                    }
+                }
                 PaintPermissionGranted => {
                     self.paint_permission = true;
                     match self.render_layer {
@@ -163,6 +172,16 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
                         }
                         None => {}
                     }
+                    // FIXME: This sends the last paint request, anticipating what
+                    // the compositor will ask for. However, even if it sends the right
+                    // tiles, the compositor still asks for them, and they will be
+                    // re-rendered redundantly.
+                    match self.last_paint_msg {
+                        Some(ref layer_buffer_set) => {
+                            self.compositor.paint(self.id, layer_buffer_set.clone(), self.epoch);                            
+                        }
+                        None => {} // Nothing to do
+                    }
                 }
                 PaintPermissionRevoked => {
                     self.paint_permission = false;
@@ -175,7 +194,7 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
         }
     }
 
-    fn render(&mut self, tiles: ~[BufferRequest], scale: f32, id: PipelineId) {
+    fn render(&mut self, tiles: ~[BufferRequest], scale: f32) {
         let render_layer;
         match self.render_layer {
             Some(ref r_layer) => {
@@ -196,15 +215,24 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
                     let width = tile.screen_rect.size.width;
                     let height = tile.screen_rect.size.height;
                     
-                    let buffer = LayerBuffer {
-                        draw_target: DrawTarget::new_with_fbo(self.opts.render_backend,
-                                                              self.share_gl_context,
-                                                              Size2D(width as i32, height as i32),
-                                                              B8G8R8A8),
-                        rect: tile.page_rect,
-                        screen_pos: tile.screen_rect,
-                        resolution: scale,
-                        stride: (width * 4) as uint
+                    let buffer = match self.buffer_map.find(tile.screen_rect.size) {
+                        Some(buffer) => {
+                            let mut buffer = buffer;
+                            buffer.rect = tile.page_rect;
+                            buffer.screen_pos = tile.screen_rect;
+                            buffer.resolution = scale;
+                            buffer
+                        }
+                        None => ~LayerBuffer {
+                            draw_target: DrawTarget::new_with_fbo(self.opts.render_backend,
+                                                                  self.share_gl_context,
+                                                                  Size2D(width as i32, height as i32),
+                                                                  B8G8R8A8),
+                            rect: tile.page_rect,
+                            screen_pos: tile.screen_rect,
+                            resolution: scale,
+                            stride: (width * 4) as uint
+                        }
                     };
                     
                     
@@ -240,17 +268,16 @@ impl<C: RenderListener + Send,T:Send+Freeze> RenderTask<C,T> {
 
             }
 
-            let layer_buffer_set = LayerBufferSet {
+            let layer_buffer_set = ~LayerBufferSet {
                 buffers: new_buffers,
             };
-            let layer_buffer_set = arc::Arc::new(layer_buffer_set);
 
             debug!("render_task: returning surface");
             if self.paint_permission {
-                self.compositor.paint(id, layer_buffer_set.clone(), self.epoch);
+                self.compositor.paint(self.id, layer_buffer_set.clone(), self.epoch);
             }
             debug!("caching paint msg");
-            self.last_paint_msg = Some((layer_buffer_set, render_layer.size));
+            self.last_paint_msg = Some(layer_buffer_set);
             self.compositor.set_render_state(IdleRenderState);
         }
     }

src/components/main/compositing/compositor_layer.rs

@@ -2,11 +2,12 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use std::cell::Cell;
 use geom::point::Point2D;
 use geom::size::Size2D;
 use geom::rect::Rect;
 use geom::matrix::identity;
-use gfx::render_task::ReRenderMsg;
+use gfx::render_task::{ReRenderMsg, UnusedBufferMsg};
 use servo_msg::compositor_msg::{LayerBuffer, LayerBufferSet, Epoch};
 use servo_msg::constellation_msg::PipelineId;
 use script::dom::event::{ClickEvent, MouseDownEvent, MouseUpEvent};
@@ -207,10 +208,13 @@ impl CompositorLayer {
                                        no quadtree initialized", self.pipeline.id),
                 Tree(ref mut quadtree) => quadtree,
             };
-            let (request, r) = quadtree.get_tile_rects_page(rect, scale);
-            redisplay = r; // workaround to make redisplay visible outside block
-            if !request.is_empty() {
-                self.pipeline.render_chan.send(ReRenderMsg(request, scale, self.pipeline.id.clone(), self.epoch));
+            let (request, unused) = quadtree.get_tile_rects_page(rect, scale);
+            redisplay = !unused.is_empty(); // workaround to make redisplay visible outside block
+            if redisplay { // send back unused tiles
+                self.pipeline.render_chan.send(UnusedBufferMsg(unused));
+            }
+            if !request.is_empty() { // ask for tiles
+                self.pipeline.render_chan.send(ReRenderMsg(request, scale, self.epoch));
             }
         }
         if redisplay {
@@ -399,7 +403,8 @@ impl CompositorLayer {
     
     // Add LayerBuffers to the specified layer. Returns false if the layer is not found.
     // If the epoch of the message does not match the layer's epoch, the message is ignored.
-    pub fn add_buffers(&mut self, pipeline_id: PipelineId, new_buffers: &LayerBufferSet, epoch: Epoch) -> bool {
+    pub fn add_buffers(&mut self, pipeline_id: PipelineId, new_buffers: ~LayerBufferSet, epoch: Epoch) -> bool {
+        let cell = Cell::new(new_buffers);
         if self.pipeline.id == pipeline_id {
             if self.epoch != epoch {
                 debug!("compositor epoch mismatch: %? != %?, id: %?", self.epoch, epoch, self.pipeline.id);
@@ -408,22 +413,26 @@ impl CompositorLayer {
             }
             { // block here to prevent double mutable borrow of self
                 let quadtree = match self.quadtree {
-                    NoTree(_, _) => fail!("CompositorLayer: cannot get buffer request for %?,
-                                           no quadtree initialized", self.pipeline.id),
+                    NoTree(_, _) => fail!("CompositorLayer: cannot add buffers, no quadtree initialized"),
                     Tree(ref mut quadtree) => quadtree,
                 };
                 
-                for buffer in new_buffers.buffers.iter() {
-                    // TODO: This may return old buffers, which should be sent back to the renderer.
-                    quadtree.add_tile_pixel(buffer.screen_pos.origin.x, buffer.screen_pos.origin.y,
-                                            buffer.resolution, ~buffer.clone());
+                let mut unused_tiles = ~[];
+                // move_rev_iter is more efficient
+                for buffer in cell.take().buffers.move_rev_iter() {
+                    unused_tiles.push_all_move(quadtree.add_tile_pixel(buffer.screen_pos.origin.x,
+                                                                       buffer.screen_pos.origin.y,
+                                                                       buffer.resolution, buffer));
+                }
+                if !unused_tiles.is_empty() { // send back unused buffers
+                    self.pipeline.render_chan.send(UnusedBufferMsg(unused_tiles));
                 }
             }
             self.build_layer_tree();
             return true;
         }
         // ID does not match ours, so recurse on descendents (including hidden children).
-        self.children.mut_iter().map(|x| &mut x.child).any(|x| x.add_buffers(pipeline_id, new_buffers, epoch))
+        self.children.mut_iter().map(|x| &mut x.child).any(|x| x.add_buffers(pipeline_id, cell.take(), epoch))
     }
 
     // Deletes a specified sublayer, including hidden children. Returns false if the layer is not found.

src/components/main/compositing/mod.rs

@@ -40,7 +40,6 @@ use servo_util::time::ProfilerChan;
 
 use extra::future::from_value;
 use extra::time::precise_time_s;
-use extra::arc;
 
 use constellation::SendableFrameTree;
 use compositing::compositor_layer::CompositorLayer;
@@ -79,7 +78,7 @@ impl RenderListener for CompositorChan {
         port.recv()
     }
 
-    fn paint(&self, id: PipelineId, layer_buffer_set: arc::Arc<LayerBufferSet>, epoch: Epoch) {
+    fn paint(&self, id: PipelineId, layer_buffer_set: ~LayerBufferSet, epoch: Epoch) {
         self.chan.send(Paint(id, layer_buffer_set, epoch))
     }
 
@@ -148,7 +147,7 @@ pub enum Msg {
     InvalidateRect(PipelineId, Rect<uint>),
 
     /// Requests that the compositor paint the given layer buffer set for the given page size.
-    Paint(PipelineId, arc::Arc<LayerBufferSet>, Epoch),
+    Paint(PipelineId, ~LayerBufferSet, Epoch),
     /// Alerts the compositor to the current status of page loading.
     ChangeReadyState(ReadyState),
     /// Alerts the compositor to the current status of rendering.
@@ -338,7 +337,7 @@ impl CompositorTask {
 
                         match compositor_layer {
                             Some(ref mut layer) => {
-                                assert!(layer.add_buffers(id, new_layer_buffer_set.get(), epoch));
+                                assert!(layer.add_buffers(id, new_layer_buffer_set, epoch));
                                 recomposite = true;
                             }
                             None => {

src/components/main/compositing/quadtree.rs

@@ -111,43 +111,48 @@ impl<T: Tile> Quadtree<T> {
 
     /// Add a tile associtated with a given pixel position and scale.
     /// If the tile pushes the total memory over its maximum, tiles will be removed
-    /// until total memory is below the maximum again.
-    pub fn add_tile_pixel(&mut self, x: uint, y: uint, scale: f32, tile: T) {
-        self.root.add_tile(x as f32 / scale, y as f32 / scale, tile, self.max_tile_size as f32 / scale);
+    /// until total memory is below the maximum again. These tiles are returned.
+    pub fn add_tile_pixel(&mut self, x: uint, y: uint, scale: f32, tile: T) -> ~[T] {
+        let (_, tiles) = self.root.add_tile(x as f32 / scale, y as f32 / scale, tile,
+                                            self.max_tile_size as f32 / scale);
+        let mut tiles = tiles;
         match self.max_mem {
             Some(max) => {
                 while self.root.tile_mem > max {
                     let r = self.root.remove_tile(x as f32 / scale, y as f32 / scale);
                     match r {
-                        (Some(_), _, _) => {}
+                        (Some(tile), _, _) => tiles.push(tile),
                         _ => fail!("Quadtree: No valid tiles to remove"),
                     }
                 }
             }
-            None => {}
+            None => {} // Nothing to do
         }
+        tiles
     }
 
     /// Add a tile associtated with a given page position.
     /// If the tile pushes the total memory over its maximum, tiles will be removed
-    /// until total memory is below the maximum again.
-    pub fn add_tile_page(&mut self, x: f32, y: f32, scale: f32, tile: T) {
-        self.root.add_tile(x, y, tile, self.max_tile_size as f32 / scale);
+    /// until total memory is below the maximum again. These tiles are returned.
+    pub fn add_tile_page(&mut self, x: f32, y: f32, scale: f32, tile: T) -> ~[T] {
+        let (_, tiles) = self.root.add_tile(x, y, tile, self.max_tile_size as f32 / scale);
+        let mut tiles = tiles;
         match self.max_mem {
             Some(max) => {
                 while self.root.tile_mem > max {
                     let r = self.root.remove_tile(x, y);
                     match r {
-                        (Some(_), _, _) => {}
+                        (Some(tile), _, _) => tiles.push(tile),
                         _ => fail!("Quadtree: No valid tiles to remove"),
                     }
                 }
             }
-            None => {}
+            None => {} // Nothing to do
         }
+        tiles
     }
 
-    /// Get the tile rect in screen and page coordinates for a given pixel position
+    /// Get the tile rect in screen and page coordinates for a given pixel position.
     pub fn get_tile_rect_pixel(&mut self, x: uint, y: uint, scale: f32) -> BufferRequest {
         self.root.get_tile_rect(x as f32 / scale, y as f32 / scale, 
                                 self.clip_size.width as f32,
@@ -155,7 +160,7 @@ impl<T: Tile> Quadtree<T> {
                                 scale, self.max_tile_size as f32 / scale)
     }
 
-    /// Get the tile rect in screen and page coordinates for a given page position
+    /// Get the tile rect in screen and page coordinates for a given page position.
     pub fn get_tile_rect_page(&mut self, x: f32, y: f32, scale: f32) -> BufferRequest {
         self.root.get_tile_rect(x, y, 
                                 self.clip_size.width as f32,
@@ -163,7 +168,7 @@ impl<T: Tile> Quadtree<T> {
                                 scale, self.max_tile_size as f32 / scale)
     }
 
-    /// Get all the tiles in the tree
+    /// Get all the tiles in the tree.
     pub fn get_all_tiles<'r>(&'r self) -> ~[&'r T] {
         self.root.get_all_tiles()
     }
@@ -189,26 +194,26 @@ impl<T: Tile> Quadtree<T> {
     }
 
     /// Given a window rect in pixel coordinates, this function returns a list of BufferRequests for tiles that
-    /// need to be rendered. It also returns a boolean if the window needs to be redisplayed, i.e. if
+    /// need to be rendered. It also returns a vector of tiles if the window needs to be redisplayed, i.e. if
     /// no tiles need to be rendered, but the display tree needs to be rebuilt. This can occur when the
     /// user zooms out and cached tiles need to be displayed on top of higher resolution tiles.
     /// When this happens, higher resolution tiles will be removed from the quadtree.
-    pub fn get_tile_rects_pixel(&mut self, window: Rect<int>, scale: f32) -> (~[BufferRequest], bool) {
-        let (ret, redisplay, _) = self.root.get_tile_rects(
+    pub fn get_tile_rects_pixel(&mut self, window: Rect<int>, scale: f32) -> (~[BufferRequest], ~[T]) {
+        let (ret, unused, _) = self.root.get_tile_rects(
             Rect(Point2D(window.origin.x as f32 / scale, window.origin.y as f32 / scale),
                  Size2D(window.size.width as f32 / scale, window.size.height as f32 / scale)),
             Size2D(self.clip_size.width as f32, self.clip_size.height as f32),
             scale, self.max_tile_size as f32 / scale, false);
-        (ret, redisplay)
+        (ret, unused)
     }
 
-    /// Same function as above, using page coordinates for the window
-    pub fn get_tile_rects_page(&mut self, window: Rect<f32>, scale: f32) -> (~[BufferRequest], bool) {
-        let (ret, redisplay, _) = self.root.get_tile_rects(
+    /// Same function as above, using page coordinates for the window.
+    pub fn get_tile_rects_page(&mut self, window: Rect<f32>, scale: f32) -> (~[BufferRequest], ~[T]) {
+        let (ret, unused, _) = self.root.get_tile_rects(
             window,
             Size2D(self.clip_size.width as f32, self.clip_size.height as f32),
             scale, self.max_tile_size as f32 / scale, false);
-        (ret, redisplay)
+        (ret, unused)
     }
 
     /// Creates a new quadtree at the specified size. This should be called when the window changes size.
@@ -357,8 +362,9 @@ impl<T: Tile> QuadtreeNode<T> {
 
     /// Add a tile associated with a given position in page coords. If the tile size exceeds the maximum,
     /// the node will be split and the method will recurse until the tile size is within limits.
-    /// Returns an the difference in tile memory between the new quadtree node and the old quadtree node.
-    fn add_tile(&mut self, x: f32, y: f32, tile: T, tile_size: f32) -> int {
+    /// Returns an the difference in tile memory between the new quadtree node and the old quadtree node,
+    /// along with any deleted tiles.
+    fn add_tile(&mut self, x: f32, y: f32, tile: T, tile_size: f32) -> (int, ~[T]) {
         debug!("Quadtree: Adding: (%?, %?) size:%?px", self.origin.x, self.origin.y, self.size);
 
         if x >= self.origin.x + self.size || x < self.origin.x
@@ -369,21 +375,28 @@ impl<T: Tile> QuadtreeNode<T> {
         if self.size <= tile_size { // We are the child
             let old_size = self.tile_mem;
             self.tile_mem = tile.get_mem();
-            self.tile = Some(tile);
-            // FIXME: This should be inline, but currently won't compile
-            let quads = [TL, TR, BL, BR];
-            for quad in quads.iter() {
-                self.quadrants[*quad as int] = None;
+            let mut unused_tiles = match replace(&mut self.tile, Some(tile)) {
+                Some(old_tile) => ~[old_tile],
+                None => ~[],
+            };
+            for child in self.quadrants.mut_iter() {
+                match *child {
+                    Some(ref mut node) => {
+                        unused_tiles.push_all_move(node.collect_tiles());
+                    }
+                    None => {} // Nothing to do
+                }
+                *child = None;
             }
             self.status = Normal;
-            self.tile_mem as int - old_size as int
+            (self.tile_mem as int - old_size as int, unused_tiles)
         } else { // Send tile to children            
             let quad = self.get_quadrant(x, y);
             match self.quadrants[quad as int] {
                 Some(ref mut child) => {
-                    let delta = child.add_tile(x, y, tile, tile_size);
+                    let (delta, unused) = child.add_tile(x, y, tile, tile_size);
                     self.tile_mem = (self.tile_mem as int + delta) as uint;
-                    delta
+                    (delta, unused)
                 }
                 None => { // Make new child      
                     let new_size = self.size / 2.0;
@@ -396,10 +409,10 @@ impl<T: Tile> QuadtreeNode<T> {
                         BL | BR => self.origin.y + new_size,
                     };
                     let mut c = ~QuadtreeNode::new_child(new_x, new_y, new_size);
-                    let delta = c.add_tile(x, y, tile, tile_size);
+                    let (delta, unused) = c.add_tile(x, y, tile, tile_size);
                     self.tile_mem = (self.tile_mem as int + delta) as uint;    
                     self.quadrants[quad as int] = Some(c);
-                    delta
+                    (delta, unused)
                 }
             }
         }
@@ -516,13 +529,13 @@ impl<T: Tile> QuadtreeNode<T> {
     }
     
     /// Given a window rect in page coordinates, returns a BufferRequest array,
-    /// a redisplay boolean, and the difference in tile memory between the new and old quadtree nodes.
+    /// an unused tile array, and the difference in tile memory between the new and old quadtree nodes.
     /// The override bool will be true if a parent node was marked as invalid; child nodes will be
     /// treated as invalid as well.
     /// NOTE: this method will sometimes modify the tree by deleting tiles.
     /// See the QuadTree function description for more details.
     fn get_tile_rects(&mut self, window: Rect<f32>, clip: Size2D<f32>, scale: f32, tile_size: f32, override: bool) ->
-        (~[BufferRequest], bool, int) {
+        (~[BufferRequest], ~[T], int) {
         
         let w_x = window.origin.x;
         let w_y = window.origin.y;
@@ -536,7 +549,7 @@ impl<T: Tile> QuadtreeNode<T> {
         if w_x + w_width < s_x || w_x > s_x + s_size
             || w_y + w_height < s_y || w_y > s_y + s_size 
             || w_x >= clip.width || w_y >= clip.height {
-            return (~[], false, 0);
+            return (~[], ~[], 0);
         }
         
         // clip window to visible region
@@ -545,7 +558,7 @@ impl<T: Tile> QuadtreeNode<T> {
         
         if s_size <= tile_size { // We are the child
             return match self.tile {
-                _ if self.status == Rendering || self.status == Hidden => (~[], false, 0),
+                _ if self.status == Rendering || self.status == Hidden => (~[], ~[], 0),
                 Some(ref tile) if tile.is_valid(scale) && !override
                 && self.status != Invalid => {
                     let redisplay = match self.quadrants {
@@ -553,20 +566,25 @@ impl<T: Tile> QuadtreeNode<T> {
                         _ => true,
                     };
                     let mut delta = 0;
+                    let mut unused_tiles = ~[];
                     if redisplay {
                         let old_mem = self.tile_mem;
-                        // FIXME: This should be inline, but currently won't compile
-                        let quads = [TL, TR, BL, BR];
-                        for quad in quads.iter() {
-                            self.quadrants[*quad as int] = None;
+                        for child in self.quadrants.mut_iter() {
+                            match *child {
+                                Some(ref mut node) => {
+                                    unused_tiles.push_all_move(node.collect_tiles());
+                                }
+                                None => {} // Nothing to do
+                            }
+                            *child = None;
                         }
                         self.tile_mem = tile.get_mem();
                         delta = self.tile_mem as int - old_mem as int;
 
                     }
-                    (~[], redisplay, delta)
+                    (~[], unused_tiles, delta)
                 }
-                _ => (~[self.get_tile_rect(s_x, s_y, clip.width, clip.height, scale, tile_size)], false, 0),
+                _ => (~[self.get_tile_rect(s_x, s_y, clip.width, clip.height, scale, tile_size)], ~[], 0),
             }
         }
         
@@ -600,8 +618,8 @@ impl<T: Tile> QuadtreeNode<T> {
         
         let quads_to_check = build_sized(4, builder);
         
-        let mut ret = ~[];
-        let mut redisplay = false;
+        let mut request = ~[];
+        let mut unused = ~[];
         let mut delta = 0;
         
         for quad in quads_to_check.iter() {
@@ -627,7 +645,7 @@ impl<T: Tile> QuadtreeNode<T> {
 
             let override = override || self.status == Invalid;
             self.status = Normal;
-            let (c_ret, c_redisplay, c_delta) = match self.quadrants[*quad as int] {
+            let (c_request, c_unused, c_delta) = match self.quadrants[*quad as int] {
                 Some(ref mut child) => child.get_tile_rects(new_window, clip, scale, tile_size, override),
                 None => {
                     // Create new child
@@ -648,11 +666,28 @@ impl<T: Tile> QuadtreeNode<T> {
             };
             
             delta = delta + c_delta;
-            ret = ret + c_ret;
-            redisplay = redisplay || c_redisplay;
+            request = request + c_request;
+            unused.push_all_move(c_unused);
         } 
         self.tile_mem = (self.tile_mem as int + delta) as uint;
-        (ret, redisplay, delta)
+        (request, unused, delta)
+    }
+
+    /// Remove all tiles from the tree. Use this to collect all tiles before deleting a branch.
+    fn collect_tiles(&mut self) -> ~[T] {
+        let mut ret = match replace(&mut self.tile, None) {
+            Some(tile) => ~[tile],
+            None => ~[],
+        };
+        for child in self.quadrants.mut_iter() {
+            match *child {
+                Some(ref mut node) => {
+                    ret.push_all_move(node.collect_tiles());
+                }
+                None => {} // Nothing to do
+            }
+        }
+        ret
     }
 
     /// Set the status of nodes contained within the rect. See the quadtree method for
@@ -745,6 +780,9 @@ pub fn test_resize() {
         fn is_valid(&self, _: f32) -> bool {
             true
         }
+        fn get_size_2d(&self) -> Size2D<uint> {
+            Size2D(0u, 0u)
+        }
     }
     
     let mut q = Quadtree::new(6, 6, 1, None);
@@ -775,6 +813,9 @@ pub fn test() {
         fn is_valid(&self, _: f32) -> bool {
             true
         }
+        fn get_size_2d(&self) -> Size2D<uint> {
+            Size2D(0u, 0u)
+        }
     }
     
     let mut q = Quadtree::new(8, 8, 2, Some(4));
@@ -797,7 +838,7 @@ pub fn test() {
 
     q.add_tile_pixel(0, 0, 0.5, T{a: 6});
     q.add_tile_pixel(0, 0, 1f32, T{a: 7});
-    let (_, redisplay) = q.get_tile_rects_pixel(Rect(Point2D(0, 0), Size2D(2, 2)), 0.5);
-    assert!(redisplay);
+    let (_, unused) = q.get_tile_rects_pixel(Rect(Point2D(0, 0), Size2D(2, 2)), 0.5);
+    assert!(!unused.is_empty());
     assert!(q.root.tile_mem == 1);
 }

src/components/msg/compositor_msg.rs

@@ -8,8 +8,6 @@ use geom::rect::Rect;
 use geom::size::Size2D;
 
 use constellation_msg::PipelineId;
-use extra::arc;
-
 
 #[deriving(Clone)]
 pub struct LayerBuffer {
@@ -31,8 +29,9 @@ pub struct LayerBuffer {
 
 /// A set of layer buffers. This is an atomic unit used to switch between the front and back
 /// buffers.
+#[deriving(Clone)]
 pub struct LayerBufferSet {
-    buffers: ~[LayerBuffer]
+    buffers: ~[~LayerBuffer]
 }
 
 /// The status of the renderer.
@@ -72,7 +71,7 @@ pub trait RenderListener {
     fn set_layer_page_size(&self, PipelineId, Size2D<uint>, Epoch);
     fn set_layer_clip_rect(&self, PipelineId, Rect<uint>);
     fn delete_layer(&self, PipelineId);
-    fn paint(&self, id: PipelineId, layer_buffer_set: arc::Arc<LayerBufferSet>, Epoch);
+    fn paint(&self, id: PipelineId, layer_buffer_set: ~LayerBufferSet, Epoch);
     fn set_render_state(&self, render_state: RenderState);
 }
 
@@ -89,6 +88,8 @@ pub trait Tile {
     fn get_mem(&self) -> uint;
     /// Returns true if the tile is displayable at the given scale
     fn is_valid(&self, f32) -> bool;
+    /// Returns the Size2D of the tile
+    fn get_size_2d(&self) -> Size2D<uint>;
 }
 
 impl Tile for ~LayerBuffer {
@@ -99,4 +100,7 @@ impl Tile for ~LayerBuffer {
     fn is_valid(&self, scale: f32) -> bool {
         self.resolution.approx_eq(&scale)
     }
+    fn get_size_2d(&self) -> Size2D<uint> {
+        self.screen_pos.size
+    }
 }