Speculatively evaluate paint functions during style.

Cargo.lock

@@ -1520,6 +1520,7 @@ dependencies = [
  "servo_geometry 0.0.1",
  "servo_url 0.0.1",
  "style 0.0.1",
+ "style_traits 0.0.1",
  "webrender_api 0.48.0 (git+https://github.com/servo/webrender)",
 ]
 
@@ -3087,6 +3088,7 @@ dependencies = [
  "heapsize_derive 0.1.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "selectors 0.19.0",
  "serde 1.0.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "servo_atoms 0.0.1",
  "webrender_api 0.48.0 (git+https://github.com/servo/webrender)",
 ]
 

components/layout/context.rs

@@ -4,7 +4,6 @@
 
 //! Data needed by the layout thread.
 
-use fnv::FnvHashMap;
 use fnv::FnvHasher;
 use gfx::display_list::{WebRenderImageInfo, OpaqueNode};
 use gfx::font_cache_thread::FontCacheThread;
@@ -25,8 +24,8 @@ use std::collections::HashMap;
 use std::hash::BuildHasherDefault;
 use std::sync::{Arc, Mutex};
 use std::thread;
+use style::context::RegisteredSpeculativePainter;
 use style::context::SharedStyleContext;
-use style::properties::PropertyId;
 
 thread_local!(static FONT_CONTEXT_KEY: RefCell<Option<FontContext>> = RefCell::new(None));
 
@@ -73,7 +72,7 @@ pub struct LayoutContext<'a> {
                                                   BuildHasherDefault<FnvHasher>>>>,
 
     /// Paint worklets
-    pub registered_painters: Arc<RwLock<FnvHashMap<Atom, RegisteredPainter>>>,
+    pub registered_painters: &'a RegisteredPainters,
 
     /// A list of in-progress image loads to be shared with the script thread.
     /// A None value means that this layout was not initiated by the script thread.
@@ -179,9 +178,11 @@ impl<'a> LayoutContext<'a> {
     }
 }
 
-/// A registered paint worklet.
+/// A registered painter
-pub struct RegisteredPainter {
+pub trait RegisteredPainter: RegisteredSpeculativePainter + Painter {}
-    pub name: Atom,
+
-    pub properties: FnvHashMap<Atom, PropertyId>,
+/// A set of registered painters
-    pub painter: Arc<Painter>,
+pub trait RegisteredPainters: Sync {
+    /// Look up a painter
+    fn get(&self, name: &Atom) -> Option<&RegisteredPainter>;
 }

components/layout/display_list_builder.rs

@@ -1181,24 +1181,18 @@ impl FragmentDisplayListBuilding for Fragment {
             .map(|argument| argument.to_css_string())
             .collect();
 
-        // Get the painter, and the computed values for its properties.
+        let mut draw_result = match state.layout_context.registered_painters.get(&name) {
-        // TODO: less copying.
+            Some(painter) => {
-        let (properties, painter) = match state.layout_context.registered_painters.read().get(&name) {
+                debug!("Drawing a paint image {}({},{}).", name, size_in_px.width, size_in_px.height);
-            Some(registered_painter) => (
+                let properties = painter.properties().iter()
-                registered_painter.properties
-                    .iter()
                     .filter_map(|(name, id)| id.as_shorthand().err().map(|id| (name, id)))
                     .map(|(name, id)| (name.clone(), style.computed_value_to_string(id)))
-                    .collect(),
+                    .collect();
-                registered_painter.painter.clone()
+                painter.draw_a_paint_image(size_in_px, device_pixel_ratio, properties, arguments)
-            ),
+            },
             None => return debug!("Worklet {} called before registration.", name),
         };
 
-        // TODO: add a one-place cache to avoid drawing the paint image every time.
-        // https://github.com/servo/servo/issues/17369
-        debug!("Drawing a paint image {}({},{}).", name, size_in_px.width, size_in_px.height);
-        let mut draw_result = painter.draw_a_paint_image(size_in_px, device_pixel_ratio, properties, arguments);
         let webrender_image = WebRenderImageInfo {
             width: draw_result.width,
             height: draw_result.height,

components/layout_thread/Cargo.toml

@@ -41,4 +41,5 @@ servo_config = {path = "../config"}
 servo_geometry = {path = "../geometry"}
 servo_url = {path = "../url"}
 style = {path = "../style"}
+style_traits = {path = "../style_traits"}
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}

components/layout_thread/lib.rs

@@ -46,6 +46,7 @@ extern crate servo_config;
 extern crate servo_geometry;
 extern crate servo_url;
 extern crate style;
+extern crate style_traits;
 extern crate webrender_api;
 
 mod dom_wrapper;
@@ -53,7 +54,7 @@ mod dom_wrapper;
 use app_units::Au;
 use dom_wrapper::{ServoLayoutElement, ServoLayoutDocument, ServoLayoutNode};
 use dom_wrapper::drop_style_and_layout_data;
-use euclid::{Point2D, Rect, Size2D, ScaleFactor};
+use euclid::{Point2D, Rect, Size2D, ScaleFactor, TypedSize2D};
 use fnv::FnvHashMap;
 use gfx::display_list::{OpaqueNode, WebRenderImageInfo};
 use gfx::font;
@@ -67,6 +68,7 @@ use layout::animation;
 use layout::construct::ConstructionResult;
 use layout::context::LayoutContext;
 use layout::context::RegisteredPainter;
+use layout::context::RegisteredPainters;
 use layout::context::heap_size_of_persistent_local_context;
 use layout::display_list_builder::ToGfxColor;
 use layout::flow::{self, Flow, ImmutableFlowUtils, MutableFlowUtils, MutableOwnedFlowUtils};
@@ -99,6 +101,8 @@ use script_layout_interface::rpc::TextIndexResponse;
 use script_layout_interface::wrapper_traits::LayoutNode;
 use script_traits::{ConstellationControlMsg, LayoutControlMsg, LayoutMsg as ConstellationMsg};
 use script_traits::{ScrollState, UntrustedNodeAddress};
+use script_traits::DrawAPaintImageResult;
+use script_traits::Painter;
 use selectors::Element;
 use servo_arc::Arc as ServoArc;
 use servo_atoms::Atom;
@@ -122,6 +126,8 @@ use std::thread;
 use style::animation::Animation;
 use style::context::{QuirksMode, ReflowGoal, SharedStyleContext};
 use style::context::{StyleSystemOptions, ThreadLocalStyleContextCreationInfo};
+use style::context::RegisteredSpeculativePainter;
+use style::context::RegisteredSpeculativePainters;
 use style::dom::{ShowSubtree, ShowSubtreeDataAndPrimaryValues, TElement, TNode};
 use style::error_reporting::{NullReporter, RustLogReporter};
 use style::invalidation::element::restyle_hints::RestyleHint;
@@ -137,6 +143,9 @@ use style::thread_state;
 use style::timer::Timer;
 use style::traversal::{DomTraversal, TraversalDriver};
 use style::traversal_flags::TraversalFlags;
+use style_traits::CSSPixel;
+use style_traits::DevicePixel;
+use style_traits::SpeculativePainter;
 
 /// Information needed by the layout thread.
 pub struct LayoutThread {
@@ -235,9 +244,9 @@ pub struct LayoutThread {
 
     webrender_image_cache: Arc<RwLock<FnvHashMap<(ServoUrl, UsePlaceholder),
                                                  WebRenderImageInfo>>>,
-    /// The executor for paint worklets.
+
-    /// Will be None if the script thread hasn't added any paint worklet modules.
+    /// The executors for paint worklets.
-    registered_painters: Arc<RwLock<FnvHashMap<Atom, RegisteredPainter>>>,
+    registered_painters: RegisteredPaintersImpl,
 
     /// Webrender interface.
     webrender_api: webrender_api::RenderApi,
@@ -520,7 +529,7 @@ impl LayoutThread {
             constellation_chan: constellation_chan.clone(),
             time_profiler_chan: time_profiler_chan,
             mem_profiler_chan: mem_profiler_chan,
-            registered_painters: Arc::new(RwLock::new(FnvHashMap::default())),
+            registered_painters: RegisteredPaintersImpl(FnvHashMap::default()),
             image_cache: image_cache.clone(),
             font_cache_thread: font_cache_thread,
             first_reflow: Cell::new(true),
@@ -605,6 +614,7 @@ impl LayoutThread {
                 visited_styles_enabled: false,
                 running_animations: self.running_animations.clone(),
                 expired_animations: self.expired_animations.clone(),
+                registered_speculative_painters: &self.registered_painters,
                 local_context_creation_data: Mutex::new(thread_local_style_context_creation_data),
                 timer: self.timer.clone(),
                 quirks_mode: self.quirks_mode.unwrap(),
@@ -616,7 +626,7 @@ impl LayoutThread {
             webrender_image_cache: self.webrender_image_cache.clone(),
             pending_images: if script_initiated_layout { Some(Mutex::new(vec![])) } else { None },
             newly_transitioning_nodes: if script_initiated_layout { Some(Mutex::new(vec![])) } else { None },
-            registered_painters: self.registered_painters.clone(),
+            registered_painters: &self.registered_painters,
         }
     }
 
@@ -738,13 +748,12 @@ impl LayoutThread {
                     .filter_map(|name| PropertyId::parse(&*name).ok().map(|id| (name.clone(), id)))
                     .filter(|&(_, ref id)| id.as_shorthand().is_err())
                     .collect();
-                let registered_painter = RegisteredPainter {
+                let registered_painter = RegisteredPainterImpl {
                     name: name.clone(),
                     properties: properties,
                     painter: painter,
                 };
-                self.registered_painters.write()
+                self.registered_painters.0.insert(name, registered_painter);
-                    .insert(name, registered_painter);
             },
             Msg::PrepareToExit(response_chan) => {
                 self.prepare_to_exit(response_chan);
@@ -1790,3 +1799,52 @@ lazy_static! {
         }
     };
 }
+
+struct RegisteredPainterImpl {
+    painter: Box<Painter>,
+    name: Atom,
+    properties: FnvHashMap<Atom, PropertyId>,
+}
+
+impl SpeculativePainter for RegisteredPainterImpl {
+    fn speculatively_draw_a_paint_image(&self, properties: Vec<(Atom, String)>, arguments: Vec<String>) {
+        self.painter.speculatively_draw_a_paint_image(properties, arguments);
+    }
+}
+
+impl RegisteredSpeculativePainter for RegisteredPainterImpl {
+    fn properties(&self) -> &FnvHashMap<Atom, PropertyId> {
+        &self.properties
+    }
+    fn name(&self) -> Atom {
+        self.name.clone()
+    }
+}
+
+impl Painter for RegisteredPainterImpl {
+    fn draw_a_paint_image(&self,
+                          size: TypedSize2D<f32, CSSPixel>,
+                          device_pixel_ratio: ScaleFactor<f32, CSSPixel, DevicePixel>,
+                          properties: Vec<(Atom, String)>,
+                          arguments: Vec<String>)
+                          -> DrawAPaintImageResult
+    {
+        self.painter.draw_a_paint_image(size, device_pixel_ratio, properties, arguments)
+    }
+}
+
+impl RegisteredPainter for RegisteredPainterImpl {}
+
+struct RegisteredPaintersImpl(FnvHashMap<Atom, RegisteredPainterImpl>);
+
+impl RegisteredSpeculativePainters for RegisteredPaintersImpl  {
+    fn get(&self, name: &Atom) -> Option<&RegisteredSpeculativePainter> {
+        self.0.get(&name).map(|painter| painter as &RegisteredSpeculativePainter)
+    }
+}
+
+impl RegisteredPainters for RegisteredPaintersImpl {
+    fn get(&self, name: &Atom) -> Option<&RegisteredPainter> {
+        self.0.get(&name).map(|painter| painter as &RegisteredPainter)
+    }
+}

components/script/dom/bindings/trace.rs

@@ -43,7 +43,7 @@ use dom::bindings::str::{DOMString, USVString};
 use dom::bindings::utils::WindowProxyHandler;
 use dom::document::PendingRestyle;
 use encoding::types::EncodingRef;
-use euclid::{Transform2D, Transform3D, Point2D, Vector2D, Rect, Size2D, ScaleFactor};
+use euclid::{Transform2D, Transform3D, Point2D, Vector2D, Rect, TypedSize2D, ScaleFactor};
 use euclid::Length as EuclidLength;
 use html5ever::{Prefix, LocalName, Namespace, QualName};
 use html5ever::buffer_queue::BufferQueue;
@@ -75,6 +75,7 @@ use script_layout_interface::reporter::CSSErrorReporter;
 use script_layout_interface::rpc::LayoutRPC;
 use script_traits::{DocumentActivity, TimerEventId, TimerSource, TouchpadPressurePhase};
 use script_traits::{UntrustedNodeAddress, WindowSizeData, WindowSizeType};
+use script_traits::DrawAPaintImageResult;
 use selectors::matching::ElementSelectorFlags;
 use serde::{Deserialize, Serialize};
 use servo_arc::Arc as ServoArc;
@@ -389,6 +390,7 @@ unsafe_no_jsmanaged_fields!(RelativePos);
 unsafe_no_jsmanaged_fields!(OpaqueStyleAndLayoutData);
 unsafe_no_jsmanaged_fields!(PathBuf);
 unsafe_no_jsmanaged_fields!(CSSErrorReporter);
+unsafe_no_jsmanaged_fields!(DrawAPaintImageResult);
 unsafe_no_jsmanaged_fields!(WebGLBufferId);
 unsafe_no_jsmanaged_fields!(WebGLFramebufferId);
 unsafe_no_jsmanaged_fields!(WebGLProgramId);
@@ -519,7 +521,14 @@ unsafe impl JSTraceable for Rect<f32> {
     }
 }
 
-unsafe impl JSTraceable for Size2D<i32> {
+unsafe impl<U> JSTraceable for TypedSize2D<i32, U> {
+    #[inline]
+    unsafe fn trace(&self, _trc: *mut JSTracer) {
+        // Do nothing
+    }
+}
+
+unsafe impl<U> JSTraceable for TypedSize2D<f32, U> {
     #[inline]
     unsafe fn trace(&self, _trc: *mut JSTracer) {
         // Do nothing

components/script/dom/paintworkletglobalscope.rs

@@ -63,6 +63,7 @@ use std::sync::mpsc;
 use std::sync::mpsc::Sender;
 use style_traits::CSSPixel;
 use style_traits::DevicePixel;
+use style_traits::SpeculativePainter;
 
 /// https://drafts.css-houdini.org/css-paint-api/#paintworkletglobalscope
 #[dom_struct]
@@ -76,6 +77,18 @@ pub struct PaintWorkletGlobalScope {
     paint_definitions: DOMRefCell<HashMap<Atom, Box<PaintDefinition>>>,
     /// https://drafts.css-houdini.org/css-paint-api/#paint-class-instances
     paint_class_instances: DOMRefCell<HashMap<Atom, Box<Heap<JSVal>>>>,
+    /// The most recent name the worklet was called with
+    cached_name: DOMRefCell<Atom>,
+    /// The most recent size the worklet was drawn at
+    cached_size: Cell<TypedSize2D<f32, CSSPixel>>,
+    /// The most recent device pixel ratio the worklet was drawn at
+    cached_device_pixel_ratio: Cell<ScaleFactor<f32, CSSPixel, DevicePixel>>,
+    /// The most recent properties the worklet was drawn at
+    cached_properties: DOMRefCell<Vec<(Atom, String)>>,
+    /// The most recent arguments the worklet was drawn at
+    cached_arguments: DOMRefCell<Vec<String>>,
+    /// The most recent result
+    cached_result: DOMRefCell<DrawAPaintImageResult>,
 }
 
 impl PaintWorkletGlobalScope {
@@ -92,6 +105,18 @@ impl PaintWorkletGlobalScope {
             image_cache: init.image_cache.clone(),
             paint_definitions: Default::default(),
             paint_class_instances: Default::default(),
+            cached_name: DOMRefCell::new(Atom::from("")),
+            cached_size: Cell::new(TypedSize2D::zero()),
+            cached_device_pixel_ratio: Cell::new(ScaleFactor::new(1.0)),
+            cached_properties: Default::default(),
+            cached_arguments: Default::default(),
+            cached_result: DOMRefCell::new(DrawAPaintImageResult {
+                width: 0,
+                height: 0,
+                format: PixelFormat::BGRA8,
+                image_key: None,
+                missing_image_urls: Vec::new(),
+            }),
         };
         unsafe { PaintWorkletGlobalScopeBinding::Wrap(runtime.cx(), global) }
     }
@@ -102,21 +127,58 @@ impl PaintWorkletGlobalScope {
 
     pub fn perform_a_worklet_task(&self, task: PaintWorkletTask) {
         match task {
-            PaintWorkletTask::DrawAPaintImage(name, size_in_px, device_pixel_ratio, properties, arguments, sender) => {
+            PaintWorkletTask::DrawAPaintImage(name, size, device_pixel_ratio, properties, arguments, sender) => {
-                let properties = StylePropertyMapReadOnly::from_iter(self.upcast(), properties);
+                let cache_hit = (&*self.cached_name.borrow() == &name) &&
-                let result = self.draw_a_paint_image(name, size_in_px, device_pixel_ratio, &*properties, arguments);
+                    (self.cached_size.get() == size) &&
+                    (self.cached_device_pixel_ratio.get() == device_pixel_ratio) &&
+                    (&*self.cached_properties.borrow() == &properties) &&
+                    (&*self.cached_arguments.borrow() == &arguments);
+                let result = if cache_hit {
+                    debug!("Cache hit on paint worklet {}!", name);
+                    self.cached_result.borrow().clone()
+                } else {
+                    debug!("Cache miss on paint worklet {}!", name);
+                    let map = StylePropertyMapReadOnly::from_iter(self.upcast(), properties.iter().cloned());
+                    let result = self.draw_a_paint_image(&name, size, device_pixel_ratio, &*map, &*arguments);
+                    if (result.image_key.is_some()) && (result.missing_image_urls.is_empty()) {
+                        *self.cached_name.borrow_mut() = name;
+                        self.cached_size.set(size);
+                        self.cached_device_pixel_ratio.set(device_pixel_ratio);
+                        *self.cached_properties.borrow_mut() = properties;
+                        *self.cached_arguments.borrow_mut() = arguments;
+                        *self.cached_result.borrow_mut() = result.clone();
+                    }
+                    result
+                };
                 let _ = sender.send(result);
             }
+            PaintWorkletTask::SpeculativelyDrawAPaintImage(name, properties, arguments) => {
+                let should_speculate = (&*self.cached_name.borrow() != &name) ||
+                    (&*self.cached_properties.borrow() != &properties) ||
+                    (&*self.cached_arguments.borrow() != &arguments);
+                if should_speculate {
+                    let size = self.cached_size.get();
+                    let device_pixel_ratio = self.cached_device_pixel_ratio.get();
+                    let map = StylePropertyMapReadOnly::from_iter(self.upcast(), properties.iter().cloned());
+                    let result = self.draw_a_paint_image(&name, size, device_pixel_ratio, &*map, &*arguments);
+                    if (result.image_key.is_some()) && (result.missing_image_urls.is_empty()) {
+                        *self.cached_name.borrow_mut() = name;
+                        *self.cached_properties.borrow_mut() = properties;
+                        *self.cached_arguments.borrow_mut() = arguments;
+                        *self.cached_result.borrow_mut() = result;
+                    }
+                }
+            }
         }
     }
 
     /// https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image
     fn draw_a_paint_image(&self,
-                          name: Atom,
+                          name: &Atom,
                           size_in_px: TypedSize2D<f32, CSSPixel>,
                           device_pixel_ratio: ScaleFactor<f32, CSSPixel, DevicePixel>,
                           properties: &StylePropertyMapReadOnly,
-                          arguments: Vec<String>)
+                          arguments: &[String])
                           -> DrawAPaintImageResult
     {
         let size_in_dpx = size_in_px * device_pixel_ratio;
@@ -131,12 +193,12 @@ impl PaintWorkletGlobalScope {
     /// https://drafts.css-houdini.org/css-paint-api/#invoke-a-paint-callback
     #[allow(unsafe_code)]
     fn invoke_a_paint_callback(&self,
-                               name: Atom,
+                               name: &Atom,
                                size_in_px: TypedSize2D<f32, CSSPixel>,
                                size_in_dpx: TypedSize2D<u32, DevicePixel>,
                                device_pixel_ratio: ScaleFactor<f32, CSSPixel, DevicePixel>,
                                properties: &StylePropertyMapReadOnly,
-                               mut arguments: Vec<String>)
+                               arguments: &[String])
                                -> DrawAPaintImageResult
     {
         debug!("Invoking a paint callback {}({},{}) at {}.",
@@ -149,7 +211,7 @@ impl PaintWorkletGlobalScope {
         // Step 2.2-5.1.
         rooted!(in(cx) let mut class_constructor = UndefinedValue());
         rooted!(in(cx) let mut paint_function = UndefinedValue());
-        let rendering_context = match self.paint_definitions.borrow().get(&name) {
+        let rendering_context = match self.paint_definitions.borrow().get(name) {
             None => {
                 // Step 2.2.
                 warn!("Drawing un-registered paint definition {}.", name);
@@ -184,7 +246,7 @@ impl PaintWorkletGlobalScope {
                 if unsafe { JS_IsExceptionPending(cx) } {
                     debug!("Paint constructor threw an exception {}.", name);
                     unsafe { JS_ClearPendingException(cx); }
-                    self.paint_definitions.borrow_mut().get_mut(&name)
+                    self.paint_definitions.borrow_mut().get_mut(name)
                         .expect("Vanishing paint definition.")
                         .constructor_valid_flag.set(false);
                     return self.invalid_image(size_in_dpx, vec![]);
@@ -206,7 +268,7 @@ impl PaintWorkletGlobalScope {
         // TODO: Step 10
         // Steps 11-12
         debug!("Invoking paint function {}.", name);
-        rooted_vec!(let arguments_values <- arguments.drain(..)
+        rooted_vec!(let arguments_values <- arguments.iter().cloned()
                     .map(|argument| CSSStyleValue::new(self.upcast(), argument)));
         let arguments_value_vec: Vec<JSVal> = arguments_values.iter()
             .map(|argument| ObjectValue(argument.reflector().get_jsobject().get()))
@@ -262,9 +324,22 @@ impl PaintWorkletGlobalScope {
         }
     }
 
-    fn painter(&self, name: Atom) -> Arc<Painter> {
+    fn painter(&self, name: Atom) -> Box<Painter> {
         // Rather annoyingly we have to use a mutex here to make the painter Sync.
-        struct WorkletPainter(Atom, Mutex<WorkletExecutor>);
+        struct WorkletPainter {
+            name: Atom,
+            executor: Mutex<WorkletExecutor>,
+        }
+        impl SpeculativePainter for WorkletPainter {
+            fn speculatively_draw_a_paint_image(&self,
+                                                properties: Vec<(Atom, String)>,
+                                                arguments: Vec<String>) {
+                let name = self.name.clone();
+                let task = PaintWorkletTask::SpeculativelyDrawAPaintImage(name, properties, arguments);
+                self.executor.lock().expect("Locking a painter.")
+                    .schedule_a_worklet_task(WorkletTask::Paint(task));
+            }
+        }
         impl Painter for WorkletPainter {
             fn draw_a_paint_image(&self,
                                   size: TypedSize2D<f32, CSSPixel>,
@@ -272,7 +347,7 @@ impl PaintWorkletGlobalScope {
                                   properties: Vec<(Atom, String)>,
                                   arguments: Vec<String>)
                                   -> DrawAPaintImageResult {
-                let name = self.0.clone();
+                let name = self.name.clone();
                 let (sender, receiver) = mpsc::channel();
                 let task = PaintWorkletTask::DrawAPaintImage(name,
                                                              size,
@@ -280,12 +355,15 @@ impl PaintWorkletGlobalScope {
                                                              properties,
                                                              arguments,
                                                              sender);
-                self.1.lock().expect("Locking a painter.")
+                self.executor.lock().expect("Locking a painter.")
                     .schedule_a_worklet_task(WorkletTask::Paint(task));
                 receiver.recv().expect("Worklet thread died?")
             }
         }
-        Arc::new(WorkletPainter(name, Mutex::new(self.worklet_global.executor())))
+        Box::new(WorkletPainter {
+            name: name,
+            executor: Mutex::new(self.worklet_global.executor()),
+        })
     }
 }
 
@@ -380,7 +458,10 @@ pub enum PaintWorkletTask {
                     ScaleFactor<f32, CSSPixel, DevicePixel>,
                     Vec<(Atom, String)>,
                     Vec<String>,
-                    Sender<DrawAPaintImageResult>)
+                    Sender<DrawAPaintImageResult>),
+    SpeculativelyDrawAPaintImage(Atom,
+                                 Vec<(Atom, String)>,
+                                 Vec<String>),
 }
 
 /// A paint definition

components/script_layout_interface/message.rs

@@ -89,7 +89,7 @@ pub enum Msg {
     UpdateScrollStateFromScript(ScrollState),
 
     /// Tells layout that script has added some paint worklet modules.
-    RegisterPaint(Atom, Vec<Atom>, Arc<Painter>),
+    RegisterPaint(Atom, Vec<Atom>, Box<Painter>),
 
     /// Send to layout the precise time when the navigation started.
     SetNavigationStart(f64),

components/script_traits/lib.rs

@@ -66,6 +66,7 @@ use std::fmt;
 use std::sync::Arc;
 use std::sync::mpsc::{Receiver, Sender, RecvTimeoutError};
 use style_traits::CSSPixel;
+use style_traits::SpeculativePainter;
 use webdriver_msg::{LoadStatus, WebDriverScriptCommand};
 use webrender_api::{ClipId, DevicePixel, ImageKey};
 use webvr_traits::{WebVREvent, WebVRMsg};
@@ -817,7 +818,7 @@ impl From<RecvTimeoutError> for PaintWorkletError {
 }
 
 /// Execute paint code in the worklet thread pool.
-pub trait Painter: Sync + Send {
+pub trait Painter: SpeculativePainter {
     /// https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image
     fn draw_a_paint_image(&self,
                           size: TypedSize2D<f32, CSSPixel>,
@@ -829,7 +830,7 @@ pub trait Painter: Sync + Send {
 
 /// The result of executing paint code: the image together with any image URLs that need to be loaded.
 /// TODO: this should return a WR display list. https://github.com/servo/servo/issues/17497
-#[derive(Debug, Deserialize, Serialize, Clone)]
+#[derive(Debug, Deserialize, Serialize, Clone, HeapSizeOf)]
 pub struct DrawAPaintImageResult {
     /// The image height
     pub width: u32,

components/style/context.rs

@@ -18,10 +18,12 @@ use font_metrics::FontMetricsProvider;
 #[cfg(feature = "gecko")] use gecko_bindings::structs;
 #[cfg(feature = "servo")] use parking_lot::RwLock;
 use properties::ComputedValues;
+#[cfg(feature = "servo")] use properties::PropertyId;
 use rule_tree::StrongRuleNode;
 use selector_parser::{EAGER_PSEUDO_COUNT, SnapshotMap};
 use selectors::matching::ElementSelectorFlags;
 use servo_arc::Arc;
+#[cfg(feature = "servo")] use servo_atoms::Atom;
 use shared_lock::StylesheetGuards;
 use sharing::StyleSharingCandidateCache;
 use std::fmt;
@@ -30,6 +32,7 @@ use std::ops;
 #[cfg(feature = "servo")] use std::sync::mpsc::Sender;
 use style_traits::CSSPixel;
 use style_traits::DevicePixel;
+#[cfg(feature = "servo")] use style_traits::SpeculativePainter;
 use stylist::Stylist;
 use thread_state;
 use time;
@@ -151,6 +154,10 @@ pub struct SharedStyleContext<'a> {
     #[cfg(feature = "servo")]
     pub expired_animations: Arc<RwLock<FnvHashMap<OpaqueNode, Vec<Animation>>>>,
 
+    /// Paint worklets
+    #[cfg(feature = "servo")]
+    pub registered_speculative_painters: &'a RegisteredSpeculativePainters,
+
     /// Data needed to create the thread-local style context from the shared one.
     #[cfg(feature = "servo")]
     pub local_context_creation_data: Mutex<ThreadLocalStyleContextCreationInfo>,
@@ -677,3 +684,19 @@ pub enum ReflowGoal {
     /// We're reflowing in order to satisfy a script query. No display list will be created.
     ForScriptQuery,
 }
+
+/// A registered painter
+#[cfg(feature = "servo")]
+pub trait RegisteredSpeculativePainter: SpeculativePainter {
+    /// The name it was registered with
+    fn name(&self) -> Atom;
+    /// The properties it was registered with
+    fn properties(&self) -> &FnvHashMap<Atom, PropertyId>;
+}
+
+/// A set of registered painters
+#[cfg(feature = "servo")]
+pub trait RegisteredSpeculativePainters: Sync {
+    /// Look up a speculative painter
+    fn get(&self, name: &Atom) -> Option<&RegisteredSpeculativePainter>;
+}

components/style/traversal.rs

@@ -12,8 +12,11 @@ use matching::{ChildCascadeRequirement, MatchMethods};
 use sharing::StyleSharingTarget;
 use smallvec::SmallVec;
 use style_resolver::StyleResolverForElement;
+#[cfg(feature = "servo")] use style_traits::ToCss;
 use stylist::RuleInclusion;
 use traversal_flags::{TraversalFlags, self};
+#[cfg(feature = "servo")] use values::Either;
+#[cfg(feature = "servo")] use values::generics::image::Image;
 
 /// A per-traversal-level chunk of data. This is sent down by the traversal, and
 /// currently only holds the dom depth for the bloom filter.
@@ -518,6 +521,11 @@ where
                    element);
             clear_descendant_data(element)
         }
+
+        // Inform any paint worklets of changed style, to speculatively
+        // evaluate the worklet code. In the case that the size hasn't changed,
+        // this will result in increased concurrency between script and layout.
+        notify_paint_worklet(context, data);
     }
 
     // Now that matching and cascading is done, clear the bits corresponding to
@@ -715,6 +723,46 @@ where
     )
 }
 
+#[cfg(feature = "servo")]
+fn notify_paint_worklet<E>(context: &StyleContext<E>, data: &ElementData)
+where
+    E: TElement,
+{
+    // We speculatively evaluate any paint worklets during styling.
+    // This allows us to run paint worklets in parallel with style and layout.
+    // Note that this is wasted effort if the size of the node has
+    // changed, but in may cases it won't have.
+    if let Some(ref values) = data.styles.primary {
+        for image in &values.get_background().background_image.0 {
+            let (name, arguments) = match *image {
+                Either::Second(Image::PaintWorklet(ref worklet)) => (&worklet.name, &worklet.arguments),
+                _ => continue,
+            };
+            let painter = match context.shared.registered_speculative_painters.get(name) {
+                Some(painter) => painter,
+                None => continue,
+            };
+            let properties = painter.properties().iter()
+                .filter_map(|(name, id)| id.as_shorthand().err().map(|id| (name, id)))
+                .map(|(name, id)| (name.clone(), values.computed_value_to_string(id)))
+                .collect();
+            let arguments = arguments.iter()
+                .map(|argument| argument.to_css_string())
+                .collect();
+            debug!("Notifying paint worklet {}.", painter.name());
+            painter.speculatively_draw_a_paint_image(properties, arguments);
+        }
+    }
+}
+
+#[cfg(feature = "gecko")]
+fn notify_paint_worklet<E>(_context: &StyleContext<E>, _data: &ElementData)
+where
+    E: TElement,
+{
+    // The CSS paint API is Servo-only at the moment
+}
+
 fn note_children<E, D, F>(
     context: &mut StyleContext<E>,
     element: E,

components/style_traits/Cargo.toml

@@ -10,7 +10,7 @@ name = "style_traits"
 path = "lib.rs"
 
 [features]
-servo = ["heapsize", "heapsize_derive", "serde", "cssparser/heapsize", "cssparser/serde", "webrender_api"]
+servo = ["heapsize", "heapsize_derive", "serde", "servo_atoms", "cssparser/heapsize", "cssparser/serde", "webrender_api"]
 gecko = []
 
 [dependencies]
@@ -23,3 +23,4 @@ heapsize_derive = {version = "0.1", optional = true}
 selectors = { path = "../selectors" }
 serde = {version = "1.0", optional = true}
 webrender_api = {git = "https://github.com/servo/webrender", optional = true}
+servo_atoms = {path = "../atoms", optional = true}

components/style_traits/lib.rs

@@ -22,11 +22,13 @@ extern crate euclid;
 extern crate selectors;
 #[cfg(feature = "servo")] #[macro_use] extern crate serde;
 #[cfg(feature = "servo")] extern crate webrender_api;
+#[cfg(feature = "servo")] extern crate servo_atoms;
 
 #[cfg(feature = "servo")] pub use webrender_api::DevicePixel;
 
 use cssparser::{CowRcStr, Token};
 use selectors::parser::SelectorParseError;
+#[cfg(feature = "servo")] use servo_atoms::Atom;
 
 /// One hardware pixel.
 ///
@@ -184,3 +186,9 @@ impl ParsingMode {
     }
 }
 
+#[cfg(feature = "servo")]
+/// Speculatively execute paint code in the worklet thread pool.
+pub trait SpeculativePainter: Send + Sync {
+    /// https://drafts.css-houdini.org/css-paint-api/#draw-a-paint-image
+    fn speculatively_draw_a_paint_image(&self, properties: Vec<(Atom, String)>, arguments: Vec<String>);
+}