layout: Do not require restyle information when not restyling (#37722)

This reduces the amount of work necessary when running layout, by making
restyle information optional in the `ReflowRequest`. When restyling
isn't
necessary, the option is `None`.

Testing: This shouldn't change any observable behavior and thus is
covered
by existing WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout/layout_impl.rs

@@ -27,7 +27,7 @@ use fxhash::FxHashMap;
 use ipc_channel::ipc::IpcSender;
 use layout_api::{
     Layout, LayoutConfig, LayoutFactory, NodesFromPointQueryType, OffsetParentResponse, QueryMsg,
-    ReflowGoal, ReflowRequest, ReflowResult, TrustedNodeAddress,
+    ReflowGoal, ReflowRequest, ReflowRequestRestyle, ReflowResult, TrustedNodeAddress,
 };
 use log::{debug, error, warn};
 use malloc_size_of::{MallocConditionalSizeOf, MallocSizeOf, MallocSizeOfOps};
@@ -586,7 +586,7 @@ impl LayoutThread {
     /// at all.
     fn can_skip_reflow_request_entirely(&self, reflow_request: &ReflowRequest) -> bool {
         // If a restyle is necessary, restyle and reflow is a necessity.
-        if reflow_request.restyle_reason.needs_restyle() {
+        if reflow_request.restyle.is_some() {
             return false;
         }
         // We always need to at least build a fragment tree.
@@ -649,9 +649,11 @@ impl LayoutThread {
         };
 
         let mut snapshot_map = SnapshotMap::new();
-        let _snapshot_setter = SnapshotSetter::new(&mut reflow_request, &mut snapshot_map);
+        let mut _snapshot_setter = None;
         let mut viewport_changed = false;
-        if reflow_request.restyle_reason.needs_restyle() {
+        if let Some(restyle) = reflow_request.restyle.as_mut() {
+            _snapshot_setter = Some(SnapshotSetter::new(restyle, &mut snapshot_map));
+
             viewport_changed = self.viewport_did_change(reflow_request.viewport_details);
             if self.update_device_if_necessary(&reflow_request, viewport_changed, &guards) {
                 if let Some(mut data) = root_element.mutate_data() {
@@ -683,7 +685,7 @@ impl LayoutThread {
                 &snapshot_map,
                 reflow_request.animation_timeline_value,
                 &reflow_request.animations,
-                match reflow_request.stylesheets_changed {
+                match reflow_request.stylesheets_changed() {
                     true => TraversalFlags::ForCSSRuleChanges,
                     false => TraversalFlags::empty(),
                 },
@@ -700,9 +702,9 @@ impl LayoutThread {
         };
 
         let mut damage = RestyleDamage::empty();
-        if reflow_request.restyle_reason.needs_restyle() {
+        if let Some(restyle) = reflow_request.restyle.as_ref() {
             damage = self.restyle_and_build_trees(
-                &reflow_request,
+                restyle,
                 root_element,
                 rayon_pool,
                 &mut layout_context,
@@ -794,7 +796,7 @@ impl LayoutThread {
             self.have_added_user_agent_stylesheets = true;
         }
 
-        if reflow_request.stylesheets_changed {
+        if reflow_request.stylesheets_changed() {
             self.stylist
                 .force_stylesheet_origins_dirty(Origin::Author.into());
         }
@@ -809,14 +811,14 @@ impl LayoutThread {
     #[servo_tracing::instrument(skip_all)]
     fn restyle_and_build_trees(
         &self,
-        reflow_request: &ReflowRequest,
+        restyle: &ReflowRequestRestyle,
         root_element: ServoLayoutElement<'_>,
         rayon_pool: Option<&ThreadPool>,
         layout_context: &mut LayoutContext<'_>,
         viewport_changed: bool,
     ) -> RestyleDamage {
         let dirty_root = unsafe {
-            ServoLayoutNode::new(&reflow_request.dirty_root.unwrap())
+            ServoLayoutNode::new(&restyle.dirty_root.unwrap())
                 .as_element()
                 .unwrap()
         };
@@ -1330,12 +1332,9 @@ struct SnapshotSetter<'dom> {
 }
 
 impl SnapshotSetter<'_> {
-    fn new(reflow_request: &mut ReflowRequest, snapshot_map: &mut SnapshotMap) -> Self {
+    fn new(restyle: &mut ReflowRequestRestyle, snapshot_map: &mut SnapshotMap) -> Self {
-        debug!(
+        debug!("Draining restyles: {}", restyle.pending_restyles.len());
-            "Draining restyles: {}",
+        let restyles = std::mem::take(&mut restyle.pending_restyles);
-            reflow_request.pending_restyles.len()
-        );
-        let restyles = std::mem::take(&mut reflow_request.pending_restyles);
 
         let elements_with_snapshot: Vec<_> = restyles
             .iter()

components/script/dom/window.rs

@@ -52,8 +52,8 @@ use js::rust::{
     MutableHandleValue,
 };
 use layout_api::{
-    FragmentType, Layout, PendingImageState, QueryMsg, ReflowGoal, ReflowRequest, RestyleReason,
+    FragmentType, Layout, PendingImageState, QueryMsg, ReflowGoal, ReflowRequest,
-    TrustedNodeAddress, combine_id_with_fragment_type,
+    ReflowRequestRestyle, RestyleReason, TrustedNodeAddress, combine_id_with_fragment_type,
 };
 use malloc_size_of::MallocSizeOf;
 use media::WindowGLContext;
@@ -2143,17 +2143,6 @@ impl Window {
             return false;
         }
 
-        let restyle_reason = document.restyle_reason();
-        document.clear_restyle_reasons();
-
-        if restyle_reason.needs_restyle() {
-            debug!("Invalidating layout cache due to reflow condition {restyle_reason:?}",);
-            // Invalidate any existing cached layout values.
-            self.layout_marker.borrow().set(false);
-            // Create a new layout caching token.
-            *self.layout_marker.borrow_mut() = Rc::new(Cell::new(true));
-        }
-
         debug!("script: performing reflow for goal {reflow_goal:?}");
         let marker = if self.need_emit_timeline_marker(TimelineMarkerType::Reflow) {
             Some(TimelineMarker::start("Reflow".to_owned()))
@@ -2166,6 +2155,15 @@ impl Window {
             debug_reflow_events(pipeline_id, &reflow_goal);
         }
 
+        let restyle_reason = document.restyle_reason();
+        document.clear_restyle_reasons();
+        let restyle = if restyle_reason.needs_restyle() {
+            debug!("Invalidating layout cache due to reflow condition {restyle_reason:?}",);
+            // Invalidate any existing cached layout values.
+            self.layout_marker.borrow().set(false);
+            // Create a new layout caching token.
+            *self.layout_marker.borrow_mut() = Rc::new(Cell::new(true));
+
             let stylesheets_changed = document.flush_stylesheets_for_reflow();
             let pending_restyles = document.drain_pending_restyles();
             let dirty_root = document
@@ -2174,19 +2172,26 @@ impl Window {
                 .or_else(|| document.GetDocumentElement())
                 .map(|root| root.upcast::<Node>().to_trusted_node_address());
 
+            Some(ReflowRequestRestyle {
+                reason: restyle_reason,
+                dirty_root,
+                stylesheets_changed,
+                pending_restyles,
+            })
+        } else {
+            None
+        };
+
         let highlighted_dom_node = document.highlighted_dom_node().map(|node| node.to_opaque());
 
         // Send new document and relevant styles to layout.
         let reflow = ReflowRequest {
-            restyle_reason,
             document: document.upcast::<Node>().to_trusted_node_address(),
-            dirty_root,
+            restyle,
-            stylesheets_changed,
             viewport_details: self.viewport_details.get(),
             origin: self.origin().immutable().clone(),
             reflow_goal,
             dom_count: document.dom_count(),
-            pending_restyles,
             animation_timeline_value: document.current_animation_timeline_value(),
             animations: document.animations().sets.clone(),
             node_to_animating_image_map: document.image_animation_manager().node_to_image_map(),

components/shared/layout/lib.rs

@@ -395,17 +395,27 @@ pub struct ReflowResult {
     pub iframe_sizes: IFrameSizes,
 }
 
-/// Information needed for a script-initiated reflow.
+/// Information needed for a script-initiated reflow that requires a restyle
+/// and reconstruction of box and fragment trees.
 #[derive(Debug)]
-pub struct ReflowRequest {
+pub struct ReflowRequestRestyle {
     /// Whether or not (and for what reasons) restyle needs to happen.
-    pub restyle_reason: RestyleReason,
+    pub reason: RestyleReason,
-    /// The document node.
-    pub document: TrustedNodeAddress,
     /// The dirty root from which to restyle.
     pub dirty_root: Option<TrustedNodeAddress>,
     /// Whether the document's stylesheets have changed since the last script reflow.
     pub stylesheets_changed: bool,
+    /// Restyle snapshot map.
+    pub pending_restyles: Vec<(TrustedNodeAddress, PendingRestyle)>,
+}
+
+/// Information needed for a script-initiated reflow.
+#[derive(Debug)]
+pub struct ReflowRequest {
+    /// The document node.
+    pub document: TrustedNodeAddress,
+    /// If a restyle is necessary, all of the informatio needed to do that restyle.
+    pub restyle: Option<ReflowRequestRestyle>,
     /// The current [`ViewportDetails`] to use for this reflow.
     pub viewport_details: ViewportDetails,
     /// The goal of this reflow.
@@ -414,8 +424,6 @@ pub struct ReflowRequest {
     pub dom_count: u32,
     /// The current window origin
     pub origin: ImmutableOrigin,
-    /// Restyle snapshot map.
-    pub pending_restyles: Vec<(TrustedNodeAddress, PendingRestyle)>,
     /// The current animation timeline value.
     pub animation_timeline_value: f64,
     /// The set of animations for this document.
@@ -428,6 +436,14 @@ pub struct ReflowRequest {
     pub highlighted_dom_node: Option<OpaqueNode>,
 }
 
+impl ReflowRequest {
+    pub fn stylesheets_changed(&self) -> bool {
+        self.restyle
+            .as_ref()
+            .is_some_and(|restyle| restyle.stylesheets_changed)
+    }
+}
+
 /// A pending restyle.
 #[derive(Debug, Default, MallocSizeOf)]
 pub struct PendingRestyle {