Bug 1709507 - Reduce allocations and memmove during scene building. r=gfx-reviewers,jrmuizel

This can make scene building up to 10% faster on some pages (such
as the youtube.com front page) that use a large number of clips.

Differential Revision: https://phabricator.services.mozilla.com/D114312

gfx/wr/webrender/src/clip.rs

@@ -722,6 +722,18 @@ impl ClipNode {
     }
 }
 
+pub struct ClipStoreStats {
+    templates_capacity: usize,
+}
+
+impl ClipStoreStats {
+    pub fn empty() -> Self {
+        ClipStoreStats {
+            templates_capacity: 0,
+        }
+    }
+}
+
 /// The main clipping public interface that other modules access.
 #[derive(MallocSizeOf)]
 #[cfg_attr(feature = "capture", derive(Serialize))]
@@ -971,18 +983,32 @@ impl ClipChainStack {
 }
 
 impl ClipStore {
-    pub fn new() -> Self {
+    pub fn new(stats: &ClipStoreStats) -> Self {
+        let mut templates = FastHashMap::default();
+        templates.reserve(stats.templates_capacity);
+
         ClipStore {
             clip_chain_nodes: Vec::new(),
             clip_node_instances: Vec::new(),
             active_clip_node_info: Vec::new(),
             active_local_clip_rect: None,
             active_pic_clip_rect: PictureRect::max_rect(),
-            templates: FastHashMap::default(),
+            templates,
             chain_builder_stack: Vec::new(),
         }
     }
 
+    pub fn get_stats(&self) -> ClipStoreStats {
+        // Selecting the smaller of the current capacity and 2*len ensures we don't
+        // retain a huge hashmap alloc after navigating away from a page with a large
+        // number of clip templates.
+        let templates_capacity = self.templates.capacity().min(self.templates.len() * 2);
+
+        ClipStoreStats {
+            templates_capacity,
+        }
+    }
+
     /// Register a new clip template for the clip_id defined in the display list.
     pub fn register_clip_template(
         &mut self,

gfx/wr/webrender/src/hit_test.rs

@@ -12,7 +12,7 @@ use crate::spatial_tree::{SpatialNodeIndex, SpatialTree};
 use crate::internal_types::{FastHashMap, LayoutPrimitiveInfo};
 use std::ops;
 use std::sync::{Arc, Mutex};
-use crate::util::LayoutToWorldFastTransform;
+use crate::util::{LayoutToWorldFastTransform, VecHelper};
 
 pub struct SharedHitTester {
     // We don't really need a mutex here. We could do with some sort of
@@ -489,12 +489,12 @@ fn add_clips(
     let template = &clip_store.templates[&clip_id];
 
     for clip in &template.clips {
-        let hit_test_clip_node = HitTestClipNode::new(
-            clip.key.into(),
-            clip.clip.spatial_node_index,
+        clip_nodes.alloc().init(
+            HitTestClipNode::new(
+                clip.key.into(),
+                clip.clip.spatial_node_index,
+            )
         );
-
-        clip_nodes.push(hit_test_clip_node);
     }
 
     // The ClipId parenting is terminated when we reach the root ClipId

gfx/wr/webrender/src/scene.rs

@@ -8,7 +8,7 @@ use api::units::*;
 use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
 use crate::render_api::MemoryReport;
 use crate::composite::CompositorKind;
-use crate::clip::ClipStore;
+use crate::clip::{ClipStore, ClipStoreStats};
 use crate::spatial_tree::SpatialTree;
 use crate::frame_builder::{ChasePrimitive, FrameBuilderConfig};
 use crate::hit_test::{HitTester, HitTestingScene, HitTestingSceneStats};
@@ -286,7 +286,7 @@ impl BuiltScene {
             output_rect: DeviceIntRect::zero(),
             background_color: None,
             prim_store: PrimitiveStore::new(&PrimitiveStoreStats::empty()),
-            clip_store: ClipStore::new(),
+            clip_store: ClipStore::new(&ClipStoreStats::empty()),
             spatial_tree: SpatialTree::new(),
             hit_testing_scene: Arc::new(HitTestingScene::new(&HitTestingSceneStats::empty())),
             tile_cache_config: TileCacheConfig::new(0),
@@ -319,6 +319,7 @@ impl BuiltScene {
         SceneStats {
             prim_store_stats: self.prim_store.get_stats(),
             hit_test_stats: self.hit_testing_scene.get_stats(),
+            clip_store_stats: self.clip_store.get_stats(),
         }
     }
 
@@ -337,6 +338,7 @@ impl BuiltScene {
 pub struct SceneStats {
     pub prim_store_stats: PrimitiveStoreStats,
     pub hit_test_stats: HitTestingSceneStats,
+    pub clip_store_stats: ClipStoreStats,
 }
 
 impl SceneStats {
@@ -344,6 +346,7 @@ impl SceneStats {
         SceneStats {
             prim_store_stats: PrimitiveStoreStats::empty(),
             hit_test_stats: HitTestingSceneStats::empty(),
+            clip_store_stats: ClipStoreStats::empty(),
         }
     }
 }

gfx/wr/webrender/src/scene_building.rs

@@ -549,7 +549,7 @@ impl<'a> SceneBuilder<'a> {
             containing_block_stack: Vec::new(),
             raster_space_stack: vec![RasterSpace::Screen],
             prim_store: PrimitiveStore::new(&stats.prim_store_stats),
-            clip_store: ClipStore::new(),
+            clip_store: ClipStore::new(&stats.clip_store_stats),
             interners,
             rf_mapper: ReferenceFrameMapper::new(),
             external_scroll_mapper: ScrollOffsetMapper::new(),