Bug 1621390 - Optimize backdrop primitives by collapsing to clear col…

…or. r=lsalzman,Bert

Differential Revision: https://phabricator.services.mozilla.com/D66362

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/6199f7b91e8bde7b7965585842d7a72e2e406e3e

webrender/src/batch.rs

@@ -19,7 +19,8 @@ use crate::internal_types::{FastHashMap, SavedTargetIndex, Swizzle, TextureSourc
 use crate::picture::{Picture3DContext, PictureCompositeMode, PicturePrimitive};
 use crate::prim_store::{DeferredResolve, EdgeAaSegmentMask, PrimitiveInstanceKind, PrimitiveVisibilityIndex, PrimitiveVisibilityMask};
 use crate::prim_store::{VisibleGradientTile, PrimitiveInstance, PrimitiveOpacity, SegmentInstanceIndex};
-use crate::prim_store::{BrushSegment, ClipMaskKind, ClipTaskIndex, VECS_PER_SEGMENT, SpaceMapper};
+use crate::prim_store::{BrushSegment, ClipMaskKind, ClipTaskIndex, PrimitiveVisibilityFlags};
+use crate::prim_store::{VECS_PER_SEGMENT, SpaceMapper};
 use crate::prim_store::image::ImageSource;
 use crate::render_target::RenderTargetContext;
 use crate::render_task_graph::{RenderTaskId, RenderTaskGraph};
@@ -193,6 +194,16 @@ impl AlphaBatchList {
         }
     }
 
+    /// Clear all current batches in this list. This is typically used
+    /// when a primitive is encountered that occludes all previous
+    /// content in this batch list.
+    fn clear(&mut self) {
+        self.current_batch_index = usize::MAX;
+        self.current_z_id = ZBufferId::invalid();
+        self.batches.clear();
+        self.item_rects.clear();
+    }
+
     pub fn set_params_and_get_batch(
         &mut self,
         key: BatchKey,
@@ -290,6 +301,14 @@ impl OpaqueBatchList {
         }
     }
 
+    /// Clear all current batches in this list. This is typically used
+    /// when a primitive is encountered that occludes all previous
+    /// content in this batch list.
+    fn clear(&mut self) {
+        self.current_batch_index = usize::MAX;
+        self.batches.clear();
+    }
+
     pub fn set_params_and_get_batch(
         &mut self,
         key: BatchKey,
@@ -502,6 +521,14 @@ impl AlphaBatchBuilder {
         }
     }
 
+    /// Clear all current batches in this builder. This is typically used
+    /// when a primitive is encountered that occludes all previous
+    /// content in this batch list.
+    fn clear(&mut self) {
+        self.alpha_batch_list.clear();
+        self.opaque_batch_list.clear();
+    }
+
     pub fn build(
         mut self,
         batch_containers: &mut Vec<AlphaBatchContainer>,
@@ -653,6 +680,14 @@ impl BatchBuilder {
         }
     }
 
+    /// Clear all current batchers. This is typically used when a primitive
+    /// is encountered that occludes all previous content in this batch list.
+    fn clear_batches(&mut self) {
+        for batcher in &mut self.batchers {
+            batcher.clear();
+        }
+    }
+
     /// Add a picture to a given batch builder.
     pub fn add_pic_to_batch(
         &mut self,
@@ -731,6 +766,16 @@ impl BatchBuilder {
         let prim_info = &ctx.scratch.prim_info[prim_instance.visibility_info.0 as usize];
         let bounding_rect = &prim_info.clip_chain.pic_clip_rect;
 
+        // If this primitive is a backdrop, that means that it is known to cover
+        // the entire picture cache background. In that case, the renderer will
+        // use the backdrop color as a clear color, and so we can drop this
+        // primitive and any prior primitives from the batch lists for this
+        // picture cache slice.
+        if prim_info.flags.contains(PrimitiveVisibilityFlags::IS_BACKDROP) {
+            self.clear_batches();
+            return;
+        }
+
         let z_id = z_generator.next();
 
         let prim_common_data = &ctx.data_stores.as_common_data(&prim_instance);

webrender/src/frame_builder.rs

@@ -15,7 +15,7 @@ use crate::gpu_types::TransformData;
 use crate::internal_types::{FastHashMap, PlaneSplitter, SavedTargetIndex};
 use crate::picture::{PictureUpdateState, SurfaceInfo, ROOT_SURFACE_INDEX, SurfaceIndex, RecordedDirtyRegion};
 use crate::picture::{RetainedTiles, TileCacheInstance, DirtyRegion, SurfaceRenderTasks, SubpixelMode};
-use crate::picture::{TileCacheLogger};
+use crate::picture::{BackdropKind, TileCacheLogger};
 use crate::prim_store::{SpaceMapper, PictureIndex, PrimitiveDebugId, PrimitiveScratchBuffer};
 use crate::prim_store::{DeferredResolve, PrimitiveVisibilityMask};
 use crate::profiler::{FrameProfileCounters, TextureCacheProfileCounters, ResourceProfileCounters};
@@ -883,14 +883,19 @@ pub fn build_render_pass(
                     Some(color) => color.a >= 1.0,
                     None => false,
                 };
-                // TODO(gw): Once we have multiple slices enabled, take advantage of
-                //           option to skip clears if the slice is opaque.
-                let clear_color = if forced_opaque {
+                let mut clear_color = if forced_opaque {
                     Some(ColorF::WHITE)
                 } else {
                     Some(ColorF::TRANSPARENT)
                 };
 
+                // If this picture cache has a valid color backdrop, we will use
+                // that as the clear color, skipping the draw of the backdrop
+                // primitive (and anything prior to it) during batching.
+                if let Some(BackdropKind::Color { color }) = tile_cache.backdrop.kind {
+                    clear_color = Some(color);
+                }
+
                 // Create an alpha batcher for each of the tasks of this picture.
                 let mut batchers = Vec::new();
                 for task_id in &task_ids {