DisplayList savelayer opacity peephole optimization (flutter#30957)

Author: flar

display_list/display_list.cc

@@ -11,6 +11,10 @@
 
 namespace flutter {
 
+const SaveLayerOptions SaveLayerOptions::kNoAttributes = SaveLayerOptions();
+const SaveLayerOptions SaveLayerOptions::kWithAttributes =
+    kNoAttributes.with_renders_with_attributes();
+
 const SkSamplingOptions DisplayList::NearestSampling =
     SkSamplingOptions(SkFilterMode::kNearest, SkMipmapMode::kNone);
 const SkSamplingOptions DisplayList::LinearSampling =

display_list/display_list.h

@@ -150,6 +150,52 @@ enum class DisplayListOpType { FOR_EACH_DISPLAY_LIST_OP(DL_OP_TO_ENUM_VALUE) };
 class Dispatcher;
 class DisplayListBuilder;
 
+class SaveLayerOptions {
+ public:
+  static const SaveLayerOptions kWithAttributes;
+  static const SaveLayerOptions kNoAttributes;
+
+  SaveLayerOptions() : flags_(0) {}
+  SaveLayerOptions(const SaveLayerOptions& options) : flags_(options.flags_) {}
+  SaveLayerOptions(const SaveLayerOptions* options) : flags_(options->flags_) {}
+
+  SaveLayerOptions without_optimizations() const {
+    SaveLayerOptions options;
+    options.fRendersWithAttributes = fRendersWithAttributes;
+    return options;
+  }
+
+  bool renders_with_attributes() const { return fRendersWithAttributes; }
+  SaveLayerOptions with_renders_with_attributes() const {
+    SaveLayerOptions options(this);
+    options.fRendersWithAttributes = true;
+    return options;
+  }
+
+  bool can_distribute_opacity() const { return fCanDistributeOpacity; }
+  SaveLayerOptions with_can_distribute_opacity() const {
+    SaveLayerOptions options(this);
+    options.fCanDistributeOpacity = true;
+    return options;
+  }
+
+  bool operator==(const SaveLayerOptions& other) const {
+    return flags_ == other.flags_;
+  }
+  bool operator!=(const SaveLayerOptions& other) const {
+    return flags_ != other.flags_;
+  }
+
+ private:
+  union {
+    struct {
+      unsigned fRendersWithAttributes : 1;
+      unsigned fCanDistributeOpacity : 1;
+    };
+    uint32_t flags_;
+  };
+};
+
 // The base class that contains a sequence of rendering operations
 // for dispatch to a Dispatcher. These objects must be instantiated
 // through an instance of DisplayListBuilder::build().

display_list/display_list_builder.cc

@@ -237,7 +237,25 @@ void DisplayListBuilder::restore() {
     layer_stack_.pop_back();
     current_layer_ = &layer_stack_.back();
     Push<RestoreOp>(0, 1);
-    if (!layer_info.has_layer) {
+    if (layer_info.has_layer) {
+      if (layer_info.is_group_opacity_compatible()) {
+        // We are now going to go back and modify the matching saveLayer
+        // call to add the option indicating it can distribute an opacity
+        // value to its children.
+        //
+        // Note that this operation cannot and does not change the size
+        // or structure of the SaveLayerOp record. It only sets an option
+        // flag on an existing field.
+        //
+        // Note that these kinds of modification operations on data already
+        // in the DisplayList are only allowed *during* the build phase.
+        // Once built, the DisplayList records must remain read only to
+        // ensure consistency of rendering and |Equals()| behavior.
+        SaveLayerOp* op = reinterpret_cast<SaveLayerOp*>(
+            storage_.get() + layer_info.save_layer_offset);
+        op->options = op->options.with_can_distribute_opacity();
+      }
+    } else {
       // For regular save() ops there was no protecting layer so we have to
       // accumulate the values into the enclosing layer.
       if (layer_info.cannot_inherit_opacity) {
@@ -249,13 +267,24 @@ void DisplayListBuilder::restore() {
   }
 }
 void DisplayListBuilder::saveLayer(const SkRect* bounds,
-                                   bool restore_with_paint) {
+                                   const SaveLayerOptions in_options) {
+  SaveLayerOptions options = in_options.without_optimizations();
+  size_t save_layer_offset = used_;
   bounds  //
-      ? Push<SaveLayerBoundsOp>(0, 1, *bounds, restore_with_paint)
-      : Push<SaveLayerOp>(0, 1, restore_with_paint);
-  CheckLayerOpacityCompatibility(restore_with_paint);
-  layer_stack_.emplace_back(true);
+      ? Push<SaveLayerBoundsOp>(0, 1, *bounds, options)
+      : Push<SaveLayerOp>(0, 1, options);
+  CheckLayerOpacityCompatibility(options.renders_with_attributes());
+  layer_stack_.emplace_back(save_layer_offset, true);
   current_layer_ = &layer_stack_.back();
+  if (options.renders_with_attributes()) {
+    // |current_opacity_compatibility_| does not take an ImageFilter into
+    // account because an individual primitive with an ImageFilter can apply
+    // opacity on top of it. But, if the layer is applying the ImageFilter
+    // then it cannot pass the opacity on.
+    if (!current_opacity_compatibility_ || current_image_filter_ != nullptr) {
+      UpdateLayerOpacityCompatibility(false);
+    }
+  }
 }
 
 void DisplayListBuilder::translate(SkScalar tx, SkScalar ty) {
@@ -455,6 +484,8 @@ void DisplayListBuilder::drawVertices(const sk_sp<SkVertices> vertices,
   Push<DrawVerticesOp>(0, 1, std::move(vertices), mode);
   // DrawVertices applies its colors to the paint so we have no way
   // of controlling opacity using the current paint attributes.
+  // Although, examination of the |mode| might find some predictable
+  // cases.
   UpdateLayerOpacityCompatibility(false);
 }
 

display_list/display_list_builder.h

@@ -153,7 +153,17 @@ class DisplayListBuilder final : public virtual Dispatcher,
   sk_sp<SkImageFilter> getImageFilter() const { return current_image_filter_; }
 
   void save() override;
-  void saveLayer(const SkRect* bounds, bool restore_with_paint) override;
+  // Only the |renders_with_attributes()| option will be accepted here. Any
+  // other flags will be ignored and calculated anew as the DisplayList is
+  // built. Alternatively, use the |saveLayer(SkRect, bool)| method.
+  void saveLayer(const SkRect* bounds, const SaveLayerOptions options) override;
+  // Convenience method with just a boolean to indicate whether the saveLayer
+  // should apply the rendering attributes.
+  void saveLayer(const SkRect* bounds, bool renders_with_attributes) {
+    saveLayer(bounds, renders_with_attributes
+                          ? SaveLayerOptions::kWithAttributes
+                          : SaveLayerOptions::kNoAttributes);
+  }
   void restore() override;
   int getSaveCount() { return layer_stack_.size(); }
 
@@ -271,11 +281,20 @@ class DisplayListBuilder final : public virtual Dispatcher,
   }
 
   struct LayerInfo {
-    LayerInfo(bool has_layer = false)
-        : has_layer(has_layer),
+    LayerInfo(size_t save_layer_offset = 0, bool has_layer = false)
+        : save_layer_offset(save_layer_offset),
+          has_layer(has_layer),
           cannot_inherit_opacity(false),
           has_compatible_op(false) {}
 
+    // The offset into the memory buffer where the saveLayer DLOp record
+    // for this saveLayer() call is placed. This may be needed if the
+    // eventual restore() call has discovered important information about
+    // the records inside the saveLayer that may impact how the saveLayer
+    // is handled (e.g., |cannot_inherit_opacity| == false).
+    // This offset is only valid if |has_layer| is true.
+    size_t save_layer_offset;
+
     bool has_layer;
     bool cannot_inherit_opacity;
     bool has_compatible_op;

display_list/display_list_canvas_dispatcher.cc

@@ -26,17 +26,42 @@ const SkPaint* DisplayListCanvasDispatcher::safe_paint(bool use_attributes) {
 
 void DisplayListCanvasDispatcher::save() {
   canvas_->save();
-  save_opacity(false);
+  // save has no impact on attributes, but it needs to register a record
+  // on the restore stack so that the eventual call to restore() will
+  // know what to do at that time. We could annotate the restore record
+  // with a flag that the record came from a save call, but it is simpler
+  // to just pass in the current opacity value as the value to be used by
+  // the children and let the utility calls notice that it didn't change.
+  save_opacity(opacity());
 }
 void DisplayListCanvasDispatcher::restore() {
   canvas_->restore();
   restore_opacity();
 }
 void DisplayListCanvasDispatcher::saveLayer(const SkRect* bounds,
-                                            bool restore_with_paint) {
-  TRACE_EVENT0("flutter", "Canvas::saveLayer");
-  canvas_->saveLayer(bounds, safe_paint(restore_with_paint));
-  save_opacity(true);
+                                            const SaveLayerOptions options) {
+  if (bounds == nullptr && options.can_distribute_opacity()) {
+    // We know that:
+    // - no bounds is needed for clipping here
+    // - the current attributes only have an alpha
+    // - the children are compatible with individually rendering with
+    //   an inherited opacity
+    // Therefore we can just use a save instead of a saveLayer and pass the
+    // intended opacity to the children.
+    canvas_->save();
+    // If the saveLayer does not use attributes, the children should continue
+    // to render with the inherited opacity unmodified. If attributes are to
+    // be applied, the children should render with the combination of the
+    // inherited opacity combined with the alpha from the current color.
+    save_opacity(options.renders_with_attributes() ? combined_opacity()
+                                                   : opacity());
+  } else {
+    TRACE_EVENT0("flutter", "Canvas::saveLayer");
+    canvas_->saveLayer(bounds, safe_paint(options.renders_with_attributes()));
+    // saveLayer will apply the current opacity on behalf of the children
+    // so they will inherit an opaque opacity.
+    save_opacity(SK_Scalar1);
+  }
 }
 
 void DisplayListCanvasDispatcher::translate(SkScalar tx, SkScalar ty) {

display_list/display_list_canvas_dispatcher.h

@@ -29,7 +29,7 @@ class DisplayListCanvasDispatcher : public virtual Dispatcher,
 
   void save() override;
   void restore() override;
-  void saveLayer(const SkRect* bounds, bool restore_with_paint) override;
+  void saveLayer(const SkRect* bounds, const SaveLayerOptions options) override;
 
   void translate(SkScalar tx, SkScalar ty) override;
   void scale(SkScalar sx, SkScalar sy) override;