Move second internal recursion to a precomputation in layer_tree_host_common.cc

cc/layer_tree_host_common.cc

Index: cc/layer_tree_host_common.cc
diff --git a/cc/layer_tree_host_common.cc b/cc/layer_tree_host_common.cc
index 8aff2029e27fd339917cf9b5afdec7beac69b28e..5d0b2d63619469c837d43c8dbafb762936ae2ff2 100644
--- a/cc/layer_tree_host_common.cc
+++ b/cc/layer_tree_host_common.cc
@@ -249,21 +249,26 @@ static bool subtreeShouldRenderToSeparateSurface(LayerType* layer, bool axisAlig
     if (!layer->filters().isEmpty() || !layer->backgroundFilters().isEmpty() || layer->filter())
         return true;
 
-    // Cache this value, because otherwise it walks the entire subtree several times.
-    int descendantsDrawContent = layer->descendantsDrawContent();
+    // Caching this value avoids unnecessary virtual function overheads.
+    int numDescendantsThatDrawContent = layer->numDescendantsThatDrawContent();
 
     // If the layer flattens its subtree (i.e. the layer doesn't preserve-3d), but it is
     // treated as a 3D object by its parent (i.e. parent does preserve-3d).
-    if (layerIsInExisting3DRenderingContext(layer) && !layer->preserves3D() && descendantsDrawContent > 0)
+    if (layerIsInExisting3DRenderingContext(layer) && !layer->preserves3D() && numDescendantsThatDrawContent > 0)
         return true;
 
     // If the layer clips its descendants but it is not axis-aligned with respect to its parent.
-    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && descendantsDrawContent > 0)
+    if (layerClipsSubtree(layer) && !axisAlignedWithRespectToParent && numDescendantsThatDrawContent > 0)
         return true;
 
-    // If the layer has opacity != 1 and does not have a preserves-3d transform style.
-    if (layer->opacity() != 1 && !layer->preserves3D() && descendantsDrawContent > 0
-        && (layer->drawsContent() || descendantsDrawContent > 1))
+    // If the layer has some translucency and does not have a preserves-3d transform style.
+    // This condition only needs a render surface if two or more layers in the
+    // subtree overlap. But checking layer overlaps is unnecesarily costly so
+    // instead we conservatively create a surface whenever at least two layers
+    // draw content for this subtree.
+    bool atLeastTwoLayersInSubtreeDrawContent = numDescendantsThatDrawContent > 0

[danakj, 2012/12/03 18:33:41]

i like this var name

+            && (layer->drawsContent() || numDescendantsThatDrawContent > 1);

[danakj, 2012/12/03 18:33:41]

the "if" seems to have disappeared entirely.

+    if (layer->opacity() != 1 && !layer->preserves3D() && atLeastTwoLayersInSubtreeDrawContent)
         return true;
 
     return false;
@@ -388,6 +393,24 @@ static inline void updateLayerContentsScale(Layer* layer, const gfx::Transform&
         replicaMaskLayer->setContentsScale(contentsScale);
 }
 
+// Recursively walks the layer tree to compute any information that is needed
+// before doing the main recursion.
+template<typename LayerType>
+static void preCalculateMetaInformation(LayerType* layer)
+{
+    int numDescendantsThatDrawContent = 0;
+
+    for (size_t i = 0; i < layer->children().size(); ++i) {
+        LayerType* childLayer = layer->children()[i];
+        preCalculateMetaInformation<LayerType>(childLayer);
+
+        numDescendantsThatDrawContent += childLayer->drawsContent() ? 1 : 0;
+        numDescendantsThatDrawContent += childLayer->numDescendantsThatDrawContent();
+    }
+
+    layer->setNumDescendantsThatDrawContent(numDescendantsThatDrawContent);
+}
+
 // Recursively walks the layer tree starting at the given node and computes all the
 // necessary transformations, clipRects, render surfaces, etc.
 template<typename LayerType, typename LayerList, typename RenderSurfaceType, typename LayerSorter>
@@ -837,6 +860,7 @@ void LayerTreeHostCommon::calculateDrawTransforms(Layer* rootLayer, const gfx::S
     // This function should have received a root layer.
     DCHECK(isRootLayer(rootLayer));
 
+    cc::preCalculateMetaInformation<Layer>(rootLayer);
     cc::calculateDrawTransformsInternal<Layer, std::vector<scoped_refptr<Layer> >, RenderSurface, void>(
         rootLayer, deviceScaleTransform, identityMatrix, identityMatrix,
         deviceViewportRect, subtreeShouldBeClipped, 0, renderSurfaceLayerList,
@@ -864,6 +888,7 @@ void LayerTreeHostCommon::calculateDrawTransforms(LayerImpl* rootLayer, const gf
     // This function should have received a root layer.
     DCHECK(isRootLayer(rootLayer));
 
+    cc::preCalculateMetaInformation<LayerImpl>(rootLayer);
     cc::calculateDrawTransformsInternal<LayerImpl, std::vector<LayerImpl*>, RenderSurfaceImpl, LayerSorter>(
         rootLayer, deviceScaleTransform, identityMatrix, identityMatrix,
         deviceViewportRect, subtreeShouldBeClipped, 0, renderSurfaceLayerList,