Suppress layer creation for descendants of GPU-rasterized layers

Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp

Index: Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
diff --git a/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp b/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
index cb11a721f59b49073306b59b5ea4be623bd6f3a5..dcd766e1a0e933a1b171661ad264c80f686d6518 100644
--- a/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
+++ b/Source/core/rendering/compositing/CompositingRequirementsUpdater.cpp
@@ -163,6 +163,7 @@ public:
         , m_subtreeIsCompositing(false)
         , m_hasUnisolatedCompositedBlendingDescendant(false)
         , m_testingOverlap(testOverlap)
+        , m_suppressLayerCreation(false)
 #ifndef NDEBUG
         , m_depth(0)
 #endif
@@ -175,6 +176,7 @@ public:
         , m_subtreeIsCompositing(other.m_subtreeIsCompositing)
         , m_hasUnisolatedCompositedBlendingDescendant(other.m_hasUnisolatedCompositedBlendingDescendant)
         , m_testingOverlap(other.m_testingOverlap)
+        , m_suppressLayerCreation(other.m_suppressLayerCreation)
 #ifndef NDEBUG
         , m_depth(other.m_depth + 1)
 #endif
@@ -186,6 +188,7 @@ public:
     bool m_subtreeIsCompositing;
     bool m_hasUnisolatedCompositedBlendingDescendant;
     bool m_testingOverlap;
+    bool m_suppressLayerCreation;
 #ifndef NDEBUG
     int m_depth;
 #endif
@@ -201,6 +204,11 @@ static bool requiresCompositingOrSquashing(CompositingReasons reasons)
     return reasons != CompositingReasonNone;
 }
 
+static bool shouldMakeDescendantsSuppressCompositedLayerCreation(CompositingReasons reasons)
+{
+    return reasons & CompositingReasonWillChangeGpuRasterizationHint;
+}
+
 static CompositingReasons subtreeReasonsForCompositing(RenderObject* renderer, bool hasCompositedDescendants, bool has3DTransformedDescendants)
 {
     CompositingReasons subtreeReasons = CompositingReasonNone;
@@ -286,12 +294,18 @@ void CompositingRequirementsUpdater::updateRecursive(RenderLayer* ancestorLayer,
 
     layer->stackingNode()->updateLayerListsIfNeeded();
 
+    bool needToRecomputeStyleReasons = layer->suppressingCompositedLayerCreation() != currentRecursionData.m_suppressLayerCreation;
+    layer->setSuppressingCompositedLayerCreation(currentRecursionData.m_suppressLayerCreation);

[abarth-chromium, 2014/04/14 20:29:53]

Why is this a property of the RenderLayer?  It seems more like a temporary
variable used by this algorithm.

[ajuma, 2014/04/15 19:11:34]

As long as we continue to have incremental compositing updates, we need to know
this elsewhere too (in particular, in the latest patch set, this is used in
RenderLayerCompositor::updateDirectCompositingReasons). If we don't take
suppression into account during incremental updates, we'll think (during the
incremental update) that a non-composited layer needs to become composited even
when it doesn't (because its only compositing reasons are being suppressed);
this will cause RenderLayerCompositor::updateLayerCompositingState to make the
wrong decisions.

+
     // Clear the flag
     layer->setHasCompositingDescendant(false);
 
     // Start by assuming this layer will not need to composite.
     CompositingReasons reasonsToComposite = CompositingReasonNone;
 
+    if (needToRecomputeStyleReasons)

[abarth-chromium, 2014/04/14 20:29:53]

There's no such thing as a need to recompute style-determined reasons.  The
style-determined reasons are just that: determined by style.  Maybe it would be
more appropriate to phrase this code in terms of how we interpret those reasons
instead of mutating state that was computed earlier in the pipeline.

[ajuma, 2014/04/15 19:11:34]

Fixed. Instead of mutating style-determined reasons, style reasons are now
stored separately in RenderLayer.

+        compositor->updateStyleDeterminedCompositingReasons(layer);
+
     // First accumulate the straightforward compositing reasons.
     CompositingReasons directReasons = m_compositingReasonFinder.directReasons(layer, m_needsToRecomputeCompositingRequirements);
 
@@ -348,6 +362,7 @@ void CompositingRequirementsUpdater::updateRecursive(RenderLayer* ancestorLayer,
     // ancestor with m_subtreeIsCompositing set to false.
     RecursionData childRecursionData(currentRecursionData);
     childRecursionData.m_subtreeIsCompositing = false;
+    childRecursionData.m_suppressLayerCreation = layer->suppressingCompositedLayerCreation() || shouldMakeDescendantsSuppressCompositedLayerCreation(directReasons);
 
     bool willBeCompositedOrSquashed = compositor->canBeComposited(layer) && requiresCompositingOrSquashing(reasonsToComposite);
     if (willBeCompositedOrSquashed) {