CSS: Avoid invalidating style when only descendants need updating

Source/core/css/invalidation/InvalidationSet.cpp

 /*
  * Copyright (C) 2014 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 34 matching lines @@
 #define TRACE_STYLE_INVALIDATOR_INVALIDATION_SELECTORPART_IF_ENABLED(element, reason, invalidationSet, singleSelectorPart) \
     if (UNLIKELY(*s_tracingEnabled)) \
         TRACE_STYLE_INVALIDATOR_INVALIDATION_SELECTORPART(element, reason, invalidationSet, singleSelectorPart);
 
 void InvalidationSet::cacheTracingFlag()
 {
     s_tracingEnabled = TRACE_EVENT_API_GET_CATEGORY_ENABLED(TRACE_DISABLED_BY_DEFAULT("devtools.timeline.invalidationTracking"));
 }
 
 InvalidationSet::InvalidationSet()
-    : m_allDescendantsMightBeInvalid(false)
+    : m_appliesDirectly(false)
+    , m_allDescendantsMightBeInvalid(false)
     , m_customPseudoInvalid(false)
     , m_treeBoundaryCrossing(false)
     , m_insertionPointCrossing(false)
 {
 }
 
 bool InvalidationSet::invalidatesElement(Element& element) const
 {
     if (m_allDescendantsMightBeInvalid)
         return true;
@@ skipping 25 matching lines @@
                 return true;
             }
         }
     }
 
     return false;
 }
 
 void InvalidationSet::combine(const InvalidationSet& other)
 {
+    if (other.appliesDirectly())
+        setAppliesDirectly();

[rune, 2015/09/16 08:44:32]

The existing code did an early return on wholeSubtreeInvalid() before copying
any other bits. Did you put this before on purpose? If not, you can move it down
where the other bits are copied.

[Eric Willigers, 2015/09/16 13:32:08]

Moved.  The original positioning, and the original naming, was motivated by
sibling invalidation: I was hoping to avoid allocating a sibling descendant
invalidation set if only the sibling itself is being invalidated. Not important:
it is clearer to instead have a sibling descendant invalidation set that is
empty and has 'invalidatesSelf'.

+
     // No longer bother combining data structures, since the whole subtree is deemed invalid.
     if (wholeSubtreeInvalid())
         return;
 
     if (other.wholeSubtreeInvalid()) {
         setWholeSubtreeInvalid();
         return;
     }
 
     if (other.customPseudoInvalid())
@@ skipping 158 matching lines @@
 {
     RefPtr<TracedValue> value = TracedValue::create();
     value->beginArray("InvalidationSet");
     toTracedValue(value.get());
     value->endArray();
     fprintf(stderr, "%s\n", value->asTraceFormat().ascii().data());
 }
 #endif // NDEBUG
 
 } // namespace blink