Avoid unnecessary updateActiveStyle comparing shadow styles.

third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2012 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:
  * 1.  Redistributions of source code must retain the above copyright
@@ skipping 280 matching lines @@
   if (!m_treeBoundaryCrossingRuleSet) {
     m_treeBoundaryCrossingRuleSet = new CSSStyleSheetRuleSubSet();
     treeScope().document().styleEngine().addTreeBoundaryCrossingScope(
         treeScope());
   }
 
   m_treeBoundaryCrossingRuleSet->push_back(
       RuleSubSet::create(parentStyleSheet, sheetIndex, ruleSetForScope));
 }
 
+bool ScopedStyleResolver::haveSameStyles(const ScopedStyleResolver* first,
+                                         const ScopedStyleResolver* second) {
+  size_t firstCount = first ? first->m_authorStyleSheets.size() : 0;
+  size_t secondCount = second ? second->m_authorStyleSheets.size() : 0;
+  if (firstCount != secondCount)
+    return false;
+  while (firstCount--) {
+    if (first->m_authorStyleSheets[firstCount]->contents() !=

[sashab, 2017/01/03 22:35:19]

I know this is what the old code did, but is there a builtin for this? If
m_authorStyleSheets return iterators, maybe there's a C++ builtin we can use.

Just feels like a lot of code for a pretty standard comparator function. Also I
think we prefer iterator for-loops to while loops.

[rune, 2017/01/03 23:35:42]

I don't think we can compare these vectors with operator== because we need to
compare sheet->contents(), not sheet. To make this code simpler to read, I guess
you'd need a vector comparator which takes a custom comparator for the elements.
I don't know if there's such a thing.

Here's an alternative:

bool ScopedStyleResolver::haveSameStyles(const ScopedStyleResolver* first,
                                         const ScopedStyleResolver* second) {
  if (!first)
    return !second || second->m_authorStyleSheets.isEmpty();
  if (!second)
    return first->m_authorStyleSheets.isEmpty();
  if (first->m_authorStyleSheets.size() != second->m_authorStyleSheets.size())
    return false;
  auto it = second->m_authorStyleSheets.begin();
  for (const auto& sheet : first->m_authorStyleSheets) {
    if (sheet->contents() != (*it++)->contents())
      return false;
  }
  return true;
}


We could skip the no resolver, no active sheet equality to make the code simpler
without losing much in practice.

[sashab, 2017/01/04 00:39:33]

Eh, that's not much nicer. Ok, I'm happy with you leaving this as they are haha,
thanks for investigating though.

+        second->m_authorStyleSheets[firstCount]->contents())
+      return false;
+  }
+  return true;
+}
+
 DEFINE_TRACE(ScopedStyleResolver::RuleSubSet) {
   visitor->trace(m_parentStyleSheet);
   visitor->trace(m_ruleSet);
 }
 
 }  // namespace blink