Bug 861449 - Incremental css::Rule destroyer. r=dbaron

Destroying large arrays of css::Rules during page teardown can take
multiple milliseconds in incremental CC. To reduce CC pauses, this
patch introduces a new class nsIncrementalClearCOMRuleArray that uses
the deferred finalizer to incrementally destroy these rule arrays.

layout/style/CSSStyleSheet.h

@@ -10,6 +10,7 @@
 #define mozilla_CSSStyleSheet_h
 
 #include "mozilla/Attributes.h"
+#include "mozilla/IncrementalClearCOMRuleArray.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/dom/Element.h"
 
@@ -19,7 +20,6 @@
 #include "nsIStyleSheet.h"
 #include "nsIDOMCSSStyleSheet.h"
 #include "nsICSSLoaderObserver.h"
-#include "nsCOMArray.h"
 #include "nsTArrayForwardDeclare.h"
 #include "nsString.h"
 #include "mozilla/CORSMode.h"
@@ -84,7 +84,7 @@ private:
   nsCOMPtr<nsIURI>       mOriginalSheetURI;  // for GetHref.  Can be null.
   nsCOMPtr<nsIURI>       mBaseURI; // for resolving relative URIs
   nsCOMPtr<nsIPrincipal> mPrincipal;
-  nsCOMArray<css::Rule>  mOrderedRules;
+  IncrementalClearCOMRuleArray mOrderedRules;
   nsAutoPtr<nsXMLNameSpaceMap> mNameSpaceMap;
   // Linked list of child sheets.  This is al fundamentally broken, because
   // each of the child sheets has a unique parent... We can only hope (and

layout/style/GroupRule.h

@@ -12,9 +12,9 @@
 #define mozilla_css_GroupRule_h__
 
 #include "mozilla/Attributes.h"
+#include "mozilla/IncrementalClearCOMRuleArray.h"
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/css/Rule.h"
-#include "nsCOMArray.h"
 #include "nsAutoPtr.h"
 #include "nsCycleCollectionParticipant.h"
 
@@ -57,7 +57,7 @@ public:
   int32_t StyleRuleCount() const { return mRules.Count(); }
   Rule* GetStyleRuleAt(int32_t aIndex) const;
 
-  typedef nsCOMArray<Rule>::nsCOMArrayEnumFunc RuleEnumFunc;
+  typedef IncrementalClearCOMRuleArray::nsCOMArrayEnumFunc RuleEnumFunc;
   bool EnumerateRulesForwards(RuleEnumFunc aFunc, void * aData) const;
 
   /*
@@ -80,7 +80,7 @@ public:
   CloneRuleInto(Rule* aRule, void* aArray)
   {
     nsRefPtr<Rule> clone = aRule->Clone();
-    static_cast<nsCOMArray<Rule>*>(aArray)->AppendObject(clone);
+    static_cast<IncrementalClearCOMRuleArray*>(aArray)->AppendObject(clone);
     return true;
   }
 
@@ -95,7 +95,7 @@ protected:
                       uint32_t* _retval);
   nsresult DeleteRule(uint32_t aIndex);
 
-  nsCOMArray<Rule> mRules;
+  IncrementalClearCOMRuleArray mRules;
   nsRefPtr<GroupRuleRuleList> mRuleCollection; // lazily constructed
 };
 

layout/style/IncrementalClearCOMRuleArray.cpp

@@ -0,0 +1,80 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/IncrementalClearCOMRuleArray.h"
+
+#include "nsCycleCollector.h"
+#include "mozilla/DeferredFinalize.h"
+#include "nsTArray.h"
+#include "nsCCUncollectableMarker.h"
+
+using namespace mozilla;
+
+typedef nsCOMArray<css::Rule> RuleArray;
+typedef nsTArray<RuleArray> RuleArrayArray;
+
+
+// These methods are based on those in DeferredFinalizerImpl.
+
+static void*
+AppendRulesArrayPointer(void* aData, void* aObject)
+{
+  RuleArrayArray* rulesArray = static_cast<RuleArrayArray*>(aData);
+  RuleArray* oldRules = static_cast<RuleArray*>(aObject);
+
+  if (!rulesArray) {
+    rulesArray = new RuleArrayArray();
+  }
+
+  RuleArray* newRules = rulesArray->AppendElement();
+  newRules->SwapElements(*oldRules);
+
+  return rulesArray;
+}
+
+// Remove up to |aSliceBudget| css::Rules from the arrays, starting at
+// the end of the last array.
+static bool
+DeferredFinalizeRulesArray(uint32_t aSliceBudget, void* aData)
+{
+  MOZ_ASSERT(aSliceBudget > 0, "nonsensical/useless call with aSliceBudget == 0");
+  RuleArrayArray* rulesArray = static_cast<RuleArrayArray*>(aData);
+
+  size_t newOuterLen = rulesArray->Length();
+
+  while (aSliceBudget > 0 && newOuterLen > 0) {
+    RuleArray& lastArray = rulesArray->ElementAt(newOuterLen - 1);
+    uint32_t innerLen = lastArray.Length();
+    uint32_t currSlice = std::min(innerLen, aSliceBudget);
+    uint32_t newInnerLen = innerLen - currSlice;
+    lastArray.TruncateLength(newInnerLen);
+    aSliceBudget -= currSlice;
+    if (newInnerLen == 0) {
+      newOuterLen -= 1;
+    }
+  }
+
+  rulesArray->TruncateLength(newOuterLen);
+
+  if (newOuterLen == 0) {
+    delete rulesArray;
+    return true;
+  }
+  return false;
+}
+
+void
+IncrementalClearCOMRuleArray::Clear()
+{
+  // Destroy the array incrementally if it is long and we
+  // haven't started shutting down.
+  if (Length() > 10 && nsCCUncollectableMarker::sGeneration) {
+    DeferredFinalize(AppendRulesArrayPointer, DeferredFinalizeRulesArray, this);
+  } else {
+    nsCOMArray<css::Rule>::Clear();
+  }
+  MOZ_ASSERT(Length() == 0);
+}

layout/style/IncrementalClearCOMRuleArray.h

@@ -0,0 +1,28 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_IncrementalClearCOMRuleArray_h
+#define mozilla_IncrementalClearCOMRuleArray_h
+
+#include "nsCOMArray.h"
+
+namespace mozilla {
+namespace css {
+class Rule;
+} // namespace css
+
+class IncrementalClearCOMRuleArray : public nsCOMArray<css::Rule>
+{
+public:
+  IncrementalClearCOMRuleArray() {}
+  ~IncrementalClearCOMRuleArray() { Clear(); }
+
+  void Clear();
+};
+
+} // namespace mozilla
+
+#endif /* !defined(mozilla_IncrementalClearCOMRuleArray_h) */

layout/style/moz.build

@@ -84,6 +84,7 @@ EXPORTS.mozilla += [
     'CSSVariableDeclarations.h',
     'CSSVariableResolver.h',
     'CSSVariableValues.h',
+    'IncrementalClearCOMRuleArray.h',
     'StyleAnimationValue.h',
 ]
 
@@ -125,6 +126,7 @@ UNIFIED_SOURCES += [
     'FontFace.cpp',
     'FontFaceSetIterator.cpp',
     'ImageLoader.cpp',
+    'IncrementalClearCOMRuleArray.cpp',
     'Loader.cpp',
     'MediaQueryList.cpp',
     'nsAnimationManager.cpp',