Bug 1530208. Fix isEqualNode to not do a bunch of string-copying. r=mccr8

Differential Revision: https://phabricator.services.mozilla.com/D21824

--HG--
extra : moz-landing-system : lando

dom/base/CharacterData.h

@@ -208,6 +208,14 @@ class CharacterData : public nsIContent {
   NS_DECL_CYCLE_COLLECTION_SKIPPABLE_SCRIPT_HOLDER_CLASS_INHERITED(
       CharacterData, nsIContent)
 
+  /**
+   * Compare two CharacterData nodes for text equality.
+   */
+  MOZ_MUST_USE
+  bool TextEquals(const CharacterData* aOther) const {
+    return mText.TextEquals(aOther->mText);
+  }
+
  protected:
   virtual ~CharacterData();
 

dom/base/nsINode.cpp

@@ -816,6 +816,12 @@ bool nsINode::IsEqualNode(nsINode* aOther) {
     return false;
   }
 
+  // Might as well do a quick check to avoid walking our kids if we're
+  // obviously the same.
+  if (aOther == this) {
+    return true;
+  }
+
   nsAutoString string1, string2;
 
   nsINode* node1 = this;
@@ -868,12 +874,10 @@ bool nsINode::IsEqualNode(nsINode* aOther) {
       case PROCESSING_INSTRUCTION_NODE: {
         MOZ_ASSERT(node1->IsCharacterData());
         MOZ_ASSERT(node2->IsCharacterData());
-        string1.Truncate();
-        static_cast<CharacterData*>(node1)->AppendTextTo(string1);
-        string2.Truncate();
-        static_cast<CharacterData*>(node2)->AppendTextTo(string2);
+        auto* data1 = static_cast<CharacterData*>(node1);
+        auto* data2 = static_cast<CharacterData*>(node2);
 
-        if (!string1.Equals(string2)) {
+        if (!data1->TextEquals(data2)) {
           return false;
         }
 

dom/base/nsTextFragment.cpp

@@ -490,3 +490,41 @@ void nsTextFragment::UpdateBidiFlag(const char16_t* aBuffer, uint32_t aLength) {
     }
   }
 }
+
+bool nsTextFragment::TextEquals(const nsTextFragment& aOther) const {
+  if (!Is2b()) {
+    // We're 1-byte.
+    if (!aOther.Is2b()) {
+      nsDependentCSubstring ourStr(Get1b(), GetLength());
+      return ourStr.Equals(
+          nsDependentCSubstring(aOther.Get1b(), aOther.GetLength()));
+    }
+
+    // We're 1-byte, the other thing is 2-byte.  Instead of implementing a
+    // separate codepath for this, just use our code below.
+    return aOther.TextEquals(*this);
+  }
+
+  nsDependentSubstring ourStr(Get2b(), GetLength());
+  if (aOther.Is2b()) {
+    return ourStr.Equals(
+        nsDependentSubstring(aOther.Get2b(), aOther.GetLength()));
+  }
+
+  // We can't use EqualsASCII here, because the other string might not
+  // actually be ASCII.  Just roll our own compare; do it in the simple way.
+  // Bug 1532356 tracks not having to roll our own.
+  if (GetLength() != aOther.GetLength()) {
+    return false;
+  }
+
+  const char16_t* ourChars = Get2b();
+  const char* otherChars = aOther.Get1b();
+  for (uint32_t i = 0; i < GetLength(); ++i) {
+    if (ourChars[i] != static_cast<char16_t>(otherChars[i])) {
+      return false;
+    }
+  }
+
+  return true;
+}

dom/base/nsTextFragment.h

@@ -236,6 +236,12 @@ class nsTextFragment final {
 
   size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
 
+  /**
+   * Check whether the text in this fragment is the same as the text in the
+   * other fragment.
+   */
+  MOZ_MUST_USE bool TextEquals(const nsTextFragment& aOther) const;
+
  private:
   void ReleaseText();