Bug 1843949 - Preallocate attribute array with the known size from parsing. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D183809

dom/base/AttrArray.cpp

@@ -302,7 +302,16 @@ bool AttrArray::GrowBy(uint32_t aGrowSize) {
     capacity = 1u << shift;
   }
 
-  CheckedUint32 sizeInBytes = capacity.value();
+  return GrowTo(capacity.value());
+}
+
+bool AttrArray::GrowTo(uint32_t aCapacity) {
+  uint32_t oldCapacity = mImpl ? mImpl->mCapacity : 0;
+  if (aCapacity <= oldCapacity) {
+    return true;
+  }
+
+  CheckedUint32 sizeInBytes = aCapacity;
   sizeInBytes *= sizeof(InternalAttr);
   if (!sizeInBytes.isValid()) {
     return false;
@@ -314,7 +323,7 @@ bool AttrArray::GrowBy(uint32_t aGrowSize) {
   }
 
   MOZ_ASSERT(sizeInBytes.value() ==
-             Impl::AllocationSizeForAttributes(capacity.value()));
+             Impl::AllocationSizeForAttributes(aCapacity));
 
   const bool needToInitialize = !mImpl;
   Impl* oldImpl = mImpl.release();
@@ -332,7 +341,7 @@ bool AttrArray::GrowBy(uint32_t aGrowSize) {
     mImpl->mAttrCount = 0;
   }
 
-  mImpl->mCapacity = capacity.value();
+  mImpl->mCapacity = aCapacity;
   return true;
 }
 

dom/base/AttrArray.h

@@ -181,9 +181,10 @@ class AttrArray {
   AttrArray(const AttrArray& aOther) = delete;
   AttrArray& operator=(const AttrArray& aOther) = delete;
 
- private:
   bool GrowBy(uint32_t aGrowSize);
+  bool GrowTo(uint32_t aCapacity);
 
+ private:
   // Tries to create an attribute, growing the buffer if needed, with the given
   // name and value.
   //

dom/base/Element.cpp

@@ -2686,6 +2686,10 @@ nsresult Element::SetAttrAndNotify(
   return NS_OK;
 }
 
+void Element::TryReserveAttributeCount(uint32_t aAttributeCount) {
+  (void)mAttrs.GrowTo(aAttributeCount);
+}
+
 bool Element::ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
                              const nsAString& aValue,
                              nsIPrincipal* aMaybeScriptedPrincipal,

dom/base/Element.h

@@ -1658,6 +1658,12 @@ class Element : public FragmentOrElement {
         SetAttr(kNameSpaceID_None, aAttr, aValue, aTriggeringPrincipal, true);
   }
 
+  /**
+   * Preallocate space in this element's attribute array for the given
+   * total number of attributes.
+   */
+  void TryReserveAttributeCount(uint32_t aAttributeCount);
+
   /**
    * Set a content attribute via a reflecting nullable string IDL
    * attribute (e.g. a CORS attribute).  If DOMStringIsNull(aValue),

parser/html/nsHtml5TreeOperation.cpp

@@ -406,6 +406,7 @@ nsresult nsHtml5TreeOperation::AddAttributes(nsIContent* aNode,
 void nsHtml5TreeOperation::SetHTMLElementAttributes(
     Element* aElement, nsAtom* aName, nsHtml5HtmlAttributes* aAttributes) {
   int32_t len = aAttributes->getLength();
+  aElement->TryReserveAttributeCount((uint32_t)len);
   for (int32_t i = 0; i < len; i++) {
     nsHtml5String val = aAttributes->getValueNoBoundsCheck(i);
     nsAtom* klass = val.MaybeAsAtom();