Bug 1382372: Add a fast-path for matching attr selectors with the empty namespace. r=bz

MozReview-Commit-ID: GSr0oGJJZRM

layout/style/ServoBindings.cpp

@@ -937,13 +937,19 @@ template <typename Implementor, typename MatchFn>
 static bool
 DoMatch(Implementor* aElement, nsIAtom* aNS, nsIAtom* aName, MatchFn aMatch)
 {
-  if (aNS) {
-    int32_t ns = nsContentUtils::NameSpaceManager()->GetNameSpaceID(aNS,
-                                                                    aElement->IsInChromeDocument());
+  if (MOZ_LIKELY(aNS)) {
+    int32_t ns = aNS == nsGkAtoms::_empty
+      ? kNameSpaceID_None
+      : nsContentUtils::NameSpaceManager()->GetNameSpaceID(
+          aNS, aElement->IsInChromeDocument());
+
+    MOZ_ASSERT(ns == nsContentUtils::NameSpaceManager()->GetNameSpaceID(
+                       aNS, aElement->IsInChromeDocument()));
     NS_ENSURE_TRUE(ns != kNameSpaceID_Unknown, false);
     const nsAttrValue* value = aElement->GetParsedAttr(aName, ns);
     return value && aMatch(value);
   }
+
   // No namespace means any namespace - we have to check them all. :-(
   BorrowedAttrInfo attrInfo;
   for (uint32_t i = 0; (attrInfo = aElement->GetAttrInfoAt(i)); ++i) {