Simplify the id fast path for non-right most selectors in querySelector.

third_party/WebKit/Source/core/dom/SelectorQuery.cpp

Index: third_party/WebKit/Source/core/dom/SelectorQuery.cpp
diff --git a/third_party/WebKit/Source/core/dom/SelectorQuery.cpp b/third_party/WebKit/Source/core/dom/SelectorQuery.cpp
index ac733e76dd62f969c43d9ace284bfba432c4bc2a..70c69a753af9bb113508ddb7356717eab6e4bb68 100644
--- a/third_party/WebKit/Source/core/dom/SelectorQuery.cpp
+++ b/third_party/WebKit/Source/core/dom/SelectorQuery.cpp
@@ -244,30 +244,22 @@ void SelectorQuery::findTraverseRootsAndExecute(
 
   for (const CSSSelector* selector = m_selectors[0]; selector;
        selector = selector->tagHistory()) {
-    if (selector->match() == CSSSelector::Id && rootNode.isInTreeScope() &&

[esprehn, 2017/03/29 19:59:43]

We only take the fast path if rootNode.isConnected() is true (inside
canUseFastPath), so we know isInTreeScope() must be true.

-        !rootNode.containingTreeScope().containsMultipleElementsWithId(
-            selector->value())) {
-      Element* element =
-          rootNode.containingTreeScope().getElementById(selector->value());
-      ContainerNode* adjustedNode = &rootNode;
-      if (element && element->isDescendantOf(&rootNode))

[esprehn, 2017/03/29 19:59:43]

if element was null, then there's no element in this TreeScope with this id, so
the selector cannot match at all, so we can just early return.

-        adjustedNode = element;
-      else if (!element || isRightmostSelector)
-        adjustedNode = nullptr;
-      if (isRightmostSelector) {

[esprehn, 2017/03/29 19:59:43]

This code was not reachable, id selectors in the right most selector are handled
by the selectorForIdLookup() fast path inside ::execute() that comes before this
function call. I've replaced this path with a DCHECK.

-        if (!adjustedNode)
-          return;
-        element = toElement(adjustedNode);
-        if (selectorMatches(*m_selectors[0], *element, rootNode))
-          SelectorQueryTrait::appendElement(output, *element);
+    const AtomicString& value = selector->value();
+
+    if (selector->match() == CSSSelector::Id &&
+        !rootNode.containingTreeScope().containsMultipleElementsWithId(value)) {
+      // Id selectors in the right most selector are handled by the caller,
+      // we should never hit them here.
+      DCHECK(!isRightmostSelector);
+      Element* element = rootNode.containingTreeScope().getElementById(value);
+      if (!element)
         return;
-      }
-
-      if (startFromParent && adjustedNode)
-        adjustedNode = adjustedNode->parentNode();
-
-      executeForTraverseRoot<SelectorQueryTrait>(adjustedNode, rootNode,
-                                                 output);
+      ContainerNode* start = &rootNode;
+      if (element->isDescendantOf(&rootNode))
+        start = element;
+      if (startFromParent)
+        start = start->parentNode();
+      executeForTraverseRoot<SelectorQueryTrait>(start, rootNode, output);
       return;
     }
 
@@ -276,8 +268,7 @@ void SelectorQuery::findTraverseRootsAndExecute(
     if (!SelectorQueryTrait::shouldOnlyMatchFirstElement && !startFromParent &&
         selector->match() == CSSSelector::Class) {
       if (isRightmostSelector) {
-        ClassElementList<AllElements> traverseRoots(rootNode,
-                                                    selector->value());
+        ClassElementList<AllElements> traverseRoots(rootNode, value);
         while (!traverseRoots.isEmpty()) {
           Element& element = *traverseRoots.next();
           if (selectorMatches(*m_selectors[0], element, rootNode))
@@ -287,10 +278,10 @@ void SelectorQuery::findTraverseRootsAndExecute(
       }
       // Since there exists some ancestor element which has the class name, we
       // need to see all children of rootNode.
-      if (ancestorHasClassName(rootNode, selector->value()))
+      if (ancestorHasClassName(rootNode, value))
         break;
 
-      ClassElementList<OnlyRoots> traverseRoots(rootNode, selector->value());
+      ClassElementList<OnlyRoots> traverseRoots(rootNode, value);
       while (!traverseRoots.isEmpty()) {
         for (Element& element :
              ElementTraversal::descendantsOf(*traverseRoots.next())) {