Allow [id=X] selectors to use the querySelector ID fast path.

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 212d353a15db86993d98e6d39140a9c352b1789c..c739f889ace3acbb4d311a3303a70ce3cb535382 100644
--- a/third_party/WebKit/Source/core/dom/SelectorQuery.cpp
+++ b/third_party/WebKit/Source/core/dom/SelectorQuery.cpp
@@ -27,6 +27,7 @@
 #include "core/dom/SelectorQuery.h"
 
 #include "bindings/core/v8/ExceptionState.h"
+#include "core/HTMLNames.h"
 #include "core/css/SelectorChecker.h"
 #include "core/css/parser/CSSParser.h"
 #include "core/dom/Document.h"
@@ -41,6 +42,8 @@
 
 namespace blink {
 
+using namespace HTMLNames;
+
 struct SingleElementSelectorQueryTrait {
     typedef Element* OutputType;
     static const bool shouldOnlyMatchFirstElement = true;
@@ -444,11 +447,17 @@ void SelectorDataList::executeSlowTraversingShadowTree(ContainerNode& rootNode,
     }
 }
 
-const CSSSelector* SelectorDataList::selectorForIdLookup(const CSSSelector& firstSelector) const
+static const CSSSelector* selectorForIdLookup(const CSSSelector& firstSelector)
 {
     for (const CSSSelector* selector = &firstSelector; selector; selector = selector->tagHistory()) {
         if (selector->match() == CSSSelector::Id)
             return selector;
+        // We only use the fast path when in standards mode where #id selectors
+        // are case sensitive, so we need the same behavior for [id=value].
+        if (selector->match() == CSSSelector::AttributeExact
+            && selector->attribute() == idAttr
+            && selector->attributeMatch() == CSSSelector::CaseSensitive)
+            return selector;

[sashab, 2016/07/12 01:37:03]

Would it be worth adding a StyleResolverStats for when this fast path is
triggered?

[esprehn, 2016/07/12 01:39:15]

It would be worth adding some metrics, but StyleResolverStats are only reported
from inside updateStyle so this wouldn't ever show up there since it's inside
querySelector(All). Recording a trace for every querySelector would probably be
pretty expensive, maybe a disabled by default one. We should consider that for a
future patch. :)

         if (selector->relation() != CSSSelector::SubSelector)
             break;
     }
@@ -473,11 +482,12 @@ void SelectorDataList::execute(ContainerNode& rootNode, typename SelectorQueryTr
     }
 
     DCHECK_EQ(m_selectors.size(), 1u);
+    DCHECK(!rootNode.document().inQuirksMode());
 
     const CSSSelector& selector = *m_selectors[0];
     const CSSSelector& firstSelector = selector;
 
-    // Fast path for querySelector*('#id'), querySelector*('tag#id').
+    // Fast path for querySelector*('#id'), querySelector*('tag#id'), querySelector*('tag[id=example]').
     if (const CSSSelector* idSelector = selectorForIdLookup(firstSelector)) {
         const AtomicString& idToMatch = idSelector->value();
         if (rootNode.treeScope().containsMultipleElementsWithId(idToMatch)) {