Landed some improvements for improving the performance of ID-rooted queries...

Landed some improvements for improving the performance of ID-rooted queries (e.g. '#foo p'). Fixes jQuery bug #4236.

src/selector.js

@@ -12,16 +12,17 @@ var chunker = /((?:\((?:\([^()]+\)|[^()]+)+\)|\[(?:\[[^[\]]*\]|['"][^'"]*['"]|[^
 
 var Sizzle = function(selector, context, results, seed) {
 	results = results || [];
-	context = context || document;
+	var origContext = context = context || document;
 
-	if ( context.nodeType !== 1 && context.nodeType !== 9 )
+	if ( context.nodeType !== 1 && context.nodeType !== 9 ) {
 		return [];
+	}
 	
 	if ( !selector || typeof selector !== "string" ) {
 		return results;
 	}
 
-	var parts = [], m, set, checkSet, check, mode, extra, prune = true;
+	var parts = [], m, set, checkSet, check, mode, extra, prune = true, contextXML = isXML(context);
 	
 	// Reset the position of the chunker regexp (start from head)
 	chunker.lastIndex = 0;
@@ -53,10 +54,19 @@ var Sizzle = function(selector, context, results, seed) {
 			}
 		}
 	} else {
+		// Take a shortcut and set the context if the root selector is an ID
+		// (but not if it'll be faster if the inner selector is an ID)
+		if ( !seed && parts.length > 1 && context.nodeType === 9 && !contextXML &&
+				Expr.match.ID.test(parts[0]) && !Expr.match.ID.test(parts[parts.length - 1]) ) {
+			var ret = Sizzle.find( parts.shift(), context, contextXML );
+			context = ret.expr ? Sizzle.filter( ret.expr, ret.set )[0] : ret.set[0];
+		}
+
+		if ( context ) {
 			var ret = seed ?
 				{ expr: parts.pop(), set: makeArray(seed) } :
-			Sizzle.find( parts.pop(), parts.length === 1 && context.parentNode ? context.parentNode : context, isXML(context) );
-		set = Sizzle.filter( ret.expr, ret.set );
+				Sizzle.find( parts.pop(), parts.length === 1 && context.parentNode ? context.parentNode : context, contextXML );
+			set = ret.expr ? Sizzle.filter( ret.expr, ret.set ) : ret.set;
 
 			if ( parts.length > 0 ) {
 				checkSet = makeArray(set);
@@ -77,7 +87,10 @@ var Sizzle = function(selector, context, results, seed) {
 					pop = context;
 				}
 
-			Expr.relative[ cur ]( checkSet, pop, isXML(context) );
+				Expr.relative[ cur ]( checkSet, pop, contextXML );
+			}
+		} else {
+			checkSet = parts = [];
 		}
 	}
 
@@ -92,7 +105,7 @@ var Sizzle = function(selector, context, results, seed) {
 	if ( toString.call(checkSet) === "[object Array]" ) {
 		if ( !prune ) {
 			results.push.apply( results, checkSet );
-		} else if ( context.nodeType === 1 ) {
+		} else if ( context && context.nodeType === 1 ) {
 			for ( var i = 0; checkSet[i] != null; i++ ) {
 				if ( checkSet[i] && (checkSet[i] === true || checkSet[i].nodeType === 1 && contains(context, checkSet[i])) ) {
 					results.push( set[i] );
@@ -110,7 +123,7 @@ var Sizzle = function(selector, context, results, seed) {
 	}
 
 	if ( extra ) {
-		Sizzle( extra, context, results, seed );
+		Sizzle( extra, origContext, results, seed );
 
 		if ( sortOrder ) {
 			hasDuplicate = false;