go/parser: ~15% faster parsing

- only compute current line position if needed
  (i.e., if a comment is present)

- added benchmark

benchmark         old ns/op    new ns/op    delta
BenchmarkParse     10902990      9313330  -14.58%

benchmark          old MB/s     new MB/s  speedup
BenchmarkParse         5.31         6.22    1.17x

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6270043

src/pkg/go/parser/parser.go

@@ -296,14 +296,14 @@ func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline
 func (p *parser) next() {
 	p.leadComment = nil
 	p.lineComment = nil
-	line := p.file.Line(p.pos) // current line
+	prev := p.pos
 	p.next0()
 
 	if p.tok == token.COMMENT {
 		var comment *ast.CommentGroup
 		var endline int
 
-		if p.file.Line(p.pos) == line {
+		if p.file.Line(p.pos) == p.file.Line(prev) {
 			// The comment is on same line as the previous token; it
 			// cannot be a lead comment but may be a line comment.
 			comment, endline = p.consumeCommentGroup(0)

src/pkg/go/parser/performance_test.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package parser
+
+import (
+	"go/token"
+	"io/ioutil"
+	"testing"
+)
+
+var src = readFile("parser.go")
+
+func readFile(filename string) []byte {
+	data, err := ioutil.ReadFile(filename)
+	if err != nil {
+		panic(err)
+	}
+	return data
+}
+
+func BenchmarkParse(b *testing.B) {
+	b.SetBytes(int64(len(src)))
+	for i := 0; i < b.N; i++ {
+		if _, err := ParseFile(token.NewFileSet(), "", src, ParseComments); err != nil {
+			b.Fatalf("benchmark failed due to parse error: %s", err)
+		}
+	}
+}