go/format: add simple benchmark framework and basic benchmark

For now, this CL adds as a first benchmark the formatting of
a 10,000 element array literal. It is easy to add additional
test cases as we see fit.

	name                   time/op
	Format/array1-10000-4    26.7ms ± 7%

	name                   speed
	Format/array1-10000-4  2.43MB/s ± 6%

	name                   alloc/op
	Format/array1-10000-4    5.52MB ± 0%

	name                   allocs/op
	Format/array1-10000-4      119k ± 0%

Updates #26528.

Change-Id: Ic8ec8f70160d122b877740412d4d4406f5f4b345
Reviewed-on: https://go-review.googlesource.com/c/153642Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/format/benchmark_test.go

+// Copyright 2018 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.
+
+// This file provides a simple framework to add benchmarks
+// based on generated input (source) files.
+
+package format_test
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"go/format"
+	"io/ioutil"
+	"testing"
+)
+
+var debug = flag.Bool("debug", false, "write .src files containing formatting input; for debugging")
+
+// array1 generates an array literal with n elements of the form:
+//
+// var _ = [...]byte{
+// 	// 0
+// 	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+// 	0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
+// 	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+// 	0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
+// 	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+// 	// 40
+// 	0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
+// 	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
+// 	...
+//
+func array1(buf *bytes.Buffer, n int) {
+	buf.WriteString("var _ = [...]byte{\n")
+	for i := 0; i < n; {
+		if i%10 == 0 {
+			fmt.Fprintf(buf, "\t// %d\n", i)
+		}
+		buf.WriteByte('\t')
+		for j := 0; j < 8; j++ {
+			fmt.Fprintf(buf, "0x%02x, ", byte(i))
+			i++
+		}
+		buf.WriteString("\n")
+	}
+	buf.WriteString("}\n")
+}
+
+var tests = []struct {
+	name string
+	gen  func(*bytes.Buffer, int)
+	n    int
+}{
+	{"array1", array1, 10000},
+	// add new test cases here as needed
+}
+
+func BenchmarkFormat(b *testing.B) {
+	var src bytes.Buffer
+	for _, t := range tests {
+		src.Reset()
+		src.WriteString("package p\n")
+		t.gen(&src, t.n)
+		data := src.Bytes()
+
+		if *debug {
+			filename := t.name + ".src"
+			err := ioutil.WriteFile(filename, data, 0660)
+			if err != nil {
+				b.Fatalf("couldn't write %s: %v", filename, err)
+			}
+		}
+
+		b.Run(fmt.Sprintf("%s-%d", t.name, t.n), func(b *testing.B) {
+			b.SetBytes(int64(len(data)))
+			b.ReportAllocs()
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				var err error
+				sink, err = format.Source(data)
+				if err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+var sink []byte