diff --git a/src/cmd/gofmt/gofmt_test.go b/src/cmd/gofmt/gofmt_test.go
index ca44f3dcf7a..d1edb7bcc16 100644
--- a/src/cmd/gofmt/gofmt_test.go
+++ b/src/cmd/gofmt/gofmt_test.go
@@ -6,18 +6,60 @@ package main
 
 import (
 	"bytes"
+	"flag"
 	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
+	"text/scanner"
 )
 
-func runTest(t *testing.T, in, out, flags string) {
+var update = flag.Bool("update", false, "update .golden files")
+
+// gofmtFlags looks for a comment of the form
+//
+//	//gofmt flags
+//
+// within the first maxLines lines of the given file,
+// and returns the flags string, if any. Otherwise it
+// returns the empty string.
+func gofmtFlags(filename string, maxLines int) string {
+	f, err := os.Open(filename)
+	if err != nil {
+		return "" // ignore errors - they will be found later
+	}
+	defer f.Close()
+
+	// initialize scanner
+	var s scanner.Scanner
+	s.Init(f)
+	s.Error = func(*scanner.Scanner, string) {}       // ignore errors
+	s.Mode = scanner.GoTokens &^ scanner.SkipComments // want comments
+
+	// look for //gofmt comment
+	for s.Line <= maxLines {
+		switch s.Scan() {
+		case scanner.Comment:
+			const prefix = "//gofmt "
+			if t := s.TokenText(); strings.HasPrefix(t, prefix) {
+				return strings.TrimSpace(t[len(prefix):])
+			}
+		case scanner.EOF:
+			return ""
+		}
+
+	}
+
+	return ""
+}
+
+func runTest(t *testing.T, in, out string) {
 	// process flags
 	*simplifyAST = false
 	*rewriteRule = ""
 	stdin := false
-	for _, flag := range strings.Split(flags, " ") {
+	for _, flag := range strings.Split(gofmtFlags(in, 20), " ") {
 		elts := strings.SplitN(flag, "=", 2)
 		name := elts[0]
 		value := ""
@@ -56,6 +98,17 @@ func runTest(t *testing.T, in, out, flags string) {
 	}
 
 	if got := buf.Bytes(); !bytes.Equal(got, expected) {
+		if *update {
+			if in != out {
+				if err := ioutil.WriteFile(out, got, 0666); err != nil {
+					t.Error(err)
+				}
+				return
+			}
+			// in == out: don't accidentally destroy input
+			t.Errorf("WARNING: -update did not rewrite input file %s", in)
+		}
+
 		t.Errorf("(gofmt %s) != %s (see %s.gofmt)", in, out, in)
 		d, err := diff(expected, got)
 		if err == nil {
@@ -67,53 +120,37 @@ func runTest(t *testing.T, in, out, flags string) {
 	}
 }
 
-var tests = []struct {
-	in, flags string
-}{
-	{"gofmt.go", ""},
-	{"gofmt_test.go", ""},
-	{"testdata/composites.input", "-s"},
-	{"testdata/slices1.input", "-s"},
-	{"testdata/slices2.input", "-s"},
-	{"testdata/ranges.input", "-s"},
-	{"testdata/old.input", ""},
-	{"testdata/rewrite1.input", "-r=Foo->Bar"},
-	{"testdata/rewrite2.input", "-r=int->bool"},
-	{"testdata/rewrite3.input", "-r=x->x"},
-	{"testdata/rewrite4.input", "-r=(x)->x"},
-	{"testdata/rewrite5.input", "-r=x+x->2*x"},
-	{"testdata/rewrite6.input", "-r=fun(x)->Fun(x)"},
-	{"testdata/rewrite7.input", "-r=fun(x...)->Fun(x)"},
-	{"testdata/rewrite8.input", "-r=interface{}->int"},
-	{"testdata/stdin*.input", "-stdin"},
-	{"testdata/comments.input", ""},
-	{"testdata/import.input", ""},
-	{"testdata/crlf.input", ""},        // test case for issue 3961; see also TestCRLF
-	{"testdata/typeswitch.input", ""},  // test case for issue 4470
-	{"testdata/emptydecl.input", "-s"}, // test case for issue 7631
-}
-
+// TestRewrite processes testdata/*.input files and compares them to the
+// corresponding testdata/*.golden files. The gofmt flags used to process
+// a file must be provided via a comment of the form
+//
+//	//gofmt flags
+//
+// in the processed file within the first 20 lines, if any.
 func TestRewrite(t *testing.T) {
-	for _, test := range tests {
-		match, err := filepath.Glob(test.in)
-		if err != nil {
-			t.Error(err)
-			continue
+	// determine input files
+	match, err := filepath.Glob("testdata/*.input")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// add larger examples
+	match = append(match, "gofmt.go", "gofmt_test.go")
+
+	for _, in := range match {
+		out := in // for files where input and output are identical
+		if strings.HasSuffix(in, ".input") {
+			out = in[:len(in)-len(".input")] + ".golden"
 		}
-		for _, in := range match {
-			out := in
-			if strings.HasSuffix(in, ".input") {
-				out = in[:len(in)-len(".input")] + ".golden"
-			}
-			runTest(t, in, out, test.flags)
-			if in != out {
-				// Check idempotence.
-				runTest(t, out, out, test.flags)
-			}
+		runTest(t, in, out)
+		if in != out {
+			// Check idempotence.
+			runTest(t, out, out)
 		}
 	}
 }
 
+// Test case for issue 3961.
 func TestCRLF(t *testing.T) {
 	const input = "testdata/crlf.input"   // must contain CR/LF's
 	const golden = "testdata/crlf.golden" // must not contain any CR's
diff --git a/src/cmd/gofmt/testdata/composites.golden b/src/cmd/gofmt/testdata/composites.golden
index b2825e732aa..fc9c98e625b 100644
--- a/src/cmd/gofmt/testdata/composites.golden
+++ b/src/cmd/gofmt/testdata/composites.golden
@@ -1,3 +1,5 @@
+//gofmt -s
+
 package P
 
 type T struct {
diff --git a/src/cmd/gofmt/testdata/composites.input b/src/cmd/gofmt/testdata/composites.input
index 7210dafc96c..fc7598af99e 100644
--- a/src/cmd/gofmt/testdata/composites.input
+++ b/src/cmd/gofmt/testdata/composites.input
@@ -1,3 +1,5 @@
+//gofmt -s
+
 package P
 
 type T struct {
diff --git a/src/cmd/gofmt/testdata/crlf.golden b/src/cmd/gofmt/testdata/crlf.golden
index 57679f770fe..193dbacc727 100644
--- a/src/cmd/gofmt/testdata/crlf.golden
+++ b/src/cmd/gofmt/testdata/crlf.golden
@@ -2,6 +2,7 @@
 	Source containing CR/LF line endings.
 	The gofmt'ed output must only have LF
 	line endings.
+	Test case for issue 3961.
 */
 package main
 
diff --git a/src/cmd/gofmt/testdata/crlf.input b/src/cmd/gofmt/testdata/crlf.input
index 61a1aa0b4ee..ae7e14dbf13 100644
--- a/src/cmd/gofmt/testdata/crlf.input
+++ b/src/cmd/gofmt/testdata/crlf.input
@@ -2,6 +2,7 @@
 	Source containing CR/LF line endings.
 	The gofmt'ed output must only have LF
 	line endings.
+	Test case for issue 3961.
 */
 package main
 
diff --git a/src/cmd/gofmt/testdata/emptydecl.golden b/src/cmd/gofmt/testdata/emptydecl.golden
index 9fe62c9738a..33d6435e0a9 100644
--- a/src/cmd/gofmt/testdata/emptydecl.golden
+++ b/src/cmd/gofmt/testdata/emptydecl.golden
@@ -1,3 +1,7 @@
+//gofmt -s
+
+// Test case for issue 7631.
+
 package main
 
 // Keep this declaration
diff --git a/src/cmd/gofmt/testdata/emptydecl.input b/src/cmd/gofmt/testdata/emptydecl.input
index d1cab00ef7e..4948a61f0de 100644
--- a/src/cmd/gofmt/testdata/emptydecl.input
+++ b/src/cmd/gofmt/testdata/emptydecl.input
@@ -1,3 +1,7 @@
+//gofmt -s
+
+// Test case for issue 7631.
+
 package main
 
 // Keep this declaration
diff --git a/src/cmd/gofmt/testdata/ranges.golden b/src/cmd/gofmt/testdata/ranges.golden
index 42168526d1d..506b3a035a3 100644
--- a/src/cmd/gofmt/testdata/ranges.golden
+++ b/src/cmd/gofmt/testdata/ranges.golden
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for range simplification.
 package p
 
diff --git a/src/cmd/gofmt/testdata/ranges.input b/src/cmd/gofmt/testdata/ranges.input
index 4b02d517520..df5f8333c21 100644
--- a/src/cmd/gofmt/testdata/ranges.input
+++ b/src/cmd/gofmt/testdata/ranges.input
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for range simplification.
 package p
 
diff --git a/src/cmd/gofmt/testdata/rewrite1.golden b/src/cmd/gofmt/testdata/rewrite1.golden
index d9beb370582..3ee5373a790 100644
--- a/src/cmd/gofmt/testdata/rewrite1.golden
+++ b/src/cmd/gofmt/testdata/rewrite1.golden
@@ -1,3 +1,5 @@
+//gofmt -r=Foo->Bar
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite1.input b/src/cmd/gofmt/testdata/rewrite1.input
index bdb894320d3..a84c8f78165 100644
--- a/src/cmd/gofmt/testdata/rewrite1.input
+++ b/src/cmd/gofmt/testdata/rewrite1.input
@@ -1,3 +1,5 @@
+//gofmt -r=Foo->Bar
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite2.golden b/src/cmd/gofmt/testdata/rewrite2.golden
index 64c67ffa67b..f980e035309 100644
--- a/src/cmd/gofmt/testdata/rewrite2.golden
+++ b/src/cmd/gofmt/testdata/rewrite2.golden
@@ -1,3 +1,5 @@
+//gofmt -r=int->bool
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite2.input b/src/cmd/gofmt/testdata/rewrite2.input
index 21171447a10..489be4e07dc 100644
--- a/src/cmd/gofmt/testdata/rewrite2.input
+++ b/src/cmd/gofmt/testdata/rewrite2.input
@@ -1,3 +1,5 @@
+//gofmt -r=int->bool
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite3.golden b/src/cmd/gofmt/testdata/rewrite3.golden
index 0d16d16011b..261a220c65d 100644
--- a/src/cmd/gofmt/testdata/rewrite3.golden
+++ b/src/cmd/gofmt/testdata/rewrite3.golden
@@ -1,3 +1,5 @@
+//gofmt -r=x->x
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite3.input b/src/cmd/gofmt/testdata/rewrite3.input
index 0d16d16011b..261a220c65d 100644
--- a/src/cmd/gofmt/testdata/rewrite3.input
+++ b/src/cmd/gofmt/testdata/rewrite3.input
@@ -1,3 +1,5 @@
+//gofmt -r=x->x
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite4.golden b/src/cmd/gofmt/testdata/rewrite4.golden
index 8dfc81a0746..b05547b4bf0 100644
--- a/src/cmd/gofmt/testdata/rewrite4.golden
+++ b/src/cmd/gofmt/testdata/rewrite4.golden
@@ -1,3 +1,5 @@
+//gofmt -r=(x)->x
+
 // 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.
diff --git a/src/cmd/gofmt/testdata/rewrite4.input b/src/cmd/gofmt/testdata/rewrite4.input
index 164cc0451f3..0817099209c 100644
--- a/src/cmd/gofmt/testdata/rewrite4.input
+++ b/src/cmd/gofmt/testdata/rewrite4.input
@@ -1,3 +1,5 @@
+//gofmt -r=(x)->x
+
 // 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.
diff --git a/src/cmd/gofmt/testdata/rewrite5.golden b/src/cmd/gofmt/testdata/rewrite5.golden
index 5a448a63d37..9beb34aee76 100644
--- a/src/cmd/gofmt/testdata/rewrite5.golden
+++ b/src/cmd/gofmt/testdata/rewrite5.golden
@@ -1,3 +1,5 @@
+//gofmt -r=x+x->2*x
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite5.input b/src/cmd/gofmt/testdata/rewrite5.input
index 0d759e69b6d..d7a6122d07a 100644
--- a/src/cmd/gofmt/testdata/rewrite5.input
+++ b/src/cmd/gofmt/testdata/rewrite5.input
@@ -1,3 +1,5 @@
+//gofmt -r=x+x->2*x
+
 // Copyright 2011 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.
diff --git a/src/cmd/gofmt/testdata/rewrite6.golden b/src/cmd/gofmt/testdata/rewrite6.golden
index e565dbdd97b..48ec9aa0df7 100644
--- a/src/cmd/gofmt/testdata/rewrite6.golden
+++ b/src/cmd/gofmt/testdata/rewrite6.golden
@@ -1,3 +1,5 @@
+//gofmt -r=fun(x)->Fun(x)
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/rewrite6.input b/src/cmd/gofmt/testdata/rewrite6.input
index 8c088b3e878..b085a84fef4 100644
--- a/src/cmd/gofmt/testdata/rewrite6.input
+++ b/src/cmd/gofmt/testdata/rewrite6.input
@@ -1,3 +1,5 @@
+//gofmt -r=fun(x)->Fun(x)
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/rewrite7.golden b/src/cmd/gofmt/testdata/rewrite7.golden
index 29babad9f94..8386a0b2a3e 100644
--- a/src/cmd/gofmt/testdata/rewrite7.golden
+++ b/src/cmd/gofmt/testdata/rewrite7.golden
@@ -1,3 +1,5 @@
+//gofmt -r=fun(x...)->Fun(x)
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/rewrite7.input b/src/cmd/gofmt/testdata/rewrite7.input
index 073e2a3e6f8..c1984708e71 100644
--- a/src/cmd/gofmt/testdata/rewrite7.input
+++ b/src/cmd/gofmt/testdata/rewrite7.input
@@ -1,3 +1,5 @@
+//gofmt -r=fun(x...)->Fun(x)
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/rewrite8.golden b/src/cmd/gofmt/testdata/rewrite8.golden
index cfc452b0310..62f0419dfb4 100644
--- a/src/cmd/gofmt/testdata/rewrite8.golden
+++ b/src/cmd/gofmt/testdata/rewrite8.golden
@@ -1,3 +1,5 @@
+//gofmt -r=interface{}->int
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/rewrite8.input b/src/cmd/gofmt/testdata/rewrite8.input
index 235efa91cc6..7964c5c75c7 100644
--- a/src/cmd/gofmt/testdata/rewrite8.input
+++ b/src/cmd/gofmt/testdata/rewrite8.input
@@ -1,3 +1,5 @@
+//gofmt -r=interface{}->int
+
 // Copyright 2013 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.
diff --git a/src/cmd/gofmt/testdata/slices1.golden b/src/cmd/gofmt/testdata/slices1.golden
index 6633a5e001e..04bc16f2160 100644
--- a/src/cmd/gofmt/testdata/slices1.golden
+++ b/src/cmd/gofmt/testdata/slices1.golden
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for slice expression simplification.
 package p
 
diff --git a/src/cmd/gofmt/testdata/slices1.input b/src/cmd/gofmt/testdata/slices1.input
index 27e9cb8fef3..1f25c43ccbc 100644
--- a/src/cmd/gofmt/testdata/slices1.input
+++ b/src/cmd/gofmt/testdata/slices1.input
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for slice expression simplification.
 package p
 
diff --git a/src/cmd/gofmt/testdata/slices2.golden b/src/cmd/gofmt/testdata/slices2.golden
index 433788e1ee6..ab657004e64 100644
--- a/src/cmd/gofmt/testdata/slices2.golden
+++ b/src/cmd/gofmt/testdata/slices2.golden
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for slice expression simplification.
 // Because of a dot import, these slices must remain untouched.
 package p
diff --git a/src/cmd/gofmt/testdata/slices2.input b/src/cmd/gofmt/testdata/slices2.input
index 433788e1ee6..ab657004e64 100644
--- a/src/cmd/gofmt/testdata/slices2.input
+++ b/src/cmd/gofmt/testdata/slices2.input
@@ -1,3 +1,5 @@
+//gofmt -s
+
 // Test cases for slice expression simplification.
 // Because of a dot import, these slices must remain untouched.
 package p
diff --git a/src/cmd/gofmt/testdata/stdin1.golden b/src/cmd/gofmt/testdata/stdin1.golden
index ff8b0b7ab48..9e4dcd20fe0 100644
--- a/src/cmd/gofmt/testdata/stdin1.golden
+++ b/src/cmd/gofmt/testdata/stdin1.golden
@@ -1,3 +1,5 @@
+	//gofmt -stdin
+
 	if x {
 		y
 	}
diff --git a/src/cmd/gofmt/testdata/stdin1.input b/src/cmd/gofmt/testdata/stdin1.input
index ff8b0b7ab48..9e4dcd20fe0 100644
--- a/src/cmd/gofmt/testdata/stdin1.input
+++ b/src/cmd/gofmt/testdata/stdin1.input
@@ -1,3 +1,5 @@
+	//gofmt -stdin
+
 	if x {
 		y
 	}
diff --git a/src/cmd/gofmt/testdata/stdin2.golden b/src/cmd/gofmt/testdata/stdin2.golden
index 7eb1b54fec0..57df3554035 100644
--- a/src/cmd/gofmt/testdata/stdin2.golden
+++ b/src/cmd/gofmt/testdata/stdin2.golden
@@ -1,4 +1,4 @@
-
+//gofmt -stdin
 
 var x int
 
diff --git a/src/cmd/gofmt/testdata/stdin2.input b/src/cmd/gofmt/testdata/stdin2.input
index 99defd2d10c..69d6bdd682e 100644
--- a/src/cmd/gofmt/testdata/stdin2.input
+++ b/src/cmd/gofmt/testdata/stdin2.input
@@ -1,4 +1,4 @@
-
+//gofmt -stdin
 
 var x int
 
diff --git a/src/cmd/gofmt/testdata/stdin3.golden b/src/cmd/gofmt/testdata/stdin3.golden
index 1bf2f5a483f..d6da0e417a0 100644
--- a/src/cmd/gofmt/testdata/stdin3.golden
+++ b/src/cmd/gofmt/testdata/stdin3.golden
@@ -1,3 +1,4 @@
+		//gofmt -stdin
 
 		/* note: no newline at end of file */
 		for i := 0; i < 10; i++ {
diff --git a/src/cmd/gofmt/testdata/stdin3.input b/src/cmd/gofmt/testdata/stdin3.input
index d963bd0d21b..ab46c1063be 100644
--- a/src/cmd/gofmt/testdata/stdin3.input
+++ b/src/cmd/gofmt/testdata/stdin3.input
@@ -1,3 +1,4 @@
+		//gofmt -stdin
 
 		/* note: no newline at end of file */
 		for i := 0; i < 10; i++ { s += i }
diff --git a/src/cmd/gofmt/testdata/stdin4.golden b/src/cmd/gofmt/testdata/stdin4.golden
index 5f73435517f..0c7acace5d0 100644
--- a/src/cmd/gofmt/testdata/stdin4.golden
+++ b/src/cmd/gofmt/testdata/stdin4.golden
@@ -1,3 +1,5 @@
+	//gofmt -stdin
+
 	// comment
 
 	i := 0
diff --git a/src/cmd/gofmt/testdata/stdin4.input b/src/cmd/gofmt/testdata/stdin4.input
index f02a54fb1a9..1fc73f31e5e 100644
--- a/src/cmd/gofmt/testdata/stdin4.input
+++ b/src/cmd/gofmt/testdata/stdin4.input
@@ -1,3 +1,5 @@
+	//gofmt -stdin
+
 	// comment
 	
 	i := 0