diff --git a/src/cmd/dist/build.go b/src/cmd/dist/build.go
index 2262a736de..2fcb12c826 100644
--- a/src/cmd/dist/build.go
+++ b/src/cmd/dist/build.go
@@ -809,11 +809,11 @@ func shouldbuild(file, dir string) bool {
 }
 
 // copy copies the file src to dst, via memory (so only good for small files).
-func copyfile(dst, src string, exec int) {
+func copyfile(dst, src string, flag int) {
 	if vflag > 1 {
 		errprintf("cp %s %s\n", src, dst)
 	}
-	writefile(readfile(src), dst, exec)
+	writefile(readfile(src), dst, flag)
 }
 
 // dopack copies the package src to dst,
diff --git a/src/cmd/dist/buildruntime.go b/src/cmd/dist/buildruntime.go
index 70aafe9183..9b8d8f0cf6 100644
--- a/src/cmd/dist/buildruntime.go
+++ b/src/cmd/dist/buildruntime.go
@@ -35,7 +35,7 @@ func mkzversion(dir, file string) {
 			"const stackGuardMultiplier = %d\n"+
 			"var buildVersion = theVersion\n", goroot_final, findgoversion(), os.Getenv("GOEXPERIMENT"), stackGuardMultiplier())
 
-	writefile(out, file, 0)
+	writefile(out, file, writeSkipSame)
 }
 
 // mkzbootstrap writes cmd/internal/obj/zbootstrap.go:
@@ -80,7 +80,7 @@ func mkzbootstrap(file string) {
 			"const goexperiment = `%s`\n",
 		goroot_final, go386, goarm, goextlinkenabled, findgoversion(), stackGuardMultiplier(), os.Getenv("GOEXPERIMENT"))
 
-	writefile(out, file, 0)
+	writefile(out, file, writeSkipSame)
 }
 
 // stackGuardMultiplier returns a multiplier to apply to the default
diff --git a/src/cmd/dist/buildtool.go b/src/cmd/dist/buildtool.go
index 2840f71749..be54ac46de 100644
--- a/src/cmd/dist/buildtool.go
+++ b/src/cmd/dist/buildtool.go
@@ -118,7 +118,7 @@ func bootstrapBuildTools() {
 	// Copy binaries into tool binary directory.
 	for _, name := range bootstrapDirs {
 		if !strings.Contains(name, "/") {
-			copyfile(pathf("%s/%s%s", tooldir, name, exe), pathf("%s/bin/%s%s", workspace, name, exe), 1)
+			copyfile(pathf("%s/%s%s", tooldir, name, exe), pathf("%s/bin/%s%s", workspace, name, exe), writeExec)
 		}
 	}
 
diff --git a/src/cmd/dist/util.go b/src/cmd/dist/util.go
index cae5d699d4..f13210f4de 100644
--- a/src/cmd/dist/util.go
+++ b/src/cmd/dist/util.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"bytes"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -245,14 +246,28 @@ func readfile(file string) string {
 	return string(data)
 }
 
-// writefile writes b to the named file, creating it if needed.  if
-// exec is non-zero, marks the file as executable.
-func writefile(b, file string, exec int) {
+const (
+	writeExec = 1 << iota
+	writeSkipSame
+)
+
+// writefile writes b to the named file, creating it if needed.
+// if exec is non-zero, marks the file as executable.
+// If the file already exists and has the expected content,
+// it is not rewritten, to avoid changing the time stamp.
+func writefile(b, file string, flag int) {
+	new := []byte(b)
+	if flag&writeSkipSame != 0 {
+		old, err := ioutil.ReadFile(file)
+		if err == nil && bytes.Equal(old, new) {
+			return
+		}
+	}
 	mode := os.FileMode(0666)
-	if exec != 0 {
+	if flag&writeExec != 0 {
 		mode = 0777
 	}
-	err := ioutil.WriteFile(file, []byte(b), mode)
+	err := ioutil.WriteFile(file, new, mode)
 	if err != nil {
 		fatal("%v", err)
 	}