diff --git a/src/cmd/go/init_test.go b/src/cmd/go/init_test.go
index ed90a77..5a5cbe5 100644
--- a/src/cmd/go/init_test.go
+++ b/src/cmd/go/init_test.go
@@ -7,6 +7,7 @@
 import (
 	"internal/testenv"
 	"os/exec"
+	"sync/atomic"
 	"testing"
 )
 
@@ -15,20 +16,27 @@
 // the benchmark if any changes were done.
 func BenchmarkExecGoEnv(b *testing.B) {
 	testenv.MustHaveExec(b)
-	b.StopTimer()
 	gotool, err := testenv.GoTool()
 	if err != nil {
 		b.Fatal(err)
 	}
-	for i := 0; i < b.N; i++ {
-		cmd := exec.Command(gotool, "env", "GOARCH")
 
-		b.StartTimer()
-		err := cmd.Run()
-		b.StopTimer()
+	// We collect extra metrics.
+	var n, userTime, systemTime int64
 
-		if err != nil {
-			b.Fatal(err)
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			cmd := exec.Command(gotool, "env", "GOARCH")
+
+			if err := cmd.Run(); err != nil {
+				b.Fatal(err)
+			}
+			atomic.AddInt64(&n, 1)
+			atomic.AddInt64(&userTime, int64(cmd.ProcessState.UserTime()))
+			atomic.AddInt64(&systemTime, int64(cmd.ProcessState.SystemTime()))
 		}
-	}
+	})
+	b.ReportMetric(float64(userTime)/float64(n), "user-ns/op")
+	b.ReportMetric(float64(systemTime)/float64(n), "sys-ns/op")
 }
diff --git a/src/cmd/go/internal/base/base.go b/src/cmd/go/internal/base/base.go
index db3ebef..004588c 100644
--- a/src/cmd/go/internal/base/base.go
+++ b/src/cmd/go/internal/base/base.go
@@ -56,6 +56,20 @@
 	// Commands initialized in package main
 }
 
+// hasFlag reports whether a command or any of its subcommands contain the given
+// flag.
+func hasFlag(c *Command, name string) bool {
+	if f := c.Flag.Lookup(name); f != nil {
+		return true
+	}
+	for _, sub := range c.Commands {
+		if hasFlag(sub, name) {
+			return true
+		}
+	}
+	return false
+}
+
 // LongName returns the command's long name: all the words in the usage line between "go" and a flag or argument,
 func (c *Command) LongName() string {
 	name := c.UsageLine
diff --git a/src/cmd/go/internal/base/goflags.go b/src/cmd/go/internal/base/goflags.go
index 3476613..f11f9a5 100644
--- a/src/cmd/go/internal/base/goflags.go
+++ b/src/cmd/go/internal/base/goflags.go
@@ -13,15 +13,7 @@
 	"cmd/go/internal/cfg"
 )
 
-var (
-	goflags   []string                // cached $GOFLAGS list; can be -x or --x form
-	knownFlag = make(map[string]bool) // flags allowed to appear in $GOFLAGS; no leading dashes
-)
-
-// AddKnownFlag adds name to the list of known flags for use in $GOFLAGS.
-func AddKnownFlag(name string) {
-	knownFlag[name] = true
-}
+var goflags []string // cached $GOFLAGS list; can be -x or --x form
 
 // GOFLAGS returns the flags from $GOFLAGS.
 // The list can be assumed to contain one string per flag,
@@ -38,22 +30,12 @@
 		return
 	}
 
-	// Build list of all flags for all commands.
-	// If no command has that flag, then we report the problem.
-	// This catches typos while still letting users record flags in GOFLAGS
-	// that only apply to a subset of go commands.
-	// Commands using CustomFlags can report their flag names
-	// by calling AddKnownFlag instead.
-	var walkFlags func(*Command)
-	walkFlags = func(cmd *Command) {
-		for _, sub := range cmd.Commands {
-			walkFlags(sub)
-		}
-		cmd.Flag.VisitAll(func(f *flag.Flag) {
-			knownFlag[f.Name] = true
-		})
+	goflags = strings.Fields(cfg.Getenv("GOFLAGS"))
+	if len(goflags) == 0 {
+		// nothing to do; avoid work on later InitGOFLAGS call
+		goflags = []string{}
+		return
 	}
-	walkFlags(Go)
 
 	// Ignore bad flag in go env and go bug, because
 	// they are what people reach for when debugging
@@ -61,11 +43,6 @@
 	// (Both will show the GOFLAGS setting if let succeed.)
 	hideErrors := cfg.CmdName == "env" || cfg.CmdName == "bug"
 
-	goflags = strings.Fields(cfg.Getenv("GOFLAGS"))
-	if goflags == nil {
-		goflags = []string{} // avoid work on later InitGOFLAGS call
-	}
-
 	// Each of the words returned by strings.Fields must be its own flag.
 	// To set flag arguments use -x=value instead of -x value.
 	// For boolean flags, -x is fine instead of -x=true.
@@ -85,7 +62,7 @@
 		if i := strings.Index(name, "="); i >= 0 {
 			name = name[:i]
 		}
-		if !knownFlag[name] {
+		if !hasFlag(Go, name) {
 			if hideErrors {
 				continue
 			}