diff --git a/src/regexp/onepass.go b/src/regexp/onepass.go
index 1b0564c3fd..1f8c1d00bd 100644
--- a/src/regexp/onepass.go
+++ b/src/regexp/onepass.go
@@ -222,9 +222,10 @@ func onePassCopy(prog *syntax.Prog) *onePassProg {
 	p := &onePassProg{
 		Start:  prog.Start,
 		NumCap: prog.NumCap,
+		Inst:   make([]onePassInst, len(prog.Inst)),
 	}
-	for _, inst := range prog.Inst {
-		p.Inst = append(p.Inst, onePassInst{Inst: inst})
+	for i, inst := range prog.Inst {
+		p.Inst[i] = onePassInst{Inst: inst}
 	}
 
 	// rewrites one or more common Prog constructs that enable some otherwise
diff --git a/src/regexp/onepass_test.go b/src/regexp/onepass_test.go
index f4e336c43b..b1caa44515 100644
--- a/src/regexp/onepass_test.go
+++ b/src/regexp/onepass_test.go
@@ -7,6 +7,7 @@ package regexp
 import (
 	"reflect"
 	"regexp/syntax"
+	"strings"
 	"testing"
 )
 
@@ -173,6 +174,7 @@ var onePassTests = []struct {
 	{`^.bc(d|e)*$`, onePass},
 	{`^(?:(?:aa)|.)$`, notOnePass},
 	{`^(?:(?:a{1,2}){1,2})$`, notOnePass},
+	{`^l` + strings.Repeat("o", 2<<8) + `ng$`, onePass},
 }
 
 func TestCompileOnePass(t *testing.T) {
@@ -223,3 +225,23 @@ func TestRunOnePass(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkCompileOnepass(b *testing.B) {
+	for _, test := range onePassTests {
+		if test.onePass == notOnePass {
+			continue
+		}
+		name := test.re
+		if len(name) > 20 {
+			name = name[:20] + "..."
+		}
+		b.Run(name, func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				if _, err := Compile(test.re); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}