diff --git a/src/cmd/compile/internal/amd64/peep.go b/src/cmd/compile/internal/amd64/peep.go
index 130f369a8b6..452f954bd82 100644
--- a/src/cmd/compile/internal/amd64/peep.go
+++ b/src/cmd/compile/internal/amd64/peep.go
@@ -823,6 +823,10 @@ func copyu(p *obj.Prog, v *obj.Addr, s *obj.Addr) int {
 		return 2
 	}
 
+	if (p.Info.Reguse|p.Info.Regset)&FtoB(int(v.Reg)) != 0 {
+		return 2
+	}
+
 	if p.Info.Flags&gc.LeftAddr != 0 {
 		if copyas(&p.From, v) {
 			return 2
diff --git a/src/cmd/compile/internal/amd64/prog.go b/src/cmd/compile/internal/amd64/prog.go
index eff6ccee5b3..ae8f5255a96 100644
--- a/src/cmd/compile/internal/amd64/prog.go
+++ b/src/cmd/compile/internal/amd64/prog.go
@@ -140,7 +140,7 @@ var progtable = [x86.ALAST]obj.ProgInfo{
 	x86.AMOVSL:     {Flags: gc.OK, Reguse: DI | SI, Regset: DI | SI},
 	x86.AMOVSQ:     {Flags: gc.OK, Reguse: DI | SI, Regset: DI | SI},
 	x86.AMOVSW:     {Flags: gc.OK, Reguse: DI | SI, Regset: DI | SI},
-	obj.ADUFFCOPY:  {Flags: gc.OK, Reguse: DI | SI, Regset: DI | SI | CX},
+	obj.ADUFFCOPY:  {Flags: gc.OK, Reguse: DI | SI, Regset: DI | SI | X0},
 	x86.AMOVSD:     {Flags: gc.SizeD | gc.LeftRead | gc.RightWrite | gc.Move},
 	x86.AMOVSS:     {Flags: gc.SizeF | gc.LeftRead | gc.RightWrite | gc.Move},
 
diff --git a/src/cmd/compile/internal/amd64/reg.go b/src/cmd/compile/internal/amd64/reg.go
index 8fab6399b1d..60822fbfe99 100644
--- a/src/cmd/compile/internal/amd64/reg.go
+++ b/src/cmd/compile/internal/amd64/reg.go
@@ -107,6 +107,7 @@ const (
 	DI  = 1 << (x86.REG_DI - x86.REG_AX)
 	SI  = 1 << (x86.REG_SI - x86.REG_AX)
 	R15 = 1 << (x86.REG_R15 - x86.REG_AX)
+	X0  = 1 << 16
 )
 
 func RtoB(r int) uint64 {
diff --git a/test/fixedbugs/issue13171.go b/test/fixedbugs/issue13171.go
new file mode 100644
index 00000000000..5d127a5426b
--- /dev/null
+++ b/test/fixedbugs/issue13171.go
@@ -0,0 +1,34 @@
+// run
+
+// Copyright 2015 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.
+
+package main
+
+// Make sure the compiler knows that DUFFCOPY clobbers X0
+
+import "fmt"
+
+//go:noinline
+func f(x float64) float64 {
+	// y is allocated to X0
+	y := x + 5
+	// marshals z before y.  Marshalling z
+	// calls DUFFCOPY.
+	return g(z, y)
+}
+
+//go:noinline
+func g(b [64]byte, y float64) float64 {
+	return y
+}
+
+var z [64]byte
+
+func main() {
+	got := f(5)
+	if got != 10 {
+		panic(fmt.Sprintf("want 10, got %f", got))
+	}
+}