diff --git a/src/cmd/compile/internal/x86/reg.go b/src/cmd/compile/internal/x86/reg.go
index b3a5fdf4e0..76d90b8e89 100644
--- a/src/cmd/compile/internal/x86/reg.go
+++ b/src/cmd/compile/internal/x86/reg.go
@@ -62,7 +62,11 @@ func regnames(n *int) []string {
 }
 
 func excludedregs() uint64 {
-	return RtoB(x86.REG_SP)
+	if gc.Ctxt.Flag_shared != 0 {
+		return RtoB(x86.REG_SP) | RtoB(x86.REG_CX)
+	} else {
+		return RtoB(x86.REG_SP)
+	}
 }
 
 func doregbits(r int) uint64 {