diff --git a/encode_opcode.go b/encode_opcode.go
index e4ca528..a7c93a4 100644
--- a/encode_opcode.go
+++ b/encode_opcode.go
@@ -6,7 +6,7 @@ import (
 	"unsafe"
 )
 
-var uintptrSize = unsafe.Sizeof(uintptr(0))
+const uintptrSize = 4 << (^uintptr(0) >> 63) // unsafe.Sizeof(uintptr(0)) but an ideal const
 
 type opcode struct {
 	op           opType // operation type