refactor codegen emitting slot indices

lib/compile/bcc.ml

@@ -5,7 +5,7 @@ module Env = Map.Make (String)
 
 type binding = {
   self : Code.regidx;
-  slot : Value.slotidx;
+  elem : Value.elem;
 }
 
 let compile modl =
@@ -23,8 +23,9 @@ let compile modl =
     | Ast.Path (Var name) -> (
         match Env.find name env with
         | exception Not_found -> Fmt.failwith "unbound: %S" name
-        | { self; slot } ->
-            emit_mov sp (Code.cst_of_int slot);
+        | { self; elem } ->
+            let idx = Code.cst (Value.of_elem elem) in
+            emit_mov sp idx;
             emit (GET (self, sp));
             Reg sp)
     | Ast.Path (Ele (obj, ele)) ->
@@ -81,9 +82,9 @@ let compile modl =
         (fun (env, n) -> function
           | Ast.Item_fun (_, _, _) | Ast.Item_exp _ -> env, n
           | Ast.Item_obj (name, _) | Ast.Item_val (name, _) ->
-              let slot = n in
-              let env = Env.add name { self; slot } env in
-              Hashtbl.add elems name (Value.Field slot);
+              let elem = Value.Field n in
+              let env = Env.add name { self; elem } env in
+              Hashtbl.add elems name elem;
               env, n + 1)
         (env, 0)
         items
@@ -97,8 +98,9 @@ let compile modl =
     (* emit constructor, compile val fields, and get result of final expression *)
     emit (CON (self, vtable));
     let emit_set name rhs =
-      let slot = (Env.find name env).slot in
-      emit_mov sp (Code.cst_of_int slot);
+      let { elem; _ } = Env.find name env in
+      let idx = Code.cst (Value.of_elem elem) in
+      emit_mov sp idx;
       emit_mov (sp + 1) rhs;
       emit (SET (self, sp))
     in

lib/runtime/code.ml

@@ -7,7 +7,12 @@ type operand =
   | Cst_int of int64
   | Reg of regidx
 
-let cst_of_int i = Cst_int (Int64.of_int i)
+let cst = function
+  | Value.Nil -> Cst_nil
+  | Value.True -> Cst_true
+  | Value.False -> Cst_false
+  | Value.Int i -> Cst_int i
+  | _ -> invalid_arg "value cannot be converted to constant operand"
 
 type basic_block = {
   mutable ins_builder : ins list;