specialize Js.Dict.fromArray to compile to a direct JS object creation when possible

Author: zth

jscomp/ml/translcore.ml

@@ -721,6 +721,13 @@ let extract_directive_for_fn exp =
   exp.exp_attributes |> List.find_map (
     fun ({txt}, payload) -> if txt = "directive" then Ast_payload.is_single_string payload else None)
 
+let args_elgible_for_dict_inlining (args : expression list) =
+  args
+  |> List.find_opt (fun (arg : expression) ->
+         match arg.exp_desc with
+         | Texp_tuple [{exp_desc = Texp_constant (Const_string _)}; _] -> false
+         | _ -> true)
+  |> Option.is_none
 
 let rec transl_exp e =
   List.iter (Translattribute.check_attribute e) e.exp_attributes;
@@ -803,6 +810,28 @@ and transl_exp0 (e : Typedtree.expression) : Lambda.lambda =
           | _ ->
               wrap (Lprim (prim, argl, e.exp_loc))
               ))
+  | Texp_apply
+      ( {exp_desc = Texp_ident (p, _, _)},
+        [(_lbl, Some {exp_desc = Texp_array args})] )
+    when Path.name p = "Js.Dict.fromArray"
+          && args_elgible_for_dict_inlining args ->
+    let fields = Array.make (List.length args) "" in
+    let args =
+      args
+      |> List.mapi (fun index (arg : expression) ->
+              match arg.exp_desc with
+              | Texp_tuple
+                  [{exp_desc = Texp_constant (Const_string (fieldName, _))}; exp]
+                ->
+                Array.set fields index fieldName;
+                exp |> transl_exp |> extract_constant
+              | _ -> assert false)
+    in
+    Lconst
+      (Const_block
+          ( Blk_record
+              {fields; mutable_flag = Mutable; record_repr = Record_regular},
+            args ))
   | Texp_apply (funct, oargs) ->
       let inlined, funct =
         Translattribute.get_and_remove_inlined_attribute funct

jscomp/test/DictTests.js

@@ -0,0 +1,9 @@
+let dictCreationCanBeInlined = Js.Dict.fromArray([
+  ("name", "hello"),
+  ("age", "what"),
+  ("more", "stuff"),
+])
+
+external imaginaryExternalArgs: array<(string, string)> = "imaginary"
+
+let dictCreationCanNotBeInlined = Js.Dict.fromArray(imaginaryExternalArgs)