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diff --git a/middle_end/flambda/initialize_symbol_to_let_symbol.ml b/middle_end/flambda/initialize_symbol_to_let_symbol.ml
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+(**************************************************************************)
+(*                                                                        *)
+(*                                 OCaml                                  *)
+(*                                                                        *)
+(*                       Pierre Chambart, OCamlPro                        *)
+(*           Mark Shinwell and Leo White, Jane Street Europe              *)
+(*                                                                        *)
+(*   Copyright 2013--2016 OCamlPro SAS                                    *)
+(*   Copyright 2014--2016 Jane Street Group LLC                           *)
+(*                                                                        *)
+(*   All rights reserved.  This file is distributed under the terms of    *)
+(*   the GNU Lesser General Public License version 2.1, with the          *)
+(*   special exception on linking described in the file LICENSE.          *)
+(*                                                                        *)
+(**************************************************************************)
+
+[@@@ocaml.warning "+a-4-9-30-40-41-42-66"]
+open! Int_replace_polymorphic_compare
+
+let constant_field (expr:Flambda.t)
+  : Flambda.constant_defining_value_block_field option =
+  match expr with
+  | Let { var; defining_expr = Const c; body = Var var' ; _ } ->
+    assert(Variable.equal var var');
+    (* This must be true since var is the only variable in scope *)
+    Some (Flambda.Const c)
+  | Let { var; defining_expr = Symbol s; body = Var var' ; _ } ->
+    assert(Variable.equal var var');
+    Some (Flambda.Symbol s)
+  | _ ->
+    None
+
+let rec loop (program : Flambda.program_body) : Flambda.program_body =
+  match program with
+  | Initialize_symbol (symbol, tag, fields, program) ->
+    let constant_fields = List.map constant_field fields in
+    begin
+      match Misc.Stdlib.List.some_if_all_elements_are_some constant_fields
+    with
+    | None ->
+      Initialize_symbol (symbol, tag, fields, loop program)
+    | Some fields ->
+      Let_symbol (symbol, Block (tag, fields), loop program)
+    end
+  | Let_symbol (symbol, const, program) ->
+    Let_symbol (symbol, const, loop program)
+  | Let_rec_symbol (defs, program) ->
+    Let_rec_symbol (defs, loop program)
+  | Effect (expr, program) ->
+    Effect (expr, loop program)
+  | End symbol ->
+    End symbol
+
+let run (program : Flambda.program) =
+  { program with
+    program_body = loop program.program_body;
+  }