path: root/src/constant_fold.ml

(**************************************************************************)
(*     Sail                                                               *)
(*                                                                        *)
(*  Copyright (c) 2013-2017                                               *)
(*    Kathyrn Gray                                                        *)
(*    Shaked Flur                                                         *)
(*    Stephen Kell                                                        *)
(*    Gabriel Kerneis                                                     *)
(*    Robert Norton-Wright                                                *)
(*    Christopher Pulte                                                   *)
(*    Peter Sewell                                                        *)
(*    Alasdair Armstrong                                                  *)
(*    Brian Campbell                                                      *)
(*    Thomas Bauereiss                                                    *)
(*    Anthony Fox                                                         *)
(*    Jon French                                                          *)
(*    Dominic Mulligan                                                    *)
(*    Stephen Kell                                                        *)
(*    Mark Wassell                                                        *)
(*                                                                        *)
(*  All rights reserved.                                                  *)
(*                                                                        *)
(*  This software was developed by the University of Cambridge Computer   *)
(*  Laboratory as part of the Rigorous Engineering of Mainstream Systems  *)
(*  (REMS) project, funded by EPSRC grant EP/K008528/1.                   *)
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(*  THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS''    *)
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(*  SUCH DAMAGE.                                                          *)
(**************************************************************************)

open Ast
open Ast_util
open Type_check
open Rewriter

module StringMap = Map.Make(String);;

(* Flag controls whether any constant folding will occur.
   false = no folding, true = perform constant folding. *)
let optimize_constant_fold = ref false

let rec fexp_of_ctor (field, value) =
  FE_aux (FE_Fexp (mk_id field, exp_of_value value), no_annot)

(* The interpreter will return a value for each folded expression, so
   we must convert that back to expression to re-insert it in the AST
   *)
and exp_of_value =
  let open Value in
  function
  | V_int n -> mk_lit_exp (L_num n)
  | V_bit Sail_lib.B0 -> mk_lit_exp L_zero
  | V_bit Sail_lib.B1 -> mk_lit_exp L_one
  | V_bool true -> mk_lit_exp L_true
  | V_bool false -> mk_lit_exp L_false
  | V_string str -> mk_lit_exp (L_string str)
  | V_record ctors ->
     mk_exp (E_record (List.map fexp_of_ctor (StringMap.bindings ctors)))
  | V_vector vs ->
     mk_exp (E_vector (List.map exp_of_value vs))
  | V_tuple vs ->
     mk_exp (E_tuple (List.map exp_of_value vs))
  | V_unit -> mk_lit_exp L_unit
  | V_attempted_read str ->
     mk_exp (E_id (mk_id str))
  | _ -> failwith "No expression for value"

(* We want to avoid evaluating things like print statements at compile
   time, so we remove them from this list of primops we can use when
   constant folding. *)
let safe_primops =
  List.fold_left
    (fun m k -> StringMap.remove k m)
    Value.primops
    [ "print_endline";
      "prerr_endline";
      "putchar";
      "print";
      "prerr";
      "print_bits";
      "print_int";
      "print_string";
      "print_real";
      "prerr_bits";
      "prerr_int";
      "prerr_string";
      "read_ram";
      "write_ram";
      "get_time_ns";
      "Elf_loader.elf_entry";
      "Elf_loader.elf_tohost"
    ]

(** We can specify a list of identifiers that we want to remove from
   the final AST here. This is useful for removing tracing features in
   optimized builds, e.g. for booting an OS as fast as possible.

   Basically we just do this by mapping

   f(x, y, z) -> ()

   when f is in the list of identifiers to be mapped to unit. The
   advantage of doing it like this is if x, y, and z are
   computationally expensive then we remove them also. String
   concatentation is very expensive at runtime so this is something we
   really want when cutting out tracing features. Obviously it's
   important that they don't have any meaningful side effects, and
   that f does actually have type unit.
*)
let opt_fold_to_unit = ref []

let fold_to_unit id =
  let remove =
    !opt_fold_to_unit
    |> List.map mk_id
    |> List.fold_left (fun m id -> IdSet.add id m) IdSet.empty
  in
  IdSet.mem id remove

let rec is_constant (E_aux (e_aux, _) as exp) =
  match e_aux with
  | E_lit _ -> true
  | E_vector exps -> List.for_all is_constant exps
  | E_record fexps -> List.for_all is_constant_fexp fexps
  | E_cast (_, exp) -> is_constant exp
  | E_tuple exps -> List.for_all is_constant exps
  | E_id id ->
     (match Env.lookup_id id (env_of exp) with
      | Enum _ -> true
      | _ -> false)
  | _ -> false

and is_constant_fexp (FE_aux (FE_Fexp (_, exp), _)) = is_constant exp

(* Wrapper around interpreter that repeatedly steps until done. *)
let rec run frame =
  match frame with
  | Interpreter.Done (state, v) -> v
  | Interpreter.Fail _ ->
     (* something went wrong, raise exception to abort constant folding *)
     assert false
  | Interpreter.Step (lazy_str, _, _, _) ->
     run (Interpreter.eval_frame frame)
  | Interpreter.Break frame ->
     run (Interpreter.eval_frame frame)
  | Interpreter.Effect_request (out, st, stack, Interpreter.Read_reg (reg, cont)) ->
     (* return a dummy value to read_reg requests which we handle above
        if an expression finally evals to it, but the interpreter
        will fail if it tries to actually use. See value.ml *)
     run (cont (Value.V_attempted_read reg) st)
  | Interpreter.Effect_request _ ->
     assert false (* effectful, raise exception to abort constant folding *)

(** This rewriting pass looks for function applications (E_app)
   expressions where every argument is a literal. It passes these
   expressions to the OCaml interpreter in interpreter.ml, and
   reconstructs the values returned back into expressions which are
   then re-typechecked and re-inserted back into the AST.

   We don't use the effect system to decide if expressions are safe to
   evaluate, because this ignores I/O, and would force us to ignore
   functions that maybe throw exceptions internally but as called are
   totally safe. Instead any exceptions during evaluation are caught,
   and the original expression is kept. Some causes of this could be:

   - Function tries to read/write register.
   - Calls an unsafe primop.
   - Throws an exception that isn't caught.
 *)

let initial_state ast env =
  Interpreter.initial_state ~registers:false ast env safe_primops

let rw_exp target ok not_ok istate =
  let evaluate e_aux annot =
    let initial_monad = Interpreter.return (E_aux (e_aux, annot)) in
    try
      begin
        let v = run (Interpreter.Step (lazy "", istate, initial_monad, [])) in
        let exp = exp_of_value v in
        try (ok (); Type_check.check_exp (env_of_annot annot) exp (typ_of_annot annot)) with
        | Type_error (env, l, err) ->
           (* A type error here would be unexpected, so don't ignore it! *)
           Reporting.warn "" l
             ("Type error when folding constants in "
              ^ string_of_exp (E_aux (e_aux, annot))
              ^ "\n" ^ Type_error.string_of_type_error err);
           not_ok ();
           E_aux (e_aux, annot)
      end
    with
    (* Otherwise if anything goes wrong when trying to constant
       fold, just continue without optimising. *)
    | _ -> E_aux (e_aux, annot)
  in

  let rw_funcall e_aux annot =
    match e_aux with
    | E_app (id, args) when fold_to_unit id ->
       ok (); E_aux (E_lit (L_aux (L_unit, fst annot)), annot)

    | E_app (id, args) when List.for_all is_constant args ->
       let env = env_of_annot annot in
       (* We want to fold all primitive operations, but avoid folding
          non-primitives that are defined in target-specific way. *)
       let is_primop =
         Env.is_extern id env "interpreter" && StringMap.mem (Env.get_extern id env "interpreter") safe_primops
       in
       if not (Env.is_extern id env target) || is_primop then
         evaluate e_aux annot
       else
         E_aux (e_aux, annot)

    | E_cast (typ, (E_aux (E_lit _, _) as lit)) -> ok (); lit

    | E_field (exp, id) when is_constant exp ->
       evaluate e_aux annot

    | E_if (E_aux (E_lit (L_aux (L_true, _)), _), then_exp, _) -> ok (); then_exp
    | E_if (E_aux (E_lit (L_aux (L_false, _)), _), _, else_exp) -> ok (); else_exp

    (* We only propagate lets in the simple case where we know that
       the id will have the inferred type of the argument. For more
       complex let bindings trying to propagate them may result in
       type errors due to how type variables are bound by let bindings
       *)
    | E_let (LB_aux (LB_val (P_aux (P_id id, _), bind), _), exp) when is_constant bind ->
       ok ();
       subst id bind exp

    | _ -> E_aux (e_aux, annot)
  in
  fold_exp { id_exp_alg with e_aux = (fun (e_aux, annot) -> rw_funcall e_aux annot)}

let rewrite_exp_once target = rw_exp target (fun _ -> ()) (fun _ -> ())

let rec rewrite_constant_function_calls' target ast =
  let rewrite_count = ref 0 in
  let ok () = incr rewrite_count in
  let not_ok () = decr rewrite_count in
  let istate = initial_state ast Type_check.initial_env in

  let rw_defs = {
      rewriters_base with
      rewrite_exp = (fun _ -> rw_exp target ok not_ok istate)
    } in
  let ast = rewrite_defs_base rw_defs ast in
  (* We keep iterating until we have no more re-writes to do *)
  if !rewrite_count > 0
  then rewrite_constant_function_calls' target ast
  else ast

let rewrite_constant_function_calls target ast =
  if !optimize_constant_fold then
    rewrite_constant_function_calls' target ast
  else
    ast