path: root/backend/Tunnelinglibs.ml

(* *************************************************************)
(*                                                             *)
(*             The Compcert verified compiler                  *)
(*                                                             *)
(*           Sylvain Boulmé  Grenoble-INP, VERIMAG             *)
(*           TODO: Proper author information                   *)
(*                                                             *)
(*  Copyright VERIMAG. All rights reserved.                    *)
(*  This file is distributed under the terms of the INRIA      *)
(*  Non-Commercial License Agreement.                          *)
(*                                                             *)
(* *************************************************************)

(*

This file implements the core functions of the tunneling passes, for both RTL
and LTL, by using a simplified CFG as a transparent interface

See [Tunneling.v] and [RTLTunneling.v]

*)

open Maps
open Camlcoq

(* type of labels in the cfg *)
type label = int * P.t

(* instructions under analyzis *)
type simple_inst = (* a simplified view of instructions *)
  BRANCH of node
| COND of node * node
| OTHER
and node = {
    lab: label;
    mutable inst: simple_inst;
    mutable link: node; (* link in the union-find: itself for non "nop"-nodes, target of the "nop" otherwise *)
    mutable dist: int;
    mutable tag: int
  }

type cfg_node = (int, node) Hashtbl.t

type positive = P.t
type integer = Z.t

(* type of the (simplified) CFG *)
type cfg = {
    nodes: cfg_node;
    mutable rems: node list; (* remaining conditions that may become lbranch or not *)
    mutable num_rems: int;
    mutable iter_num: int (* number of iterations in elimination of conditions *)
  }

module Tunneling = functor
  (LANG: sig
    type code_unit (* the type of a node of the code cfg (an instruction or a bblock *)
    type funct
  end)
  (OPT: sig
    val langname: string
    val limit_tunneling: int option (* for debugging: [Some x] limit the number of iterations *)
    val debug_flag: bool ref
    val final_dump: bool (* set to true to have a more verbose debugging *)
  end)
  (FUNS: sig
    (* build [c.nodes] and accumulate in [acc] conditions at beginning of LTL basic-blocks *)
    val build_simplified_cfg: cfg -> node list -> positive -> LANG.code_unit -> node list

    val print_code_unit: cfg -> bool -> int * LANG.code_unit -> bool

    val fn_code: LANG.funct -> LANG.code_unit PTree.t
    val fn_entrypoint: LANG.funct -> positive

    val check_code_unit: (positive * integer) PTree.t -> positive -> LANG.code_unit -> unit
  end)
-> struct

let debug fmt =
  if !OPT.debug_flag then Printf.eprintf fmt
  else Printf.ifprintf stderr fmt

exception BugOnPC of int

let lab_i (n: node): int = fst n.lab
let lab_p (n: node): P.t = snd n.lab

let rec target c n = (* inspired from the "find" of union-find algorithm *)
  match n.inst with
  | COND(s1,s2) ->
     if n.link != n
     then update c n
     else if n.tag < c.iter_num then (
       (* we try to change the condition ... *)
       n.tag <- c.iter_num; (* ... but at most once by iteration *)
       let ts1 = target c s1 in
       let ts2 = target c s2 in
       if ts1 == ts2 then (n.link <- ts1; ts1) else n
     ) else n
  | _ ->
     if n.link != n
     then update c n
     else n
and update c n =
  let t = target c n.link in
  n.link <- t; t

let get_node c p =
  let li = P.to_int p in
  try
    Hashtbl.find c.nodes li
  with
    Not_found ->
      let rec n = { lab = (li, p); inst = OTHER; link = n ; dist = 0; tag = 0 }  in
      Hashtbl.add c.nodes li n;
      n

let set_branch c p s =
  let li = P.to_int p in
  try
    let n = Hashtbl.find c.nodes li in
    n.inst <- BRANCH s;
    n.link <- target c s
  with
    Not_found ->
      let n = { lab = (li,p); inst = BRANCH s; link = target c s; dist = 0; tag = 0 } in
      Hashtbl.add c.nodes li n



(* try to change a condition into a branch
[acc] is the current accumulator of conditions to consider in the next iteration of repeat_change_cond
*)
let try_change_cond c acc pc =
  match pc.inst with
  | COND(s1,s2) ->
     let ts1 = target c s1 in
     let ts2 = target c s2 in
     if ts1 == ts2 then (
       pc.link <- ts1;
       c.num_rems <- c.num_rems - 1;
       acc
     ) else
       pc::acc
  | _  -> raise (BugOnPC (lab_i pc)) (* COND expected *)

(* repeat [try_change_cond] until no condition is changed into a branch *)
let rec repeat_change_cond c =
  c.iter_num <- c.iter_num + 1;
  debug "++ %sTunneling.branch_target %d: remaining number of conds to consider = %d\n" OPT.langname (c.iter_num) (c.num_rems);
  let old =  c.num_rems in
  c.rems <- List.fold_left (try_change_cond c) [] c.rems;
  let curr = c.num_rems in
  let continue =
    match OPT.limit_tunneling with
    | Some n -> curr < old && c.iter_num < n
    | None -> curr < old
  in
  if continue
  then repeat_change_cond c


(* compute the final distance of each nop nodes to its target *)
let undef_dist = -1
let self_dist = undef_dist-1
let rec dist n =
  if n.dist = undef_dist
  then (
    n.dist <- self_dist; (* protection against an unexpected loop in the data-structure *)
    n.dist <-
      (match n.inst with
       | OTHER -> 0
       | BRANCH p -> 1 + dist p
       | COND (p1,p2) -> 1 + (max (dist p1) (dist p2)));
    n.dist
  ) else if n.dist=self_dist then raise (BugOnPC (lab_i n))
    else n.dist

let final_export f c =
  let count = ref 0 in
  let filter_nops_init_dist _ n acc =
    let tn = target c n in
    if tn == n
    then (
      n.dist <- 0; (* force [n] to be a base case in the recursion of [dist] *)
      acc
    ) else (
      n.dist <- undef_dist; (* force [dist] to compute the actual [n.dist] *)
      count := !count+1;
      n::acc
    )
  in
  let nops = Hashtbl.fold filter_nops_init_dist c.nodes [] in
  let res = List.fold_left (fun acc n -> PTree.set (lab_p n) (lab_p n.link, Z.of_uint (dist n)) acc) PTree.empty nops in
  debug "* %sTunneling.branch_target: final number of eliminated nops = %d\n"
  OPT.langname !count;
  res

(*********************************************)
(*** START: printing and debugging functions *)

let string_of_labeli nodes ipc =
  try
    let pc = Hashtbl.find nodes ipc in
    if pc.link == pc
    then Printf.sprintf "(Target@%d)" (dist pc)
    else Printf.sprintf "(Nop %d @%d)" (lab_i pc.link) (dist pc)
  with
    Not_found -> ""

let print_cfg (f: LANG.funct) c  =
  let a = Array.of_list (PTree.fold (fun acc pc cu -> (P.to_int pc,cu)::acc) (FUNS.fn_code f) []) in
  Array.fast_sort (fun (i1,_) (i2,_) -> i2 - i1) a;
  let ep = P.to_int (FUNS.fn_entrypoint f) in
  debug "entrypoint: %d %s\n" ep (string_of_labeli c.nodes ep);
  let println = Array.fold_left (FUNS.print_code_unit c) false a in
  (if println then debug "\n");debug "remaining cond:";
  List.iter (fun n -> debug "%d " (lab_i n)) c.rems;
  debug "\n"

(*************************************************************)
(* Copy-paste of the extracted code of the verifier          *)
(* with [raise (BugOnPC (P.to_int pc))] instead of [Error.*] *)

(** val check_code : coq_UF -> code -> unit res **)

let check_code td c =
  PTree.fold (fun _ pc cu -> FUNS.check_code_unit td pc cu) c (())

(*** END: copy-paste & debugging functions *******)

let branch_target f =
  debug "* %sTunneling.branch_target: starting on a new function\n" OPT.langname;
  if OPT.limit_tunneling <> None then debug "* WARNING: limit_tunneling <> None\n";
  let c = { nodes = Hashtbl.create 100; rems = []; num_rems = 0; iter_num = 0 } in
  c.rems <- PTree.fold (FUNS.build_simplified_cfg c) (FUNS.fn_code f) [];
  repeat_change_cond c;
  let res = final_export f c in
  if !OPT.debug_flag then (
    try
      check_code res (FUNS.fn_code f);
      if OPT.final_dump then print_cfg f c;
    with e -> (
      print_cfg f c;
      check_code res (FUNS.fn_code f)
    )
  );
  res

end