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|
(* *********************************************************************)
(* *)
(* The Compcert verified compiler *)
(* *)
(* Xavier Leroy, INRIA Paris-Rocquencourt *)
(* *)
(* Copyright Institut National de Recherche en Informatique et en *)
(* Automatique. All rights reserved. This file is distributed *)
(* under the terms of the GNU General Public License as published by *)
(* the Free Software Foundation, either version 2 of the License, or *)
(* (at your option) any later version. This file is also distributed *)
(* under the terms of the INRIA Non-Commercial License Agreement. *)
(* *)
(* *********************************************************************)
(* A simple control flow analysis for C statements.
Main purpose: emit warnings for _Noreturn functions. *)
open C
open Cutil
(* Statements are abstracted as "flow transformers":
functions from possible inputs to possible outcomes.
Possible inputs are:
- start normally at the beginning of the statement
- start at a "case" or "default" label because of an enclosing switch
Possible outputs are:
- terminate normally and fall through
- terminate early on "break"
- terminate early on "continue"
- terminate early on "return"
*)
type flow = bool -> (* start normally at beginning of statement *)
bool -> (* start at "case"/"default" label *)
int
(* Outputs are encoded as bit masks in an integer *)
let _None = 0
let _Fallthrough = 1
let _Break = 2
let _Continue = 4
let _Return = 8
let can flag out = out land flag <> 0
let add flag out = out lor flag
let remove flag out = out land (lnot flag)
let join out1 out2 = out1 lor out2
(* Some elementary flows *)
let normal : flow = fun i is -> if i then _Fallthrough else _None
let break : flow = fun i is -> if i then _Break else _None
let continue : flow = fun i is -> if i then _Continue else _None
let return : flow = fun i is -> if i then _Return else _None
let noflow : flow = fun i is -> _None
(* Some flow transformers *)
(* Sequential composition *)
let seq (s1: flow) (s2: flow) : flow = fun i is ->
let o1 = s1 i is in
let o2 = s2 (can _Fallthrough o1) is in
join (remove _Fallthrough o1) o2
(* Nondeterministic choice *)
let either (s1: flow) (s2: flow) : flow = fun i is ->
join (s1 i is) (s2 i is)
(* If-then-else, with special cases for conditions that are always true
or always false. *)
let resolve_test env e =
match Ceval.integer_expr env e with
| None -> None
| Some n -> Some (n <> 0L)
let if_ env e (s1: flow) (s2: flow) : flow =
match resolve_test env e with
| Some true -> s1
| Some false -> s2
| None -> either s1 s2
(* Convert "continue" into "fallthrough". Typically applied to a loop body. *)
let catch_continue (s: flow) : flow = fun i is ->
let o = s i is in
if can _Continue o then add _Fallthrough (remove _Continue o) else o
(* Convert "continue" into "fallthrough". Typically applied to a loop. *)
let catch_break (s: flow) : flow = fun i is ->
let o = s i is in
if can _Break o then add _Fallthrough (remove _Break o) else o
(* For goto-labeled statements we assume they can always be entered normally. *)
let label (s: flow) : flow = fun i is -> s true is
(* For "case" and "default" labeled statements, we assume they can be entered
normally as soon as the nearest enclosing "switch" can be entered normally. *)
let case (s: flow) : flow = fun i is -> s is is
let switch (s: flow) : flow = fun i is -> s false i
(* This is the flow for an infinite loop with body [s].
There is no fallthrough exit, but all other exits from [s] are preserved. *)
let loop (s: flow) : flow = fun i is ->
let o = s i is in
if (not i) && can _Fallthrough o then
(* Corner case: on the first iteration, [s] was not entered normally,
but it exits by fallthrough. Then on the next iteration [s] is
entered normally. So, we need to recompute the flow of [s]
assuming it is entered normally. *)
s true is
else
(* In all other cases, iterating [s] once more does not produce new
exits that we did not compute on the first iteration. Just remove
the fallthrough exit. *)
remove _Fallthrough o
(* This is the main analysis function. Given a C statement [s] it returns
a flow that overapproximates the behavior of [s]. *)
let rec outcomes env s : flow =
match s.sdesc with
| Sskip ->
normal
| Sdo {edesc = ECall(fn, args)} ->
if find_custom_attributes ["noreturn"; "__noreturn__"]
(attributes_of_type env fn.etyp) = []
then normal else noflow
| Sdo e ->
normal
| Sseq(s1, s2) ->
seq (outcomes env s1) (outcomes env s2)
| Sif(e, s1, s2) ->
if_ env e (outcomes env s1) (outcomes env s2)
| Swhile(e, s) ->
catch_break (
loop (
if_ env e
(catch_continue (outcomes env s)) (* e is true: execute body [s] *)
break)) (* e is false: exit loop *)
| Sdowhile(s, e) ->
catch_break (
loop (
seq (catch_continue (outcomes env s)) (* do the body *)
(if_ env e normal break))) (* then continue or break *)
| Sfor(s1, e, s2, s3) ->
seq (outcomes env s1) (* initialization, runs once *)
(catch_break (
loop (
if_ env e (* e is true: do body, do next *)
(seq (catch_continue (outcomes env s2)) (outcomes env s3))
break))) (* e is false: exit loop *)
| Sbreak ->
break
| Scontinue ->
continue
| Sswitch(e, s) ->
catch_break (switch (outcomes env s))
| Slabeled(Slabel _, s) ->
label (outcomes env s)
| Slabeled((Scase _ | Sdefault), s) ->
case (outcomes env s)
| Sgoto _ ->
noflow
| Sreturn opte ->
return
| Sblock sl ->
outcomes_block env sl
| Sdecl dcl ->
normal
| Sasm _ ->
normal
and outcomes_block env sl =
match sl with
| [] ->
normal
| s1 :: sl ->
seq (outcomes env s1) (outcomes_block env sl)
(* This is the entry point in this module. Given the body of a function,
estimate if and how this function can return. *)
let function_returns env body =
let o = outcomes env body true false in
(* First boolean is: function can return
Second boolean is: function can return by fall-through *)
(can _Fallthrough o || can _Return o, can _Fallthrough o)
|
