From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (from majordomo@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id PAA28929; Thu, 27 Sep 2001 15:28:12 +0200 (MET DST) X-Authentication-Warning: pauillac.inria.fr: majordomo set sender to owner-caml-list@pauillac.inria.fr using -f Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id PAA28925 for ; Thu, 27 Sep 2001 15:28:11 +0200 (MET DST) Received: from smtp1.cswv.com (smtp1.cswv.com [4.17.129.17]) by concorde.inria.fr (8.11.1/8.10.0) with ESMTP id f8RDSAX09866 for ; Thu, 27 Sep 2001 15:28:11 +0200 (MET DST) Received: from smtp1.cswv.com ([4.17.129.17]) by smtp1.cswv.com with Microsoft SMTPSVC(5.5.1877.197.19); Thu, 27 Sep 2001 09:28:58 -0400 Received: FROM exchange1.cswv.com BY smtp1.cswv.com ; Thu Sep 27 09:28:57 2001 -0400 Received: by exchange1.cswv.com with Internet Mail Service (5.5.2653.19) id ; Thu, 27 Sep 2001 09:32:27 -0400 Message-ID: From: "Krishnaswami, Neel" To: caml-list@inria.fr Subject: Re: [Caml-list] Error Reporting Date: Thu, 27 Sep 2001 09:32:26 -0400 MIME-Version: 1.0 X-Mailer: Internet Mail Service (5.5.2653.19) Content-Type: text/plain; charset="windows-1252" Sender: owner-caml-list@pauillac.inria.fr Precedence: bulk David McClain [mailto:barabh@qwest.net] wrote: > > I am looking for some ideas on handling program error reporting for > a compiler that is built largely on CPS. Since there are no stack > frames to crawl for traceback, I have a first cut based on keeping a > finite length queue of last visited closures. But this becomes > problematic when there are circularities in the code, i.e., > recursion. Also, I think I have way too much error tracking going on > at each incremental execution step and so runtime performance is cut > back compared to what it could be. Since you're using CPS, it seems like you have most of the infrastructure you need already. If you track source locations from parsing down to code generation, you can place a source location into each frame as you allocate it. Then, when you have an error, you can simply walk back down the contination, emitting the source location of each frame. Since the continuation represents the rest of the computation, you automatically get the exact execution context of the erroneous statement. The overhead for this is one word per frame, plus initializing it at each continuation allocation. This is probably acceptable unless you are writing a really high-performance compiler. Also, tail-call optimization will make the traceback only approximate, but this is IME not a big deal in practice. Below is a small CPS-style lambda calculus interpreter that uses this idea to generate tracebacks of its evaluation. It's a treecode interpreter, but the ideas ought to carry over directly to bytecode too. -*-*-*- type var = string type pos = int (* Expressions carry the source location with them *) type expr = | Lit of pos * int | Var of pos * var | Fun of pos * var * expr | App of pos * expr * expr (* Continuation frames have the position as an extra argument, so that the continuation as a whole contains the backtrace info. Primitive functions take the continuation as an extra argument so that they can generate a backtrace on an error. *) type denot = | Int of int | Closure of env * var * expr | Primop of (denot -> cont -> denot) and frame = | KFun of pos * env * expr | KArg of pos * env * denot and env = (var * denot) list and cont = frame list (* Here are the guts of the machinery to generate a backtrace. We need to extract a position from a frame and then just map over the whole continuation to get the backtrace. *) exception Eval_error of string * pos list let framepos frame = match frame with | KFun(pos, _, _) -> pos | KArg(pos, _, _) -> pos let backtrace k = List.map framepos k (* We augment variable lookup so that failed lookups generate backtraces. If you check statically for unbound variables this is unneccesary. *) let bind v d r = (v,d) :: r let lookup v r k = try List.assoc v r with Not_found -> raise (Eval_error("Unbound variable " ^ v, backtrace k)) (* In the interpreter proper, the only place where a backtrace is generated is when an argument is applied to a non-function. If an if-statement is added, then that should be too. Likewise, if exceptions are added then an uncaught exception should generate one also. *) let rec eval e r k = match e with | Lit(pos, n) -> next (Int n) k | Var(pos, v) -> next (lookup v r k) k | Fun(pos, formal, body) -> next (Closure(r, formal, body)) k | App(pos, f, arg) -> eval f r (KFun(pos, r, arg) :: k) and next d k = match k with | [] -> d | KFun(pos, r, arg) :: k' -> eval arg r k' | KArg(pos, r, func) :: k' -> (match func with | Int _ -> raise (Eval_error("Not a function!", backtrace k)) | Closure(r, formal, body) -> eval body (bind formal d r) k' | Primop f -> next (f d k) k') -- Neel Krishnaswami neelk@cswcasa.com ------------------- Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr