From: Stefan Monnier <monnier@iro.umontreal.ca>
To: Alan Mackenzie <acm@muc.de>
Cc: Rocky Bernstein <rocky@gnu.org>, emacs-devel <emacs-devel@gnu.org>
Subject: Re: Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects]
Date: Fri, 20 Mar 2020 17:30:47 -0400 [thread overview]
Message-ID: <jwvy2ruu41w.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <20200320201005.GC5255@ACM> (Alan Mackenzie's message of "Fri, 20 Mar 2020 20:10:05 +0000")
>> I think fat-cons cells are cheap to implement (with (hopefully) no
>> performance impact when not used .....
> They may be cheap to implement in themselves, but adapting the entire
> byte compiler and all our macros to the heavily restricted semantics
> they would impose would be an enormous job.
The idea is that you want to make it work acceptably even if only some
of the cons-cells are fat. This way, as you adapt the existing code to
pay attention/preserve fat-cons-cells, your location information gets
more and more precise, but even before you've done this enormous job,
you already get some of the benefit.
> I've tried something similar, and gave up in exhaustion.
If you want "exact" results, then you'll get tired long before getting
there, yes. But it's not needed.
> Where does this 99.9% come from? How is this cons tracking you're
> proposing supposed to work, when there are an infinite number of
> occurrences of the likes of
>
> (cons (car form) (cdr form))
>
> in our code?
This still preserves info inside the fat-cons-cells contained in (car
form) and (cdr form), so it's not as bad as it looks.
Of course, when such code is applied recursively on all sub-expressions
(i.e. in a code-walker such as macroexpand-all, cconv, and byte-opt)
then we lose all the info, so we do need to change those before we can
benefit, but AFAICT those 3 are the only crucial ones (there are a few
other code-walkers around, such as generator.el) and hopefully some of
that rewrite can be made fairly mechanically.
> Are you saying that this is how other Lisp compilers deal with source
> code positions? How do they deal with the difficult problem of user
> macros?
Not sure about Common-Lisp, but Scheme systems deal with it by
distinguishing "sexp" from "syntax objects" where syntax objects are
basically sexps wrapped (recursively) within location wrappers.
> I think there's quite a bit of doubt as to whether this could work
> effectively in Emacs.
I have no doubt that it can work.
I am not sure it'll be acceptable, OTOH, because it will depend on the
overhead it will impose on the execution of the byte-compiler.
> The way to dispel this doubt is for Somebody (tm) to implement it.
Exactly.
> To which the answer is to install the working solution pending the
> implementation of something better, after which it can be superseded.
Ever heard of temporary hacks that end up permanent?
Take for example the issue of .... oh, I don't know ... line numbers in
error messages? ;-)
To a large extent the reason we don't have better line-numbers right now
is because of the hack we accepted some years ago, so now instead of
working on "giving line-numbers in error messages", we're reduced to
"improve the precision of line-numbers in error messages" which is not
nearly as pressing an issue.
Stefan
prev parent reply other threads:[~2020-03-20 21:30 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-19 15:10 GNU is looking for Google Summer of Code Projects Rocky Bernstein
2020-03-19 17:35 ` Stefan Monnier
2020-03-19 17:56 ` Andrea Corallo
2020-03-19 18:05 ` Andrea Corallo
2020-03-19 18:19 ` Rocky Bernstein
2020-03-19 21:26 ` Stefan Monnier
2020-03-19 21:45 ` Andrea Corallo
2020-03-19 23:07 ` Rocky Bernstein
2020-03-19 20:34 ` Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects] Alan Mackenzie
2020-03-19 20:43 ` Andrea Corallo
2020-03-20 19:18 ` Alan Mackenzie
2020-03-21 11:22 ` Andrea Corallo
2020-03-21 15:30 ` Correct line/column numbers in byte compiler messages Alan Mackenzie
2020-03-21 16:28 ` Andrea Corallo
2020-03-21 18:37 ` Andrea Corallo
2020-03-21 20:19 ` Alan Mackenzie
2020-03-21 21:08 ` Andrea Corallo
2020-03-21 23:39 ` Andrea Corallo
2020-03-22 11:26 ` Alan Mackenzie
2020-03-19 20:56 ` Correct line/column numbers in byte compiler messages [Was: GNU is looking for Google Summer of Code Projects] Rocky Bernstein
2020-03-19 22:05 ` Stefan Monnier
2020-03-20 19:25 ` Alan Mackenzie
2020-03-19 21:41 ` Stefan Monnier
2020-03-19 22:09 ` Stefan Monnier
2020-03-20 20:10 ` Alan Mackenzie
2020-03-20 21:23 ` Rocky Bernstein
2020-03-20 21:27 ` Clément Pit-Claudel
2020-03-20 23:46 ` Stefan Monnier
2020-03-20 21:30 ` Stefan Monnier [this message]
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