From: Pip Cet <pipcet@gmail.com>
To: Stefan Monnier <monnier@iro.umontreal.ca>
Cc: 34757@debbugs.gnu.org, chuntaro@sakura-games.jp
Subject: bug#34757: Invalid bytecode from byte compiler
Date: Sat, 16 Mar 2019 16:51:13 +0000 [thread overview]
Message-ID: <CAOqdjBepQQroLqafjqPuhsevvwWCkt8kpPOR1dfO5b9HM9A3ew@mail.gmail.com> (raw)
In-Reply-To: <jwvh8c39b5r.fsf-monnier+emacs@gnu.org>
On Fri, Mar 15, 2019 at 8:30 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>
> > Just to be sure I understand correctly, you would like to remove the
> > decompilation of trivial function calls entirely?
>
> Yes, tho the main motivation was to try and figure out what the
> decompilation is useful for.
Thanks for explaining!
> This only affects "open code" (i.e. not the content of functions, which
> are already never decompiled), so the impact should be minor and it
> removes a rather complicated and brittle chunk of code whose purpose is
> not clear. It was originally introduced when we didn't have
> byte-compiled function objects, so its main purpose was one of
> performance, to avoid pessimizing the code by replacing trivial function
> calls with more costly (byte-code "...") expressions but nowadays such
> (byte-code "...") expressions only occur basically at the top-level of
> .elc files where such pessimization would be unnoticeable in terms
> of performance.
I agree completely, for what it's worth.
> It does impact the readability of .elc files, OTOH, so I'm not
> completely happy with the result when considering the few cases where
> I was happy to be able to make sense of a .elc file to better understand
> the source of a problem (after all, that's why I wrote the
> elisp-byte-code-mode).
I can speak only for myself, but I think the byte-compiler "magically"
deciding to turn (rare) top-level non-defuns into plain code rather
than byte code is confusing. It's much better with your patches.
> > It seems the special case is necessary to avoid compilation errors,
>
> I haven't found it to be really necessary, no.
Well, you fixed it with the second patch.
> > and that it's used for (byte-compile 3), so I think the comment could
> > be improved a little.
>
> (byte-compile 3) seems to work correctly here even without the
> special case. It returns (byte-code "\300\207" [3] 1) which is indeed
> a correct expression that evaluates to 3 (just like the argument to
> `byte-compile` was an expression whose evaluation returns 3).
No argument here. The case branch affects what happens when
(byte-compile 3) is called, but isn't necessary for the result to be
correct, and the comment can be misread to imply it's irrelevant in
this case.
> Let's not forget that what `byte-compile` tries to do is to preserve the
> invariant that
>
> (eval EXP) ≃ (eval (byte-compile EXP))
I think byte-compile does different things for different arguments:
the behavior for symbols and other expressions is simply different.
> This said, if you remove the special case, you will bump into
> a corner-case bug in `byte-compile` which happens because that function
> incorrectly considers that `byte-compile-top-level` returns a value
> whereas in reality it returns an expression (just like `byte-compile`):
> the decompilation happens to turn expressions that return constant
> values (like byte-compiled functions) into their value (as an
> optimization which relies on the fact that those objects are
> self-evaluating), so if you remove it, you then bump into this bug of
> byte-compile. The patch below would fix this bug.
I don't think that was a bug, but it was an unfortunate wrinkle in the
(undocumented) API of byte-compile-top-level.
> But indeed the decompilation of constants is handy since that's what
> people expect from `byte-compile`. When I (byte-compile '(lambda (x)
> (foo))) I expect to receive a byte-compiled function, and not
> a byte-code expression which when evaluated will return that
> byte-compiled function.
I think it's more than handy: it's how I'd read the current
documentation, and how I'd expect a function called byte-compile to
behave.
> > I'm not sure this case can actually happen without doing something
> > silly, but (byte-compile '(internal-get-closed-var 0)) throws an error
> > with Stefan's patch, because the byte code is (byte-constant . 0)
> > (byte-return).
>
> This source code is arguably invalid, so it's not a real problem, but
The source code is invalid, but the LAP code is valid-looking, and I
can't conclude it cannot be generated by valid source code being
passed to `byte-compile' somehow.
> diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
> index f46cab2c17..ae17553d0c 100644
> --- a/lisp/emacs-lisp/bytecomp.el
> +++ b/lisp/emacs-lisp/bytecomp.el
> @@ -2674,7 +2674,11 @@ byte-compile
> (setq fun (byte-compile-top-level fun nil 'eval)))
> (if macro (push 'macro fun))
> (if (symbolp form)
> - (fset form fun)
> + ;; byte-compile returns an *expression* equivalent to the
I think this would be clearer if it read "byte-compile-top-level
returns an *expression*..."
next prev parent reply other threads:[~2019-03-16 16:51 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-05 8:01 bug#34757: Invalid bytecode from byte compiler chuntaro
2019-03-08 13:18 ` Eli Zaretskii
2019-03-08 13:50 ` Michael Heerdegen
2019-03-08 14:36 ` Eli Zaretskii
2019-03-08 21:13 ` Pip Cet
2019-03-15 8:08 ` Eli Zaretskii
2019-03-15 14:08 ` Stefan Monnier
2019-03-15 19:40 ` Pip Cet
2019-03-15 20:30 ` Stefan Monnier
2019-03-16 16:51 ` Pip Cet [this message]
2019-06-13 11:44 ` Pip Cet
2019-07-27 21:30 ` Stefan Monnier
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAOqdjBepQQroLqafjqPuhsevvwWCkt8kpPOR1dfO5b9HM9A3ew@mail.gmail.com \
--to=pipcet@gmail.com \
--cc=34757@debbugs.gnu.org \
--cc=chuntaro@sakura-games.jp \
--cc=monnier@iro.umontreal.ca \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.