GNU bug report logs - #51982
Erroneous handling of local variables in byte-compiled nested lambdas

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Package: emacs;

Reported by: Paul Pogonyshev <pogonyshev <at> gmail.com>

Date: Fri, 19 Nov 2021 20:32:02 UTC

Severity: normal

Tags: patch

Fixed in version 29.1

Done: Lars Ingebrigtsen <larsi <at> gnus.org>

Bug is archived. No further changes may be made.

Full log

Message #67 received at 51982 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Michael Heerdegen <michael_heerdegen <at> web.de>,
 Paul Pogonyshev <pogonyshev <at> gmail.com>, 51982 <at> debbugs.gnu.org
Subject: Re: bug#51982: Erroneous handling of local variables in byte-compiled
 nested lambdas
Date: Thu, 2 Dec 2021 10:13:31 +0100

> Yes, and I can confirm that both transformations (let* -> let and the inverse) seem to generate almost or exactly identical bytecode for lexbind code. If you can find an example where it's worse, I'd like to know.

And by "yes" I mean "no", since the identical bytecode is only true in the absence of mutation which, as always, throws a spanner in the works. For example, while

(let ((x (sin y)) (y (cos x)))
  (+ x y))))
->
(let* ((x0 (sin y)) (y (cos x)) (x x0))
  (+ x y))))

is a transformation with identical bytecode, mutating one of the variables:

(let ((x (sin y)) (y (cos x)))
  (setq x (1+ x))
  (+ x y))))
->
(let* ((x0 (sin y)) (y (cos x)) (x x0))
  (setq x (1+ x))
  (+ x y))))

will prevent the intermediate from being eliminated. Not a disaster by any means but not good enough as a general method of translation.
This bug report was last modified 2 years and 249 days ago.

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