GNU bug report logs - #42147
28.0.50; pure vs side-effect-free, missing optimizations?

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#42147: closed (28.0.50; pure vs side-effect-free, missing
 optimizations?)
Date: Tue, 07 Jul 2020 15:56:01 +0000

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Your message dated Tue, 7 Jul 2020 17:54:56 +0200
with message-id <F368819C-EE60-4172-B2F0-95560C406AED <at> acm.org>
and subject line Re: bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?
has caused the debbugs.gnu.org bug report #42147,
regarding 28.0.50; pure vs side-effect-free, missing optimizations?
to be marked as done.

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From: Andrea Corallo <andrea_corallo <at> yahoo.it>
To: "bug-gnu-emacs <at> gnu.org" <bug-gnu-emacs <at> gnu.org>
Subject: 28.0.50; pure vs side-effect-free, missing optimizations?
Date: Tue, 30 Jun 2020 22:27:12 +0000 (UTC)

Hi all,

I was looking in byte-opt.el to how are classified a number of
functions.

My understanding is that pure functions should be technically a subset
of side-effect-and-error-free for which the environment has no influence
on the value the function evaluates to.  For this reason they can be
constant folded in the compile time if possible.

Now in pure functions I see we do not have a lot of functions that (to
my understanding) would classify for that.  I'm thinking to simple
predicates acting on immutable objects as consp or fixnump to give an
example.

Shouldn't we move these into the pure class?

ATM for instance this does not get optimized:

(defun foo ()
  (fixnump 3))

In case this makes sense I'll be happy to work on it and prepare a
patch.

Thanks

  Andrea

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From: Mattias Engdegård <mattiase <at> acm.org>
To: Andrea Corallo <andrea_corallo <at> yahoo.it>
Cc: 42147-done <at> debbugs.gnu.org, Paul Eggert <eggert <at> cs.ucla.edu>,
 Stefan Monnier <monnier <at> iro.umontreal.ca>, Andrea Corallo <akrl <at> sdf.org>
Subject: Re: bug#42147: 28.0.50; pure vs side-effect-free, missing
 optimizations?
Date: Tue, 7 Jul 2020 17:54:56 +0200

6 juli 2020 kl. 23.23 skrev Andrea Corallo <andrea_corallo <at> yahoo.it>:

> Yes I am happy :) Not sure about the floating point discussion
> originated from this but on my side this bug can be closed.

Then closed it is. I would happily write something in NEWS but -- as Eli noted -- for any noticeable change in behaviour to occur, many conditions need to be met, several of which are quite unlikely.

More improvements to the constant-folding are possible and desirable. For example, I have a patch that deals with constant expressions in let-bindings, so that

(let ((x (+ 1 2)))
  (f x))

simplifies to (f 3), with the variable x removed. This in turn generates more opportunities for further simplification and dead-code elimination. Tell me if you are interested.

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