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

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From: Philipp Stephani <p.stephani2 <at> gmail.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: Mattias Engdegård <mattiase <at> acm.org>, Paul Eggert <eggert <at> cs.ucla.edu>, Andrea Corallo <andrea_corallo <at> yahoo.it>, 42147 <at> debbugs.gnu.org
Subject: bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?
Date: Sat, 25 Jul 2020 22:11:52 +0200

Am Sa., 25. Juli 2020 um 22:07 Uhr schrieb Stefan Monnier
<monnier <at> iro.umontreal.ca>:
>
> > That sounds like a very reasonable definition. Do you think it's
> > equivalent to my hypothesis and/or to the current behavior of the byte
> > optimizer?
>
> Probably not exactly: there might be functions which don't always
> "preserve `eql`" but for which we decide nevertheless that it's OK to
> precompute them at compile time for pragmatic reasons.
>
> E.g. `concat`.


I don't think we can really do that, as that would allow the byte
compiler to introduce bugs in the code, right? The manual states that
"This function [concat] always constructs a new string that is not
‘eq’ to any existing string" so I don't see how it could ever be pure.

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