From: "Mattias Engdegård" <mattiase@acm.org>
To: Pip Cet <pipcet@gmail.com>
Cc: 38708@debbugs.gnu.org
Subject: bug#38708: [PATCH] Deduplicate flonum and bignum constants in bytecode
Date: Sat, 28 Dec 2019 16:49:05 +0100 [thread overview]
Message-ID: <096F68FE-0CB3-45CD-AB14-89BB5447484C@acm.org> (raw)
In-Reply-To: <CAOqdjBcnL=jZD=+u+TbpUs4hJ4mR+tZFjKkzPWcjWDoPz_zCUw@mail.gmail.com>
27 dec. 2019 kl. 18.07 skrev Pip Cet <pipcet@gmail.com>:
> I don't have a strong opinion about this (well, I do, actually: 'eq
> and 'eql should be equal), but my impression from the last time this
> was discussed is that the problems this causes (different code
> behavior for byte-compiled code versus evaluated code) outweighed the
> benefits (very tiny code size reduction).
Thank you for inspecting my change! And sorry, I didn't know this had been debated before. Is there a record of that discussion anywhere?
> Most importantly, I think that we should be able to be define
>
> (defun f () (eq 18446744073709551616 18446744073709551616))
>
> That function should always return t on sane systems that have eq =
> eql, and always return nil on systems that have <64 bits in a fixnum
> and the old-style eq.
I'm not sure I understand. Surely such a criterion imposes a rather low limit on permissible optimisations? For example, shouldn't
(eq (ash 1 x) (ash 1 x))
be allowed to be optimised to t (after CSE, say), even if x can be 64, despite the fact that interpreted or low-optimised compiled code would yield nil in that case?
Perhaps the change should really be done on the emacs-27 branch, to avoid changing bignum behaviour, but that is just a slightly weaker version of the same restriction. Unless we decide to turn eq into a synonym for eql, eq is a one-sided conservative approximation of eql for bignums and flonums.
> Anyway, I still think the right course of action here is to fix (or
> deprecate) eq rather than changing minor details of the byte compiler
> in incompatible ways. However, if we decide the gain is significant
> for floating point numbers, let's restrict this to floating point
> numbers and leave bignums alone?
What would anyone gain from such a restriction? And the change is minor because it's a small thing to do; what I thought looked like an obvious oversight, or one that made more sense back when Elisp didn't have bignums.
next prev parent reply other threads:[~2019-12-28 15:49 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-22 16:42 bug#38708: [PATCH] Deduplicate flonum and bignum constants in bytecode Mattias Engdegård
2019-12-27 14:24 ` Mattias Engdegård
2019-12-27 17:07 ` Pip Cet
2019-12-28 15:49 ` Mattias Engdegård [this message]
2019-12-28 16:36 ` Pip Cet
2019-12-28 18:50 ` Mattias Engdegård
2019-12-28 19:07 ` Eli Zaretskii
2019-12-29 17:29 ` Pip Cet
2019-12-29 22:30 ` Mattias Engdegård
2019-12-30 15:46 ` Eli Zaretskii
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