From: Paul Eggert <eggert@cs.ucla.edu>
To: Stefan Monnier <monnier@iro.umontreal.ca>, emacs-devel@gnu.org
Subject: Re: Floating-point constant folding in Emacs byte compiler
Date: Thu, 22 Mar 2018 22:22:25 -0700 [thread overview]
Message-ID: <d4edaa6a-3f30-9abc-4e7a-366762fcdd23@cs.ucla.edu> (raw)
In-Reply-To: <jwvh8p7k2j4.fsf-monnier+gmane.emacs.devel@gnu.org>
Stefan Monnier wrote:
> I'm pretty sure those runtime differences are not on-purpose
Although that's most likely true for the examples I gave, it's a problematic
assumption for code that intentionally inspects runtime behavior.
I mention this because my recently-installed byte-opt patch fixed a problem of
this sort with lsh. byte-opt was evaluating (lsh -1 -1) at compile-time, whereas
the intent of the (lsh -1 -1) was to calculate most-positive-fixnum at run-time.
It's not hard to imagine similar sorts of code to inspect properties of the
machine's floating-point behavior, code that would yield the wrong value if
compiled on a machine with different floating-point properties.
Nowadays Emacs hosts almost invariably use IEEE floating point, and I suppose we
could hardwire that assumption into Emacs. Even so, IEEE floating-point is not
bit-for-bit identical on all platforms, so if we're constant-folding
floating-point expressions surely we should at least document that we're doing
so, so that programmers know that they can't rely on constant expressions
yielding a different value than what run-time evaluation would deliver.
next prev parent reply other threads:[~2018-03-23 5:22 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-22 23:04 Floating-point constant folding in Emacs byte compiler Paul Eggert
2018-03-23 1:26 ` Stefan Monnier
2018-03-23 5:22 ` Paul Eggert [this message]
2018-03-23 8:24 ` Eli Zaretskii
2018-03-23 20:00 ` Paul Eggert
2018-03-23 8:15 ` Eli Zaretskii
2018-03-23 20:52 ` Pip Cet
2018-03-24 6:25 ` Eli Zaretskii
2018-03-26 9:39 ` Robert Pluim
2018-03-26 15:13 ` Eli Zaretskii
2018-03-26 15:57 ` Robert Pluim
2018-03-26 16:02 ` Eli Zaretskii
2018-03-26 18:23 ` Pip Cet
2018-03-26 18:29 ` Eli Zaretskii
2018-03-27 0:28 ` Paul Eggert
2018-03-27 23:28 ` Paul Eggert
2018-03-30 16:26 ` Pip Cet
2018-03-30 16:31 ` Noam Postavsky
2018-03-30 16:39 ` Paul Eggert
2018-04-02 10:56 ` Pip Cet
2018-04-02 11:22 ` Eli Zaretskii
2018-04-02 11:42 ` Pip Cet
2018-04-02 12:50 ` Eli Zaretskii
2018-04-02 14:50 ` Stefan Monnier
2018-04-02 15:02 ` Pip Cet
2018-04-02 12:57 ` Noam Postavsky
2018-04-02 13:30 ` Eli Zaretskii
2018-04-02 14:48 ` Stefan Monnier
2018-04-02 19:20 ` Paul Eggert
2018-04-02 19:39 ` Pip Cet
2018-04-02 19:58 ` Eli Zaretskii
2018-04-02 20:55 ` Pip Cet
[not found] ` <<83y3i568i0.fsf@gnu.org>
2018-04-02 13:37 ` Drew Adams
2018-04-02 14:05 ` Eli Zaretskii
2018-04-02 14:54 ` Pip Cet
2018-04-02 15:02 ` Drew Adams
2018-03-26 17:52 ` Stefan Monnier
2018-03-26 18:30 ` Eli Zaretskii
2018-03-27 0:08 ` Paul Eggert
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