Subject: 27.1; (+ -0.0) returns +0.0 when compiled

From: Mattias Engdegård <[email-masked]>
Date: Wed, 29 Jul 2020 14:07:32 +0200

Unary +, *, min and max, all of which should be identity for numbers, convert -0.0 to +0.0 when byte-compiled:

(defun f (x) (+ x))
(f -0.0)
=> -0.0
(byte-compile 'f)
(f -0.0)
=> 0.0

The reason is that byte-compile-associative transforms (+ x), (* x), (min x) and (max x) into (+ x 0).

No patch yet (sorry!) but I'm not sure what would be the best way to go about it. Some possibilities:

A. Use a full 1-argument call, like (+ x). This is more expensive (about 1.8×) since the general function call mechanism has to be used.
B. Use (* x 1) instead; this appears to work. This is also more expensive (1.6×); not sure why.
C. Add a new byte-op. Fast but probably overkill.

Better suggestions welcome!