Re: Floating-point constant folding in Emacs byte compiler

[Paul Eggert <eggert@cs.ucla.edu>]

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On 04/02/2018 07:48 AM, Stefan Monnier wrote:
> we're not free to
> merge two strings just because they're `equal`, whereas for floats
> we are.
Yes, that's the key point here. I see that the Elisp documentation does 
not specify how eq behaves on floats with the same values, so I took a 
crack at specifying this the same way that Common Lisp and Scheme do by 
installing the attached into master. I doubt whether Common Lisp/Scheme 
semantics here will break real Elisp code in any significant way.

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From b89189f686a599817d8cdcc81038b62a38a7baa7 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 2 Apr 2018 12:19:00 -0700
Subject: [PATCH] Clarify eq on floats

* doc/lispref/objects.texi (Equality Predicates):
Say that two floats with the same values might or might not be eq.
---
 doc/lispref/objects.texi | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/doc/lispref/objects.texi b/doc/lispref/objects.texi
index af740625ad..78a7dccc88 100644
--- a/doc/lispref/objects.texi
+++ b/doc/lispref/objects.texi
@@ -2083,6 +2083,10 @@ Equality Predicates
 necessarily @code{eq} to each other: they are @code{eq} only if they
 are the same object, meaning that a change in the contents of one will
 be reflected by the same change in the contents of the other.
+For other types of objects whose contents cannot be changed (e.g.,
+floats), two arguments with the same contents might or might not be
+the same object, and @code{eq} returns @code{t} or @code{nil}
+depending on whether the Lisp interpreter created one object or two.
 
 @example
 @group
@@ -2095,6 +2099,12 @@ Equality Predicates
      @result{} t
 @end group
 
+@group
+(eq 3.0 3.0)
+     @result{} t @r{or} nil
+;; @r{The result is implementation-dependent.}
+@end group
+
 @group
 (eq "asdf" "asdf")
      @result{} nil
-- 
2.14.3