Fix bug with eql etc. on NaNs

Fix a bug where eql, sxhash-eql, memql, and make-hash-table
were not consistent on NaNs.  Likewise for equal,
sxhash-equal, member, and make-hash-table.  Some of these
functions ignored NaN significands, whereas others treated
them as significant.  It's more logical to treat significands
as significant, and this typically makes eql a bit more
efficient on floats, with just one integer comparison instead
of one to three floating-point comparisons.
* doc/lispref/numbers.texi (Float Basics): Document that
NaNs are never numerically equal, but might be eql.
* src/fns.c (WORDS_PER_DOUBLE): Move to top level of this file.
(union double_and_words): Now named, and at the top level of this file.
(same_float): New function.
(Fmemql, Feql, internal_equal, cmpfn_eql): Use it, so that
the corresponding functions treat NaNs consistently.
(sxhash_float): Simplify based on above-mentioned changes.

* test/src/fns-tests.el (fns-tests-equality-nan): New test.

1 files changed, 7 insertions, 2 deletions

diff --git a/doc/lispref/numbers.texi b/doc/lispref/numbers.texi
index 2fed2b642fd..6c51b849d35 100644
--- a/doc/lispref/numbers.texi
+++ b/doc/lispref/numbers.texi
@@ -232,13 +232,18 @@ distinguish them.
 @cindex negative infinity
 @cindex infinity
 @cindex NaN
+@findex eql
+@findex sxhash-eql
   The @acronym{IEEE} floating-point standard supports positive
 infinity and negative infinity as floating-point values.  It also
 provides for a class of values called NaN, or ``not a number'';
 numerical functions return such values in cases where there is no
 correct answer.  For example, @code{(/ 0.0 0.0)} returns a NaN@.
-Although NaN values carry a sign, for practical purposes there is no other
-significant difference between different NaN values in Emacs Lisp.
+A NaN is never numerically equal to any value, not even to itself.
+NaNs carry a sign and a significand, and non-numeric functions like
+@code{eql} and @code{sxhash-eql} treat two NaNs as equal when their
+signs and significands agree.  Significands of NaNs are
+machine-dependent and are not directly visible to Emacs Lisp.
 
 Here are read syntaxes for these special floating-point values: