Fix rounding error in ‘ceiling’ etc.

Without this fix, (ceiling most-negative-fixnum -1.0) returns
most-negative-fixnum instead of correctly signaling range-error,
and similarly for floor, round, and truncate.
* configure.ac (trunc): Add a check, since Gnulib’s doc says
‘trunc’ is missing from MSVC 9.  The Gnulib doc says ‘trunc’ is
also missing from some other older operating systems like Solaris
9 which I know we don’t care about any more, so MSVC is the only
reason to worry about ‘trunc’ here.
* src/editfns.c (styled_format): Formatting a float with %c is now an
error.  The old code did not work in general, because FIXNUM_OVERFLOW_P
had rounding errors.  Besides, the "if (FLOATP (...))" was in there
only as a result of my misunderstanding old code that I introduced
2011.  Although %d etc. is sometimes used on floats that represent
huge UIDs or PIDs etc. that do not fit in fixnums, this cannot
happen with characters.
* src/floatfns.c (rounding_driver): Rework to do the right thing
when the intermediate result equals 2.305843009213694e+18, i.e.,
is exactly 1 greater than MOST_POSITIVE_FIXNUM on a 64-bit host.
Simplify so that only one section of code checks for overflow,
rather than two.
(double_identity): Remove.  All uses changed to ...
(emacs_trunc): ... this new function.  Add replacement for
platforms that lack ‘trunc’.
* src/lisp.h (FIXNUM_OVERFLOW_P, make_fixnum_or_float):
Make it clear that the arg cannot be floating point.
* test/src/editfns-tests.el (format-c-float): New test.
* test/src/floatfns-tests.el: New file, to test for this bug.

2 files changed, 31 insertions, 0 deletions

diff --git a/test/src/editfns-tests.el b/test/src/editfns-tests.el
index 7b4f41aab5d..14124ef85fb 100644
--- a/test/src/editfns-tests.el
+++ b/test/src/editfns-tests.el
@@ -133,4 +133,7 @@
     (should (string= (buffer-string) "éä\"ba÷"))
     (should (equal (transpose-test-get-byte-positions 7) '(1 3 5 6 7 8 10)))))
 
+(ert-deftest format-c-float ()
+  (should-error (format "%c" 0.5)))
+
 ;;; editfns-tests.el ends here
diff --git a/test/src/floatfns-tests.el b/test/src/floatfns-tests.el
new file mode 100644
index 00000000000..a2116a59459
--- /dev/null
+++ b/test/src/floatfns-tests.el
@@ -0,0 +1,28 @@
+;;; floatfn-tests.el --- tests for floating point operations
+
+;; Copyright 2017 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+(require 'ert)
+
+(ert-deftest divide-extreme-sign ()
+  (should-error (ceiling most-negative-fixnum -1.0))
+  (should-error (floor most-negative-fixnum -1.0))
+  (should-error (round most-negative-fixnum -1.0))
+  (should-error (truncate most-negative-fixnum -1.0)))
+
+(provide 'floatfns-tests)