Make logcount act like CL on negative arg

Behaving like Common Lisp is less likely to lead to surprises,
as it yields the same answers on 32- vs 64-bit machines.
* doc/lispref/numbers.texi (Bitwise Operations):
Document behavior on negative integers.
* src/data.c (Flogcount):
Behave like Common Lisp for negative arguments.
* test/src/data-tests.el (data-tests-popcnt)
(data-tests-logcount): Test negative args too.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/data.c b/src/data.c
index fd8cdd19aa2..2e7f3e017be 100644
--- a/src/data.c
+++ b/src/data.c
@@ -3071,11 +3071,13 @@ usage: (logxor &rest INTS-OR-MARKERS)  */)
 
 DEFUN ("logcount", Flogcount, Slogcount, 1, 1, 0,
        doc: /* Return population count of VALUE.
-If VALUE is negative, the count is of its two's complement representation.  */)
+This is the number of one bits in the two's complement representation
+of VALUE.  If VALUE is negative, return the number of zero bits in the
+representation.  */)
   (Lisp_Object value)
 {
   CHECK_NUMBER (value);
-  EMACS_UINT v = XUINT (value);
+  EMACS_INT v = XINT (value) < 0 ? -1 - XINT (value) : XINT (value);
   return make_number (EMACS_UINT_WIDTH <= UINT_WIDTH
                       ? count_one_bits (v)
                       : EMACS_UINT_WIDTH <= ULONG_WIDTH