add alternative pylong_join() implementation (slower than what's there), purely for documentation

Cython/Utility/__init__.py

@@ -7,3 +7,19 @@ def pylong_join(count, digits_ptr='digits', join_type='unsigned long'):
     return ('(' * (count * 2) + "(%s)" % join_type + ' | '.join(
         "%s[%d])%s)" % (digits_ptr, _i, " << PyLong_SHIFT" if _i else '')
         for _i in range(count-1, -1, -1)))
+
+
+# although it could potentially make use of data independence,
+# this implementation is a bit slower than the simpler one above
+def _pylong_join(count, digits_ptr='digits', join_type='unsigned long'):
+    """
+    Generate an unrolled shift-then-or loop over the first 'count' digits.
+    Assumes that they fit into 'join_type'.
+    """
+    def shift(n):
+        # avoid compiler warnings for overly large shifts that will be discarded anyway
+        return " << (%d * PyLong_SHIFT < 8 * sizeof(%s) ? %d * PyLong_SHIFT : 0)" % (n, join_type, n) if n else ''
+
+    return '(%s)' % ' | '.join(
+        "(((%s)%s[%d])%s)" % (join_type, digits_ptr, i, shift(i))
+        for i in range(count-1, -1, -1))