7.4 Floating-point numbers

7.4.5 A known problem

Type and representation information is not being propagated from a declared variable ofmultiple-value-setq ormultiple-value-bind onto the expression supplying the value(s). Therefore, code such as the following fails to be optimized:

 (defun baz (x y)
   (declare (fixnum x y))
   (multiple-value-bind (z w)
                        (round x y)
     (declare (fixnum z w))
     (list (+ z 1) (+ w 1.0))))

Below are two examples of code that is successfully optimized:

 (defun bar (x y) (declare (fixnum x y))
    (multiple-value-bind (z w)
                         (the (values fixnum fixnum) (round x y))
     (list (+ z 1) (+ w 1.0))))

 (defun foo  (x y) (declare (fixnum x y))
   (let  ((z (round x y)))
     (declare (fixnum z))
     (+ z 1)))

In some cases, it may be just as efficient to re-calculate the remainder, for example, as follows:

     (defun foobar  (x y)
       (declare (float x y))
       (let* ((quo (round x y))
              (rem (- x (* y (float quo)))))
         (declare (fixnum quo)
                  (float rem))
         (list (+ quo 1) (+ rem 1.0))))

The Advanced User's Guide - 9 SEP 1996

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