9.6.2 Floating point optimization

The float declaration allows generation of more efficient code using float numbers. It reduces allocation during float calculations. It is best used with safety=0 . For example:

(progn

  (setf a 

        (make-array 1000 

                    :initial-element 1D0

                    :element-type 'double-float))

  nil ; to avoid printing the large array

  )

(compile 

 (defun test (a)

   (declare (optimize (speed 3) (safety 0) (float 0)))

   (declare (type (simple-array double-float  (1000))

                  a))

   (let ((sum 0D0))

     (declare (type double-float sum))

     (dotimes (i 1000)

       (incf sum (the double-float (aref a i))))

     sum)))

(time (test a))

=>

Timing the evaluation of (TEST A)

user time    =      0.000

system time  =      0.000

Elapsed time =   0:00:00

Allocation   = 16 bytes standard / 0 bytes conses

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