[Openmcl-devel] To sqrt or to expt, that is the question

Gary King gwking at cs.umass.edu
Mon Dec 22 17:52:44 PST 2003

I was looking at some code where someone had used expt to compute the 
square root of a number. "Hmm, I thought, it all depends on the 
compiler and the implementation, but I would have thought that (sqrt x) 
would be faster than (expt x 0.5)." So I tried the following, 
admittedly unscientific, test:

  (let ((x 0d0))
    (loop repeat 100000 do
          (setf x (expt 101 0.5)))

  (let ((x 0d0))
    (loop repeat 100000 do
          (setf x (sqrt 101)))

To my surprise in both MCL and OpenMCL, not only was expt faster, but 
sqrt conses. Note that I tried the same test in SBCL and found that 
there was no significant difference between expt and sqrt. This 
probably isn't a big deal, but it might be worth optimizing sqrt in MCL 
at some point...
Gary Warren King, Lab Manager
EKSL East, University of Massachusetts * 413 577 0176

"no art, however minor, demands less than total dedication if you want 
to excel in it."
   -- Alberti

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