Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!ll-xn!seibert
From: seibert@XN.LL.MIT.EDU (seibert)
Newsgroups: comp.sys.ti.explorer
Subject: fixnum vs single-float multiplies
Message-ID: <1576@xn.LL.MIT.EDU>
Date: 19 Oct 89 14:49:56 GMT
Organization: MIT Lincoln Laboratory, Lexington, MA
Lines: 70

Hello lispm experts.  I have a conundrum that the local folks haven't been
able to explain to me.  I hope you can help.  I'm investigating fixnum vs 
single-float multiplies.  It appears to me that a single-float multiply is 
faster than a fixnum multiply.  I've appended to this message the (short) test
functions I'm using and a sample output.  I understand why using bignums is
slower than fixnums, and the progression of CPU timings as the number of bits
used in the integer representation increases, but not why the single-float
multiply appears to be in the same ball-park as the fixnum multiply.

I've tried the tests with and without:  declarations, setfs, ash, and multiple
repetitions within the timing brackets.  These results are from an Explorer I,
but I get essentially the same results on my Symbolics machine.  

The idea of the code is to take a "nbrr" and time how much CPU time is needed
to square it using a single-float representation and a range of integer rep'ns.
Here's the code....

(defvar bbits 8)
(defvar right -7)
(defvar nbrr (/ 3.0))
(defvar str t)
(defmacro i* (i j) `(the fixnum
			 (ash (* (the fixnum ,i)
				 (the fixnum ,j))
			      (the fixnum right))))

(defun foo ()
  (let ((inbr (floor (* nbrr (ash 1 (1- bbits)))))
	(result nil))
    (timeit ()
      (setf result (i* inbr inbr)))
    (format str "   ~3d bits  result: ~10,7f ~12A; ~12A inbr ~d"
	    bbits (* result (/ 1.0 (ash 1 (1- bbits)))) 
            (type-of result) (type-of inbr) inbr)))
  
(defun foobar ()
  (do ((bbits 4 (+ bbits  4)))
      ((= bbits 60))
    (setf right (- 1 bbits))
    (foo))
  (format str "~%floating point... ")
  (let ((nbrw nbrr)
	(result nil))
    (timeit ()
      (setf result (* nbrw nbrw)))
    (format str "             result: ~10,7f ~12A; ~12A nbr ~20,17f" 
            result (type-of result) (type-of nbrw) nbrw)))

When I invoke foobar, here's the output....

CPU: 18.0 us     4 bits  result:  0.0000000 FIXNUM      ; FIXNUM       inbr 2
CPU: 19.0 us     8 bits  result:  0.1015625 FIXNUM      ; FIXNUM       inbr 42
CPU: 18.0 us    12 bits  result:  0.1108398 FIXNUM      ; FIXNUM       inbr 682
CPU: 56.0 us    16 bits  result:  0.1110840 FIXNUM      ; FIXNUM       inbr 10922
CPU: 60.0 us    20 bits  result:  0.1111088 FIXNUM      ; FIXNUM       inbr 174762
CPU: 62.0 us    24 bits  result:  0.1111110 FIXNUM      ; FIXNUM       inbr 2796202
CPU: 67.0 us    28 bits  result:  0.1111111 FIXNUM      ; BIGNUM       inbr 44739244
CPU: 54.0 us    32 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 715827904
CPU: 91.0 us    36 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 11453246464
CPU: 90.0 us    40 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 183251943424
CPU: 90.0 us    44 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 2932031094784
CPU: 89.0 us    48 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 46912497516544
CPU: 95.0 us    52 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 750599960264704
CPU: 95.0 us    56 bits  result:  0.1111111 BIGNUM      ; BIGNUM       inbr 12009599364235264
floating point... 
CPU: 25.0 us             result:  0.1111111 SINGLE-FLOAT; SINGLE-FLOAT nbr  0.33333334000000000

Thanks for your help.  _Ms
-- 
Michael Seibert         seibert@xn.ll.mit.edu        ll-xn!seibert