Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!hplabs!sdcrdcf!sdcsmb!sea!eggert
From: eggert@sea.sm.unisys.com (Paul Eggert)
Newsgroups: comp.lang.lisp
Subject: Re: Lisp vs. C Floating Point (Suns)
Message-ID: <6@sea.sm.unisys.com>
Date: 30 Jan 88 19:17:18 GMT
References: <536@spar.SPAR.SLB.COM>
Reply-To: eggert@sea.sm.unisys.com (Paul Eggert)
Organization: Unisys Santa Monica
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In article <536@spar.SPAR.SLB.COM> malcolm@spar.SPAR.SLB.COM (Malcolm Slaney)
gives a C version on of the Barrow/Gabriel/Lucid FFT benchmark, and concludes
that Lisp compilers now generate code as fast as the code that C compilers
generate.  But his C version's register declarations could stand improvements;
in particular, the key scalar variables should be put in registers.  Given the
changes to his program suggested at the end of this message, C's performance
goes up 15-20% on a Sun-3 with 68881, so his table should be revised.  (I lack
access to the other kinds of Suns he benchmarked and so will omit their
figures.)

		Time (in seconds) to execute 10 iterations of a 1024 point FFT

					Sun-3/160	Sun-3/160
					(original)	(revised)

	Double 68881 C			5.0		4.1
	Single 68881 C			4.8		4.2
	Single 68881 Lucid 2.1.1	4.7		4.7

Even with these changes, I would like to see a better benchmark for relating
the quality of C compilers versus Lisp compilers.  The FFT benchmark is
dominated by floating point work, which is relatively insensitive to the choice
of language and compiler.  Even if we were to pick a different Gabriel
benchmark, blindly translating an existing Lisp program into C (as Slaney has
done) biases the results in favor of Lisp.

*** fftorig.c	Sat Jan 30 09:09:15 1988
--- fft.c	Sat Jan 30 10:13:45 1988
***************
*** 46,52 ****
   */
  	register float *ar = areal, *ai = aimag;
! 	register int	i = 1, j = 0, k = 0, m = 0; 
! 	int	n = 1024, nv2 = 512, le = 0,
! 		le1 = 0, ip = 0;
! 	float	ur = 0.0, ui = 0.0, wr = 0.0, wi = 0.0, tr = 0.0, ti = 0.0;
  	register int	l;
--- 46,53 ----
   */
+ 	register int i = 1, ip = 0;
  	register float *ar = areal, *ai = aimag;
! 	register float ur = 0, ui = 0, tr = 0, ti = 0;
! 	register int le1 = 0, le = 0, n = 1024, j = 0, k = 0, m = 0; 
! 	register int nv2 = n>>1;
! 	register float wr = 0, wi = 0;
  	register int	l;