Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!sun-spots-request
From: achhabra@uceng.uc.edu (atul k chhabra)
Newsgroups: comp.sys.sun
Subject: Fortran 300 times faster than C for computing cosine (on Sun3 & 4)
Keywords: Software
Message-ID: <764@uceng.UC.EDU>
Date: 21 Mar 89 11:24:44 GMT
Sender: usenet@rice.edu
Organization: Univ. of Cincinnati, College of Engg.
Lines: 67
Approved: Sun-Spots@rice.edu
Original-Date: 7 Mar 89 23:12:30 GMT
X-Sun-Spots-Digest: Volume 7, Issue 203, message 7 of 14

I chanced upon a segment of code that runs approximately 300 times faster
in FORTRAN than in C. I have tried the code on Sun3(OS3.5) and on
Sun4(OS4.0) (of course, on Sun4 the -f68881 flag was not used.) The
results are similar on both machines. Can anyone enlighten me on this
bizzare result?

Listing of cosc.c:
__________

/*
 * Compile using:
 *	cc -f68881 -O -o cosc cosc.c -lm.
 */

#include <math.h>

main()
{
    int i;
    float tmp;

    for(i=0;i<262144;i++)
	tmp=cos(2.5)*cos(2.5)*cos(2.5)*cos(2.5);
}

Listing of cosf.f
__________

c
c	Compile using:
c		f77 -f68881 -O -o cosf cosf.f
c
	program cosf
	integer i
	real tmp

	do 10 i=1,262144
		tmp=cos(2.5)*cos(2.5)*cos(2.5)*cos(2.5)
10	continue
	end

Timings on Sun3(OS3.5):

% time cosc
55.6u 1.0s 1:49 51% 24+8k 12+1io 0pf+0w
^^^^^
% time cosf
0.2u 0.0s 0:00 75% 16+8k 4+0io 0pf+0w
^^^^

[[ This is a good one!  You've gotten tricked by one of the most common
optimization snares around.  For f77, "-O" defaults to "-O3", which
includes global optimization.  Presumably it also does dead code
elimination.  But what constitutes dead code?  Well, code that performs
calculations that aren't used.  How do you define a value that has been
used?  Ultimately, if it has an effect on the outside world, or more
specifically, if it or something it effects is written or printed.  Your
cosf.f does neither; "tmp" is never used for anything important, so your
entire program gets optimized away!  Add "print *,tmp" after the body of
the loop and cosf.f takes alot longer.  Interestingly enough, it still
appears to be quicker than cosc.c.  --wnl ]]

Atul Chhabra, Dept. of Electrical & Computer Engineering, ML 030,
University of Cincinnati, Cincinnati, OH 45221-0030.

voice: (513)556-4766  INTERNET: achhabra@ucesp1.ece.uc.edu [129.137.33.114]
                                OR achhabra@uceng.uc.edu [129.137.33.1]