Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!radar!cadillac!Pkg.Mcc.COM!steve
From: steve@Pkg.Mcc.COM (Steve Madere)
Newsgroups: comp.lang.c++
Subject: Re: why is this program slow?
Message-ID: <1991Jan9.101212@Pkg.Mcc.COM>
Date: 9 Jan 91 16:12:12 GMT
References: <1991Jan9.002244.23398@news.cs.indiana.edu>
Sender: news@cadillac.CAD.MCC.COM
Reply-To: steve@Pkg.Mcc.COM (Steve Madere)
Lines: 94

In article <1991Jan9.002244.23398@news.cs.indiana.edu>,
shirley@iuvax.cs.indiana.edu (peter shirley) writes:
| I have a C++ program that takes about 170% as long as a similar C
program.
| I've run it on a vax and a sun with and without big-O.
| I've inlined everything I can, and I don't understand the slowdown.
| Could someone enlighten me?  (yes, I looked at the C code generated
| by CC, and am too dumb to understand it).
| 

<Stuff deleted>
<C program segment folows>
|     for ( i = 0; i < 1000000; i++)
|     {
|         a[0] = a[0] + b[0];
|         a[1] = a[1] + b[1];
|         a[2] = a[2] + b[2];
|     }

<C++ program segment follows>
| inline void vector::operator=(vector& v)
| {
| 
|    data[0] = v.data[0];
|    data[1] = v.data[1];
|    data[2] = v.data[2];
| 
| }
| 
| 
| 
| inline vector operator+(vector& u, vector& v)
| {
|     vector temp;
| 
|     temp.data[0] = u.data[0] + v.data[0];
|     temp.data[1] = u.data[1] + v.data[1];
|     temp.data[2] = u.data[2] + v.data[2];
|     return temp;
| }

<Stuff deleted>
|     for (int i = 0; i < 1000000; i++)
|         a = a + b;
| 
|     cerr << a << "\n";
| }

At this point the cause of the speed problems should be obvious.
Your C++ program is not quite equivalent to your C program.
To make them equivalent, you should have the following C program
segment substituted for the one above.

for (i=0;i<1000000;i++)
{
  temp = calloc(3,sizeof(*temp));
  temp[0] = a[0] + b[0];
  temp[1] = a[1] + b[1];
  temp[2] = a[2] + b[2];
  a[0]=temp[0];
  a[1]=temp[1];
  a[2]=temp[2];
  free(temp);
}

Once this is done, I think your two programs will probably run
at the same speed :-(.

This is a problem with C++ that I have known about for some time.
The lure of simplified notation leads one to try to implement
vector and matrix operations with special classes that will hide
the complexity.  Unfortunately, all of those temporary variables
that must be created to realize any mathematical operations really
waste CPU time.  It takes a finite amount of time to allocate space,
copy data, and free up the space again.  In many cases it is a
significant percentage of the total run time of the program.

A friend of mine at UCSD has worked on this problem for some time
and concluded that what one needs is some kind of matrix awareness
in the compiler to recognize when operations can be somehow 
combined.  Clearly this is not the kind of thing that C++ was designed
to do.  It is appropriate to add matrix awareness to a computational
language like FORTRAN but C++ is not a scientific computation
language.  (no flames please, I just mean that its PRIMARY purpose
is not scientific computation).

Maybe what one needs is a preprocessed language that takes matrix
operations, turns them into optimized C code which you can 
link with your C++ program.  

Any suggestions from the peanut gallery?

Steve Madere
steve@pkg.mcc.com