Path: utzoo!censor!geac!torsqnt!hybrid!scifi!bywater!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: why is this program slow?
Message-ID: <547@taumet.com>
Date: 9 Jan 91 16:38:34 GMT
References: <1991Jan9.002244.23398@news.cs.indiana.edu>
Organization: Taumetric Corporation, San Diego
Lines: 57

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.

>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;
>}

This is your problem.  You create a local object, then return it by
value.  This must then be copied into the target variable.  The
equivalent C code looks like this:

typedef struct { double data[3]; } vector;

vector plus( vector* this, vector* u, vector* v)
{
    ...the body is the same as above...
}

main()
{
    vector u, v, w;
    ...
    w = plus(&w, &u, &v);
    ...
}

Compare
    w = plus(&w, &u, &v);
to
    w.data[0] = u.data[0] + v.data[0];
    w.data[1] = u.data[1] + v.data[1];
    w.data[2] = u.data[2] + v.data[2];
There is an extra structure copy using plus() instead of direct code,
and the copy is in the inner loop.

Try rewriting operator+ as:

inline vector operator+(vector& u, vector& v)
{
    return vector(u.data[0] + v.data[0],
		  u.data[1] + v.data[1],
		  u.data[2] + v.data[2]);
}

Many C++ compilers are smart enough not to create a temporary object,
but to construct the new vector directly in the target variable.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com