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From: ggs@ulysses.att.com (Griff Smith)
Newsgroups: comp.lang.c++
Subject: Re: why is this program slow?
Summary: not doing the same thing
Message-ID: <14167@ulysses.att.com>
Date: 9 Jan 91 14:48:52 GMT
References: <1991Jan9.002244.23398@news.cs.indiana.edu>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 37

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.

The C program is written in terms of basic operations that the optimizer
can easily improve; the C++ version isn't.  For example:

>         a[0] = a[0] + b[0];

This is better written as

	  a[0] += b[0];

though modern compilers can usually make the two cases equivalent.  The
C++ version is not so fortunate.  You write

>         a = a + b;

Since `a' is a structure, your `+' operator first has to store the sum in
your `temp', then your `=' operator must copy it into `a'.  This takes a
string move at best, and a slow function call at worst.  I tried redefining
your `+' operator to be a `vector& +=' operator and my execution time on
a tahoe became 6.0 seconds instead of 14.5.  That's still not quite as
good as 4.5 for the C version, but it explains most of the difference.

> Could someone enlighten me?  (yes, I looked at the C code generated
> by CC, and am too dumb to understand it).

C++ output isn't intelligible to most mortals; this mortal frequently
has trouble with C++ input.  I often find it easier to use adb to look
at the assembly language.
-- 
Griff Smith	AT&T (Bell Laboratories), Murray Hill
Phone:		1-201-582-7736
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