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From: dweller@news.imagin.net (David Weller)
Subject: Re: C's register keyword
Date: 1997/09/08
Message-ID: <5v1k1v$3mq6$1@prime.imagin.net>#1/1
X-Deja-AN: 270773194
References: <5uqhm2$t48$1@goanna.cs.rmit.edu.au>
 <EG71Ex.4D6.0.-s@inmet.camb.inmet.com>
Organization: ImagiNet Communications Ltd, Arlington, Texas
Newsgroups: comp.lang.ada
Date: 1997-09-08T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <EG71Ex.4D6.0.-s@inmet.camb.inmet.com>,
Tucker Taft <stt@houdini.camb.inmet.com> wrote:
>Tristan Ludowyk (ludowyk@yallara.cs.rmit.edu.au) wrote:
>: is there an ada equivelent to 'register' in C/C++?
>
>In Ada 95, any local variable not declared "aliased" is a candidate
>for living in a machine register.  The compiler decides based
>on usage.
>

As a modest, and perhaps slightly contrived example:
C version:

	for (int i = 0; i < 10000; i++)
		for (int j = 0; j < 10000; j++)
				/*some nontrivial statement*/

Ada version:

	for i in 0..9999 loop
		for j in 0..9999 loop
			--some nontrivial statement
		end loop;
	end loop;

Using gcc/GNAT, you find that the Ada executable yields faster
performance (assuming same optimization levels and identical times for
the "some nontrivial statement".  Only when you declare the i and j
variables in C as with the "register" keyword do you see identical
performance.

This is, IMHO, a fundamental point behind Ada -- the compiler can (and
does!) perform several optimizations that would otherwise be impossible
in C without "aggressive" programming (and considering that C requires
the developer to program "defensively", it should come as no surprise
that these highly skilled "aggressive-defensive" programmers also tend
to be "manic-depressive" :-)

As Tuck points out, "aliased" variables in Ada behave much like
"un-registered" variables in C, but also have a LOT more "protection"
to them than C variables.

Of course, having nattered away like this, it all really boils down to
the one mantra all programers should (and rarely) follow:

	Measure everything!


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