no noalias not negligible: more red herrings, conclusion unchanged

[Chris Torek]

In article <54713 at sun.uucp> dgh%dgh at Sun.COM (David Hough) writes:
>... the following versions of ROLLED source; the first results in unrolled
>object code; the second and most other equivalent ones do not:

[Some text deleted.  First version:]
>        i = 0;
>        do {
>                dy[i] = dy[i] + da * dx[i];
>                i++;
>        } while (i < n);

[Second:]
>        i = 0;
>        do {
>                dy[i] = dy[i] + da * dx[i];
>        } while (i++ < n);

This illustrates a very important point.  These two loops are *not*
equivalent!  If n is 2, the first loop executes twice (as it should);
the second executes three times.  So remember:

  Make it right before you make it fast, and do try not to break it
  when you make it fast. :-)

(I still do not understand the insistence on avoiding the form

	do {
		dy[i] += da * dx[i];
	} while (++i < n);

which is much easier to read, and remains correct.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at mimsy.umd.edu	Path:	uunet!mimsy!chris