Re: Faster make depend

Olaf Titz (
28 Jul 1996 12:13:47 +0200

Matti E Aarnio <> wrote:
> In addition, calculating inverse ("1/x") of constant (at
> compile time), and then doing multiplication with it is
> (at Alphas) blinding fast... Sure it overflows, but the
> result is correct for division :-) You can't get reminder

Don't do such tricks without proper regard for precision. If you
compute x/10 with FP arithmetic, the result is _exact_ e.g. for x=10
or x=5. If you optimize this to be x*0.1, however, the result is
_never_ exact because there is no exact binary FP representation for
0.1. (In simple cases you have the chance that the round-off errors
compensate, but you can _never_ be sure of that - in precision
calculations, round-off errors are always accounted for the worst

The numerics people would be very upset if someone implemented the
above "optimization" in a compiler without telling them...


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