Messages in this thread | | | From | Olaf Titz <> | Subject | Re: Faster make depend | Date | 28 Jul 1996 12:13:47 +0200 |
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Matti E Aarnio <mea@mea.cc.utu.fi> 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 case.)
The numerics people would be very upset if someone implemented the above "optimization" in a compiler without telling them...
olaf -- ___ Olaf.Titz@inka.de or @{stud,informatik}.uni-karlsruhe.de ____ __ o <URL:http://www.inka.de/~bigred/> <IRC:praetorius> __/<_ >> Just as long as the wheels keep on turning round _)>(_)______________ I will live for the groove 'til the sun goes down << ____
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