Re: Rounding with Div and Mod operators

Christopher Glaeser <cdg@nullstone.com>
16 May 1999 15:26:59 -0400

          From comp.compilers
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Re: Rounding with Div and Mod operators Scott.Daniels@Acm.Org (Scott.David.Daniels) (1999-05-16)
Re: Rounding with Div and Mod operators cdg@nullstone.com (Christopher Glaeser) (1999-05-16)
Re: Rounding with Div and Mod operators johan.persson@mbox319.swipnet.se (Johan Persson) (1999-05-16)
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[6 later articles]
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From: Christopher Glaeser <cdg@nullstone.com>
Newsgroups: comp.compilers
Date: 16 May 1999 15:26:59 -0400
Organization: Nullstone Corporation
References: 99-05-039
Keywords: arithmetic, design

> What is interesting about this rule is there seem to be two ways of
> rounding that satisfy it when n or d are negative - either we round
> integers to the next lowest value or we round towards zero.
> My question is: which rounding system is preferred and does it matter?


Which ever rounding direction you choose, make sure the implementation
is correct. Several years ago I looked at the integer divide code
generated by twenty commercial C compilers to fine tune our
performance test suite, and twelve of these compilers generated
incorrect code. The most common defect was related to power-of-two
optimization such that (x / 2) generated different results than (x /
y) when y=2.


Best,
Christopher Glaeser mailto:cdg@nullstone.com
Nullstone Corporation http://www.nullstone.com

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