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Re: dividing by a power of two

To: bousch@topo.math.u-psud.fr (Thierry Bousch)
Subject: Re: dividing by a power of two
From: roemer@informatik.uni-frankfurt.de
Date: Fri, 16 Dec 94 17:24:04 MET
Cc: mint@atari.archive.umich.edu
In-reply-to: <199412161521.QAA20992@topo.matups.fr>; from "Thierry Bousch" at Dec 16, 94 4:21 pm
Mailer: Elm [revision: 70.85]
Posted-date: Fri, 16 Dec 94 17:24:04 MET

> I've noticed this too. The problem is that (x/16) is not equal to (x>>4)
> when `x' is negative, so gcc has to test for the sign of `x'. This

Not if gcc uses "asr" (arithmetic shift right). This takes care to copy the
sign bit to all bits shifted in from the left, so "asr #1, -16" gives -8.

The PureC compiler has this same oddity, it does well for x*16 and some such
but not for divisions by powers of two...

Kay.
--
Kay Roemer              roemer@informatik.uni-frankfurt.de
"If I ever meet ..."    http://www.uni-frankfurt.de/~roemer/
"... myself I'll hit myself so hard I won't know who hit me" (Zaphod)

Follow-Ups:
- Re: dividing by a power of two
  - From: lux@heidelbg.ibm.com (Wolfgang Lux)

References:
- dividing by a power of two
  - From: Thierry Bousch <bousch@topo.math.u-psud.fr>

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