Re: Q: division vs multiplication

kptben@aol.com (KPT Ben)
Mon, 17 Apr 1995 00:19:16 GMT

          From comp.compilers

Related articles
[6 earlier articles]
Re: Q: division vs multiplication brandis@inf.ethz.ch (1995-04-04)
Re: Q: division vs multiplication Terje.Mathisen@hda.hydro.com (1995-04-06)
Re: Q: division vs multiplication meissner@cygnus.com (Mike Meissner) (1995-04-16)
Re: Q: division vs multiplication martens@cis.ohio-state.edu (1995-04-16)
Re: Q: division vs multiplication jmccarty@spdmail.spd.dsccc.com (1995-04-18)
Re: Q: division vs multiplication leichter@zodiac.rutgers.edu (1995-04-11)
Re: Q: division vs multiplication kptben@aol.com (1995-04-17)
Re: Q: division vs multiplication pcg@aber.ac.uk (1995-04-17)
Re: Q: division vs multiplication gsc@magna.com.au (1995-04-18)
Re: Q: division vs multiplication jbuck@Synopsys.COM (1995-04-28)
Re: Q: division vs multiplication davidm@flora.Rational.com (1995-04-28)
Re: Q: division vs multiplication Roger@natron.demon.co.uk (Roger Barnett) (1995-04-28)
Re: Q: division vs multiplication jmccarty@spdmail.spd.dsccc.com (1995-04-29)
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Newsgroups: comp.compilers
From: kptben@aol.com (KPT Ben)
Keywords: arithmetic
Organization: America Online, Inc. (1-800-827-6364)
References: 95-04-080
Date: Mon, 17 Apr 1995 00:19:16 GMT

martens@cis.ohio-state.edu (Jeff Martens) wrote:


>kptben@aol.com (KPT Ben) writes:
>>When using 32-bit integer math on the PowerPC 601, a brute-force divide
is
>>36 cycles; a multiply is 5 cycles, and bitshifts are 1 cycle.
>>Multiplication and division by powers of two can be done with a single
>>bitshift instruction on the 601 (two instructions are necessary when
>>dividing a signed integer by a power of 2).


>You have to be careful here, though, because a right arithmetic
>shift of an odd negative integer will not give the same result as
>division by 2. For example:


  >-1 div 2 is 0
  >ashr(-1) is -1


On the PPC, you can divide any signed 32-bit integer by 2^n by using (in
PPC assembly):


srawi dst, src, n //arithmetic-shift (generates carry bit if dst < 0)
addze dst, dst //add carry bit to dst


and it will give identical results to dividing by 2^n.


--
Ben Weiss
HSC Software
--


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