Re: Q: division vs multiplication

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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