Re: Q: division vs multiplication

>t.hulek@imperial.ac.uk (Mr Tomas Hulek) wrote:
>[Can one optimize floating division by powers of two into something like a
>shift?]


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
--


-- Jeff (jmartens@nimue.hood.edu)
--

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