Thorsten Froehlich wrote:

> In article <3fc8a358@news.povray.org> , Wolfgang Wieser <wwi### [at] gmxde>
> wrote:
> 
>> Is there any reason to use
>>   a/=2.0;
>> instead of
>>   a*=0.5;
>> in case a is a float or double variable?
>>
>> After all, 0.5 has an exact representation as float (being
>> 1 * 2^-1). On current CPUs, the latter is faster by a factor
>> of three, IIRC.
> 
> Unless you depend on certain precision side effects, the multiplication
> will be sufficient. 
>
It would be nice if you could elaborate on this. 

I can imagine that
  a*=0.2;
results in a precision degeneration compared to
  a/=5.0;
(because 0.2 cannot be represented exactly a dual system). 

But since 2 and 0.5 can be represented exactly as a float/double, 
I cannot imagine any precision decrease when using 
  a*=0.5;
instead of
  a/=2;
(It is simply a decrease-exponent-by-one operation.)

> And it isn't faster by a specific factor.  The issues behind
> the speed difference are much more complex.  On modern processors,
> multiplications can commonly be fully pipelines, while divisions are an
> iterative process that is not pipelined (because it would take too many
> gates).
> 
The factor three or four was not a specific measurement but a rule of 
thumb for people doing numerics. It should be a good estimate for 
everyday life ;)

Wolfgang

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