[fpc-devel] Floating point precision

[Colin Western]

Jonas Maebe wrote:
> 
> On 06 Apr 2007, at 21:36, Colin Western wrote:
> 
>> Can I ask what determines the precision of floating point calculations 
>> are done in? It seems that fpc treats (for example, with J declared as 
>> integer)
>>
>> 1/sqrt(J+1.0) as single
>>
>> but
>>
>> 1/sqrt(J+1) as double
> 
> In 2.1.2 and 2.2, both will be evaluated as single (except on 80x87, 
> because it is not possible to tell the 80x87 to perform a particular 
> calculation using only single/double precision).

The current SVN (which is the only one I am using at the moment) does 
give the results at different precisions as shown for x86_64 and PPC - I 
don't know if that should be considered as a bug. (I understand that the 
older 386 architecture will normally use double or extended all the time).

Having said that, for numerical programming if double is wanted, the 
evaluation as double in the absence of specific precision information is 
actually desirable, and avoids some hard to find errors. Values like 
Sqrt(2) are common, which I should presumably write as Sqrt(Double(2)) 
to be sure I don't loose precision. It came up because my application 
(which requires double rather than single accuracy) was giving slightly 
different results for different architectures. I have now gone through 
and fixed the differences I can find, and the results are now the same, 
but it is difficult to be sure I have caught all the problem areas.

Is there some way of optionally making double the default for constants 
(rather than single) as a way of avoiding these errors?

Colin