Re: Ada decimal types

["REH" <me@you.com>]

"Simon Wright" <simon@pushface.org> wrote in message 
news:m2k68zm9xl.fsf@grendel.local...
> "REH" <me@you.com> writes:
>
>> "Keith Thompson" <kst-u@mib.org> wrote in message
>> news:lnmzdvttoc.fsf@nuthaus.mib.org...
>
>>> And it's the small, not the delta, that needs to be 1.0e-6.  If you
>>> specify a delta of 1.0e-6 for an ordinary fixed-point type without
>>> specifying the small, you'll (probabaly) get a small of 2.0**-20.
>>> (As you probably know.)
>>
>> It's not an binary fixed point, but a decimal fixed point.  I believe the
>> small has to be a power of ten, not two.  Anyways, that is a good point.
>> For a decimal type, do I need to specify the small?
>
> On GNAT, Duration (which is a plain fixed-point type) is effectively
> implemented as
>
>   type Duration is delta 1.0e-9;
>   for Duration'Small use 1.0e-9;
>   for Duration'Size use 64;
>
> which means that the lsb is 1 nanosecond. I'm pretty sure that a
> compiler has to do what you tell it here (or reject the unit).
>
> It may well be that a decimal fixed point type would have exactly the
> same effect but I have never used one.
>
> type My_Duration is digits 9 delta 1.0e-9;
> for My_Duration'Size use 64;
>
> One point we came unstuck over: depending on your machine, the
> alignment of a 64-bit value may end up as 8 bytes, whereas the
> alignment of your POSIX-like representation would be 4 bytes.
>
> Had you considered just using Duration?

The problem with that is that different compilers that we use define 
different precisions for duration.  Also, duration is a binary fixed point. 
A don't want to use those because precision may be lost.  Whether or not the 
loss will be an issue, I do not know but I would rather not risk it.  Of 
course I just may be worrying over nothing.

Thanks.

REH