ES Decimal status

David-Sarah Hopwood david.hopwood at industrial-designers.co.uk
Fri Sep 12 16:01:22 PDT 2008


Sam Ruby wrote:
> On Fri, Sep 12, 2008 at 4:02 PM, David-Sarah Hopwood
> <david.hopwood at industrial-designers.co.uk> wrote:
>> Sam Ruby wrote:
>>> Description of ES 3.1m decimal support
>>>
>>>      number and decimal are distinct primitive data types.  The
>>>      former is based on IEEE 754 binary 64 floating point, the latter
>>>      is based on IEEE 754r decimal 128 floating point.
>> [...]
>>>      Conversion from number to decimal is precise and will round trip.
>> Conversion of number to decimal is not precise.
> 
> I chose my words carefully :-)
> 
> http://en.wikipedia.org/wiki/Accuracy_and_precision

I agree that decimal has a precision greater than number, but you said
that "Conversion from number to decimal is precise" without qualification.
In the case of a deterministic conversion rather than a measurement, I
think it is quite unclear to use the term "precise" in this context
(as opposed to "repeatable" or "reproducible"), and I don't think that
the spec should express it in that way.

>> I'm also not sure whether the round-trip property covers all NaNs
>> (preserving payload), denormal, or unnormal values.
> 
> NaN payloads should be preserved (decimal has quite a few more bits).
> 
> The results, however, wouldn't preserve the "normalness" (or lack
> there of) of the original input.

Thanks for the clarification. Does IEEE 754r (or the ES3.1m interpretation/
profile of it) specify that NaN payloads shall be preserved in a way that
round-trips?

-- 
David-Sarah Hopwood


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