Question/request/bug(?) re. floating-point in dmd

[Walter Bright]

On 10/29/2013 5:08 AM, Don wrote:
> On Wednesday, 23 October 2013 at 16:50:52 UTC, Walter Bright wrote:
>> On 10/23/2013 9:22 AM, David Nadlinger wrote:
>>> On Wednesday, 23 October 2013 at 16:15:56 UTC, Walter Bright wrote:
>>>> A D compiler is allowed to compute floating point results at arbitrarily large
>>>> precision - the storage size (float, double, real) only specify the minimum
>>>> precision.
>>>>
>>>> This behavior is fairly deeply embedded into the front end, optimizer, and
>>>> various back ends.
>>>
>>> I know we've had this topic before, but just for the record, I'm still not sold
>>> on the idea of allowing CTFE to yield different results than runtime execution.
>>
>> Java initially tried to enforce a maximum precision, and it was a major
>> disaster for them. If I have been unable to convince you, I suggest reviewing
>> that case history.
>>
>> Back when I designed and built digital electronics boards, it was beaten into
>> my skull that chips always get faster, never slower, and the slower parts
>> routinely became unavailable. This means that the circuits got designed with
>> maximum propagation delays in mind, and with a minimum delay of 0. Then, when
>> they work with a slow part, they'll still work if you swap in a faster one.
>>
>> FP precision is the same concept. Swap in more precision, and your correctly
>> designed algorithm will still work.
>
>
> THIS IS COMPLETELY WRONG. You cannot write serious floating-point code under
> such circumstances. This takes things back to the bad old days before IEEE,
> where results were implementation-dependent.
>
> We have these wonderful properties, float.epsilon, etc, which allow code to
> adapt to machine differences. The correct approach is to write generic code
> which will give full machine precision and will work on any machine
> configuration. That's actually quite easy.
>
> But to write code which will function correctly when an unspecified and
> unpredictable error can be added to any calculation -- I believe that's
> impossible. I don't know how to write such code.

Unpredictable, sure, but it is unpredictable in that the error is less than a 
guaranteed maximum error. The error falls in a range 0<=error<=epsilon. As an 
analogy, all engineering parts are designed with a maximum deviation from the 
ideal size, not a minimum deviation.