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.
