Casting double to ulong weirdness

[via Digitalmars-d]

On Tuesday, 25 August 2015 at 17:40:06 UTC, Steven Schveighoffer 
wrote:
> I'll note that D does exactly what C does in the case where you 
> are using 80-bit floating point numbers.

I don't think C specifies how it should be done, but some 
compilers have a "precise" compilation flag that is supposed to 
retain order and accurate intermediate rounding.

> IMO, these two operations should be the same. If the result of 
> an expression is detected to be double, then it should behave 
> like one. You can't have the calculation done in 80-bit mode, 
> and then magically throw away the rounding to get to 64-bit 
> mode.

Yes, that is rather obvious. IEEE754-2008 go much further than 
that, though. It requires that all arithmetic have correct 
rounding. Yes, I am aware that the D specification allows higher 
precision, but it seems to me that this neither gets you 
predictable results or maximum performance. And what is the point 
of being able to set the rounding mode if you don't know the bit 
width used?

It is a practical issue in all simulations where you want 
reproducible results. If D is meant for scientific computing it 
should support correct rounding and reproducible results. If D is 
meant for gaming it should provide ways of expressing minimum 
precision or other ways of loosening the accuracy where needed.  
I'm not really sure which group the current semantics appeals to. 
I personally either want reproducible or very fast...