bigfloat II

[Don]

Paul D. Anderson wrote:
> Joel C. Salomon Wrote:
> 
>> Paul D. Anderson wrote:
>>> Multiplying two floats produces a number whose potential precision is the sum of the operands' precision. We need a method to determine what the precision of the product should be. Not that it's difficult to come up with an answer -- but we have to agree on something.
>> Not more precision than the input data deserve.  The decimal
>> floating-point numbers of the new IEEE 754-2008 carry with them a notion
>> of “how precise is this result?”; this might be a good starting point
>> for discussion.
>>
>> —Joel Salomon
> 
> The implementation will comply with IEEE 754-2008. I just wanted to illustrate that precision can depend on the operation as well as the operands.
> 
> Paul

I'm not sure why you think there needs to be a precision depending on 
the operation.
The IEEE 754-2008 has the notion of "Widento" precision, but AFAIK it's 
primarily to support x87 -- it's pretty clear that the normal mode of 
operation is to use the precision which is the maximum precision of the 
operands. Even when there is a Widento mode, it's only provided for the 
non-storage formats (Ie, 80-bit reals only).

I think it should operate the way int and long does:
typeof(x*y) is x if x.mant_dig >= y.mant_dig, else y.
What you might perhaps do is have a global setting for adjusting the 
default size of a newly constructed variable. But it would only affect 
constructors, not temporaries inside expressions.