[Cfp-interest 1803] Re: Exact subnormal results

[David Hough CFP]

Two problems in 754 - since 1977 - 
have been that the underflow exception can arise
without raising any flag, and it can arise 
without producing a subnormal or zero result.

This first case, associated with exact subnormal result,
is unlike any other 754 exception.
However with alternate exception handling one can detect the first case.

The second case means that underflow can't be detected just by looking for a
subnormal or zero result.
Worse, in binary, a minimum normalized result handled by default
might be accompanied by
an underflow flag in some implementations and not others.

This has always been problematic to map into languages in a way that applies
to non-754 implementations.

I consider the definition of underflow to be a bad design choice in 754,
but it can't be fixed while maintaining upward compatibility of implementations.