Refine documentation of libm exceptions goals [committed]

[Rich Felker]

On Wed, Feb 18, 2015 at 06:45:38PM +0000, Joseph Myers wrote:
> On Wed, 18 Feb 2015, Szabolcs Nagy wrote:
> 
> > * Joseph Myers <joseph@codesourcery.com> [2015-02-18 17:25:38 +0000]:
> > > On Wed, 18 Feb 2015, Szabolcs Nagy wrote:
> > > 
> > > > this is an interesting issue because iso c annex f forbids the
> > > > omission of underflow right now, which is hard to achive unless
> > > > libm checks <= DBL_MIN instead of the sensible < DBL_MIN and
> > > > raises spurious underflow for a lot of exact (or non-tiny) DBL_MIN
> > > > results (which is allowed by the standard)
> > > 
> > > I don't believe it forbids computing an imprecise result that happens to 
> > > be (exactly) DBL_MIN or more, and so doesn't underflow.
> > > 
> > 
> > well, there are no precision requirements on not exactly-defined math
> > functions, but with that interpretation an implementation which never
> > raises underflow is also correct: it can claim that it exactly computed
> > an incorrect result so there is no precision loss and thus no underflow
> 
> Yes.  This falls under the general implementation-defined accuracy of libm 
> functions.

I think this is a stretch to say that implementation-defined accuracy
allows an implementation to falsely report a result as exact when it's
not. For all functions in the math library it's relatively trivial to
know that the result is non-exact; only a small number of functions in
a small number of cases even admit exact results.

Rich