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Re: Refine documentation of libm exceptions goals [committed]


* 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

but this kind of makes underflow exceptions in libm unreliable from the
application point of view: they can be omitted anywhere and raised
spuriously anywhere (unless the implementation documents further guarantees
like glibc does).

i consider this as a loophole in the spec


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