[Cfp-interest 1801] Re: Exact subnormal results

[Jim Thomas]

> On Oct 20, 2020, at 5:02 PM, Damian McGuckin <damianm at esi.com.au> wrote:
> 
> 
> Hi Jim,
> 
> On Tue, 20 Oct 2020, Jim Thomas wrote:
> 
>> The definitions are in 7.12.1 in
>> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2573.pdf
>> I assume you?re asking about the C standard.
> 
> Because if you look there, the word underflow is used long before it appears in any definition.
> 
> On page 450,
> 
> 	The "underflow" floating-point exception is raised whenever a
> 	result is tiny (essentially subnormal or zero) and suffers loss of
> 	accuracy.

This is in Annex F and is suppose to reflect the IEC 60559 definition. An update here is needed.

The discussion Fred is raising is about the C definition of underflow, in 7.12.1. Ideally, the C definition would not be inconsistent with the (correct) IEC 60559 one.

> 
> Given:
> 
> 	0.25 * 2**emin
> 
> does this overflow. Because it is exact.  But is Page 450 the strict definition. Because, that answers Fred's question (and agrees with Fred).
> 
> I actually meant in the latest IEEE 754 (or IEC).

See 7.5 in

https://wiki.edg.com/pub/CFP/WebHome/754-2019.pdf <https://wiki.edg.com/pub/CFP/WebHome/754-2019.pdf>

- Jim Thomas

> 
> Sorry, if I seem rushed. I am off to two meetings and did not have time to look it up as thoroughly as I would have liked. But I wanted to get the email out before your part of the world went to bed.
> 
> Regards - Damian
> 
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