Docs for isIdentical say:

   Same as ==, except that positive and negative zero are not identical,
   and two NANs are identical if they have the same 'payload'.

However, it returns false for NaN's with different signbits but same payload.  Should this be the case?

Ran into this because isIdentical is used in unittests to compare NaN's but I find an occassional test fails due to signbit for some operations on LDC ARM.  I wrote an isNaNWithPayload() predicate for the failing tests, but wonder if isIdentical should ignore signbit for NaNs or have its docs changed.
-- 
Dan

On Monday, 22 June 2015 at 08:10:20 UTC, Dan Olson wrote:
> Docs for isIdentical say:
>
>    Same as ==, except that positive and negative zero are not identical,
>    and two NANs are identical if they have the same 'payload'.
>
> However, it returns false for NaN's with different signbits but same payload.  Should this be the case?
>
> Ran into this because isIdentical is used in unittests to compare NaN's but I find an occassional test fails due to signbit for some operations on LDC ARM.  I wrote an isNaNWithPayload() predicate for the failing tests, but wonder if isIdentical should ignore signbit for NaNs or have its docs changed.

Well, given the name of the function and the fact that it's clearly trying to check for NaN equality rather than treating their comparison as always false, I would have expected that it would be true for NaNs if they were absolutely identical, which presumably includes the signbit, but I really don't know much about how NaNs are implemented, so I don't know what the implications of that are. Still, it seems odd to compare part of the NaN for equality but not all of it.

- Jonathan M Davis

"Jonathan M Davis" <jmdavisProg@gmx.com> writes:

> On Monday, 22 June 2015 at 08:10:20 UTC, Dan Olson wrote:
>> Docs for isIdentical say:
>>
>>    Same as ==, except that positive and negative zero are not
>> identical,
>>    and two NANs are identical if they have the same 'payload'.
>>
>> However, it returns false for NaN's with different signbits but same payload.  Should this be the case?
>>
>> Ran into this because isIdentical is used in unittests to compare NaN's but I find an occassional test fails due to signbit for some operations on LDC ARM.  I wrote an isNaNWithPayload() predicate for the failing tests, but wonder if isIdentical should ignore signbit for NaNs or have its docs changed.
>
> Well, given the name of the function and the fact that it's clearly trying to check for NaN equality rather than treating their comparison as always false, I would have expected that it would be true for NaNs if they were absolutely identical, which presumably includes the signbit, but I really don't know much about how NaNs are implemented, so I don't know what the implications of that are. Still, it seems odd to compare part of the NaN for equality but not all of it.

Yeah, based on the name "isIdentical", its current behaviour

   assert(!isIdentical(-NaN(0xabc), NaN(0xabc));

does seem to correct.  In that case the documentation could say "NaNs are identical if they have the same payload and signbit".

On Mon, 22 Jun 2015 09:41:08 +0000, Jonathan M Davis wrote:

> Still, it seems odd to compare
> part of the NaN for equality but not all of it.

it's very useful for NaN-boxing.