You miss my point; or more likely I didn't make it clear enough.

The point is that it is trivial to assign a signalling NaN to an NV; the problem arises when you try to verify/prove that you've done so. because having got the value in there, in order to access it, almost any code you use to get access to the NV (including unpack), will use SvNVX(), and that macro performs the (pointless) floating point addition, which will mean the SNaN will have been coerced to a QNaN before you can check that the assignment was successful; thus you will forever think the assignment failed, when in fact it succeeded, and it is only the attempt to verify it fails.

Though I suppose another view might be: can you judge the assignment to be successful, if you can never get the value you assigned, back, without it having been corrupted.

Either of these should produce an SNaN under 32-bit:

printf "%f\n", unpack 'd', pack 'VV', 0x00000001, 0x7FF00000;;
1.#QNAN0

printf "%f\n", unpack 'F', pack 'VV', 0x00000001, 0x7FF00000;;
1.#QNAN0

[download]

They don't, because the unpack (via the SvNVX() macro) adds -0.0 to the value generated by the pack. And that's a bug.

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In reply to Re^8: unintentional conversion of signaling NaN to quiet NaN by BrowserUk
in thread unintentional conversion of signaling NaN to quiet NaN by pryrt

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