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Use of SNaN

Vincent Lefèvre wrote:
> Sorry, I should have been more accurate.  I meant that it can be
> possible in languages (at least in C), partly based on the fact
> that IEEE 754 specifies the sNaN encoding.  In C, this can be
> done by storing a sequence of bytes using a union.  Of course,
> this can only be implementation-defined because IEEE 754 specifies
> the encoding only as a bit-string (not as a sequence of bytes).

Unfortunately sNaN encoding is specified only for DFP by 754-2008.
For BFP the encoding is only suggested:  the 1st bit of the trailing
significand SHOULD be 1 for qNaN and 0 for sNaN.  That's because
there is existing hardware (HP's PA-RISC, I believe) that uses the
opposite polarity.

I just re-read 6.2.1 and noticed that even the position of the bit
is only a SHOULD, never mind the polarity.  But I think everybody
does use the same bit.  (This is why stds-754 is CCed.)

Michel.
---Sent: 2012-01-09 16:58:28 UTC