On 8/20/2016 3:16 PM, Martin Buchholz wrote:


On Sat, Aug 20, 2016 at 12:32 PM, Martin Buchholz <marti...@google.com <mailto:marti...@google.com>> wrote:


    Quibble: "guaranteed" may be true in practice,
    but longBitsToDouble is technically permitted to reset the sign
    bit back to 1 if the hardware is hostile. Because of this, I would
    simply remove the word "guaranteed".


After reading IEEE 754 2008 6.2.1. , I now believe signalling NaNs have to use the significand bits; the sign bit is free to use; and so I retract my quibble, and am happy with the word "guaranteed".

Right; the sign bit of a NaN has very little meaning. The hardware guys like to be able to do things like for a floating-point multiply "sign bit of output = XOR of sign bits of inputs" without having to do inconvenient NaN checks :-) The NaN payload for retrospective diagnostics is all in the significand.

Cheers,

-Joe

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