Are the bit patterns of NaNs really hardware-dependent?

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This is what §2.3.2 of the JVM 7 spec has to say about it:

The elements of the double value set are exactly the values that can be represented
using the double floating-point format defined in the IEEE 754 standard, except
that there is only one NaN value (IEEE 754 specifies 253-2 distinct NaN values).

and §2.8.1:

The Java Virtual Machine has no signaling NaN value.

So technically there is only one NaN. But §4.2.3 of the JLS also says (right after your quote):

For the most part, the Java SE platform treats NaN values of a given type as though collapsed into a single canonical value, and hence this specification normally refers to an arbitrary NaN as though to a canonical value.

However, version 1.3 of the Java SE platform introduced methods enabling the programmer to distinguish between NaN values: the Float.floatToRawIntBits and Double.doubleToRawLongBits methods. The interested reader is referred to the specifications for the Float and Double classes for more information.

Which I take to mean exactly what you and CandiedOrange propose: It is dependent on the underlying processor, but Java treats them all the same.

But it gets better: Apparently, it is entirely possible that your NaN values are silently converted to different NaNs, as described in Double.longBitsToDouble():

Note that this method may not be able to return a double NaN with exactly same bit pattern as the long argument. IEEE 754 distinguishes between two kinds of NaNs, quiet NaNs and signaling NaNs. The differences between the two kinds of NaN are generally not visible in Java. Arithmetic operations on signaling NaNs turn them into quiet NaNs with a different, but often similar, bit pattern. However, on some processors merely copying a signaling NaN also performs that conversion. In particular, copying a signaling NaN to return it to the calling method may perform this conversion. So longBitsToDouble may not be able to return a double with a signaling NaN bit pattern. Consequently, for some long values, doubleToRawLongBits(longBitsToDouble(start)) may not equal start. Moreover, which particular bit patterns represent signaling NaNs is platform dependent; although all NaN bit patterns, quiet or signaling, must be in the NaN range identified above.

For reference, there is a table of the hardware-dependant NaNs here. In summary:

- x86:     
   quiet:      Sign=0  Exp=0x7ff  Frac=0x80000
   signalling: Sign=0  Exp=0x7ff  Frac=0x40000
- PA-RISC:               
   quiet:      Sign=0  Exp=0x7ff  Frac=0x40000
   signalling: Sign=0  Exp=0x7ff  Frac=0x80000
- Power:
   quiet:      Sign=0  Exp=0x7ff  Frac=0x80000
   signalling: Sign=0  Exp=0x7ff  Frac=0x5555555500055555
- Alpha:
   quiet:      Sign=0  Exp=0      Frac=0xfff8000000000000
   signalling: Sign=1  Exp=0x2aa  Frac=0x7ff5555500055555

So, to verify this you would really need one of these processors and go try it out. Also any insights on how to interpret the longer values for the Power and Alpha architectures are welcome.

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