[addaon]

Don't have the spec handy, but specifically binary operations combining two NaN inputs must result in one of the input NaNs. For all of Intel SSE, AMD SSE, PowerPC, and ARM, the left hand operand is returned if both are signaling or both or quiet. x87 does weird things (but when doesn't it?), and ARM does weird things when mixing signaling and quiet NaNs.

[ekelsen]

I also don't have access to the spec, but the people writing Rust do and they claim this: "IEEE makes almost no guarantees about the sign and payload bits of the NaN"

https://rust-lang.github.io/rfcs/3514-float-semantics.html

See also this section of wikipedia https://en.wikipedia.org/wiki/NaN#Canonical_NaN

"On RISC-V, most floating-point operations only ever generate the canonical NaN, even if a NaN is given as the operand (the payload is not propagated)."

And from the same article:

"IEEE 754-2008 recommends, but does not require, propagation of the NaN payload." (Emphasis mine)

I call bullshit on the statement "specifically binary operations combining two NaN inputs must result in one of the input NaNs." It is definitely not in the spec.

[j16sdiz]

Blame the long and confusing language in spec:

> For an operation with quiet NaN inputs, other than maximum and minimum operations, if a floating-point result is to be delivered the result shall be a quiet NaN which should be one of the input NaNs.

The same document say:

> shall -- indicates mandatory requirements strictly to be followed in order to conform to the standard and from which no deviation is permitted (“shall” means “is required to”)

> should -- indicates that among several possibilities, one is recommended as particularly suitable, without mentioning or excluding others; or that a certain course of action is preferred but not necessarily required; or that (in the negative form) a certain course of action is deprecated but not prohibited (“should” means “is recommended to”)

i.e. It required to be a quiet NaN, and recommended to use one of the input NaN.

[ekelsen]

Thanks for the direct evidence that the output NaN is not required to be one of the input NaNs.