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enumerate the two parts of the NaN rules

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Ralf Jung 2024-08-30 11:31:36 +02:00
parent 08fadfd8d8
commit f6b7727605
1 changed files with 20 additions and 19 deletions
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39
library/core/src/primitive_docs.rs
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@ -1235,26 +1235,27 @@ mod prim_f16 {}
/// operations are guaranteed to exactly preserve the bit pattern of their input except for possibly
/// changing the sign bit.
///
/// The following rules apply when a NaN value is returned from an arithmetic operation: the result
/// has a non-deterministic sign. The quiet bit and payload are non-deterministically chosen from
/// the following set of options:
/// The following rules apply when a NaN value is returned from an arithmetic operation:
/// - The result has a non-deterministic sign.
/// - The quiet bit and payload are non-deterministically chosen from
/// the following set of options:
///
/// - **Preferred NaN**: The quiet bit is set and the payload is all-zero.
/// - **Quieting NaN propagation**: The quiet bit is set and the payload is copied from any input
/// operand that is a NaN. If the inputs and outputs do not have the same payload size (i.e., for
/// `as` casts), then
/// - If the output is smaller than the input, low-order bits of the payload get dropped.
/// - If the output is larger than the input, the payload gets filled up with 0s in the low-order
/// bits.
/// - **Unchanged NaN propagation**: The quiet bit and payload are copied from any input operand
/// that is a NaN. If the inputs and outputs do not have the same size (i.e., for `as` casts), the
/// same rules as for "quieting NaN propagation" apply, with one caveat: if the output is smaller
/// than the input, droppig the low-order bits may result in a payload of 0; a payload of 0 is not
/// possible with a signaling NaN (the all-0 significand encodes an infinity) so unchanged NaN
/// propagation cannot occur with some inputs.
/// - **Target-specific NaN**: The quiet bit is set and the payload is picked from a target-specific
/// set of "extra" possible NaN payloads. The set can depend on the input operand values.
/// See the table below for the concrete NaNs this set contains on various targets.
/// - **Preferred NaN**: The quiet bit is set and the payload is all-zero.
/// - **Quieting NaN propagation**: The quiet bit is set and the payload is copied from any input
/// operand that is a NaN. If the inputs and outputs do not have the same payload size (i.e., for
/// `as` casts), then
/// - If the output is smaller than the input, low-order bits of the payload get dropped.
/// - If the output is larger than the input, the payload gets filled up with 0s in the low-order
/// bits.
/// - **Unchanged NaN propagation**: The quiet bit and payload are copied from any input operand
/// that is a NaN. If the inputs and outputs do not have the same size (i.e., for `as` casts), the
/// same rules as for "quieting NaN propagation" apply, with one caveat: if the output is smaller
/// than the input, droppig the low-order bits may result in a payload of 0; a payload of 0 is not
/// possible with a signaling NaN (the all-0 significand encodes an infinity) so unchanged NaN
/// propagation cannot occur with some inputs.
/// - **Target-specific NaN**: The quiet bit is set and the payload is picked from a target-specific
/// set of "extra" possible NaN payloads. The set can depend on the input operand values.
/// See the table below for the concrete NaNs this set contains on various targets.
///
/// In particular, if all input NaNs are quiet (or if there are no input NaNs), then the output NaN
/// is definitely quiet. Signaling NaN outputs can only occur if they are provided as an input