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Diffstat (limited to 'library/core/src/primitive_docs.rs')
| -rw-r--r-- | library/core/src/primitive_docs.rs | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/library/core/src/primitive_docs.rs b/library/core/src/primitive_docs.rs index 09ebef89fb0..7f35c7789e2 100644 --- a/library/core/src/primitive_docs.rs +++ b/library/core/src/primitive_docs.rs @@ -1190,6 +1190,11 @@ mod prim_f16 {} /// portable or even fully deterministic! This means that there may be some /// surprising results upon inspecting the bit patterns, /// as the same calculations might produce NaNs with different bit patterns. +/// This also affects the sign of the NaN: checking `is_sign_positive` or `is_sign_negative` on +/// a NaN is the most common way to run into these surprising results. +/// (Checking `x >= 0.0` or `x <= 0.0` avoids those surprises, but also how negative/positive +/// zero are treated.) +/// See the section below for what exactly is guaranteed about the bit pattern of a NaN. /// /// When a primitive operation (addition, subtraction, multiplication, or /// division) is performed on this type, the result is rounded according to the @@ -1211,6 +1216,79 @@ mod prim_f16 {} /// *[See also the `std::f32::consts` module](crate::f32::consts).* /// /// [wikipedia]: https://en.wikipedia.org/wiki/Single-precision_floating-point_format +/// +/// # NaN bit patterns +/// +/// This section defines the possible NaN bit patterns returned by non-"bitwise" floating point +/// operations. The bitwise operations are unary `-`, `abs`, `copysign`; those are guaranteed to +/// exactly preserve the bit pattern of their input except for possibly changing the sign bit. +/// +/// A floating-point NaN value consists of: +/// - a sign bit +/// - a quiet/signaling bit +/// - a payload, which makes up the rest of the significand (i.e., the mantissa) except for the +/// quiet/signaling bit. +/// +/// Rust assumes that the quiet/signaling bit being set to `1` indicates a quiet NaN (QNaN), and a +/// value of `0` indicates a signaling NaN (SNaN). In the following we will hence just call it the +/// "quiet bit". +/// +/// The following rules apply when a NaN value is returned: 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. +/// +/// 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 +/// value. Similarly, if all input NaNs are preferred (or if there are no input NaNs) and the target +/// does not have any "extra" NaN payloads, then the output NaN is guaranteed to be preferred. +/// +/// The non-deterministic choice happens when the operation is executed; i.e., the result of a +/// NaN-producing floating point operation is a stable bit pattern (looking at these bits multiple +/// times will yield consistent results), but running the same operation twice with the same inputs +/// can produce different results. +/// +/// These guarantees are neither stronger nor weaker than those of IEEE 754: IEEE 754 guarantees +/// that an operation never returns a signaling NaN, whereas it is possible for operations like +/// `SNAN * 1.0` to return a signaling NaN in Rust. Conversely, IEEE 754 makes no statement at all +/// about which quiet NaN is returned, whereas Rust restricts the set of possible results to the +/// ones listed above. +/// +/// Unless noted otherwise, the same rules also apply to NaNs returned by other library functions +/// (e.g. `min`, `minimum`, `max`, `maximum`); other aspects of their semantics and which IEEE 754 +/// operation they correspond to are documented with the respective functions. +/// +/// When a floating-point operation is executed in `const` context, the same rules apply: no +/// guarantee is made about which of the NaN bit patterns described above will be returned. The +/// result does not have to match what happens when executing the same code at runtime, and the +/// result can vary depending on factors such as compiler version and flags. +/// +/// ### Target-specific "extra" NaN values +// FIXME: Is there a better place to put this? +/// +/// | `target_arch` | Extra payloads possible on this platform | +/// |---------------|---------| +/// | `x86`, `x86_64`, `arm`, `aarch64`, `riscv32`, `riscv64` | None | +/// | `sparc`, `sparc64` | The all-one payload | +/// | `wasm32`, `wasm64` | If all input NaNs are quiet with all-zero payload: None.<br> Otherwise: all possible payloads. | +/// +/// For targets not in this table, all payloads are possible. + #[stable(feature = "rust1", since = "1.0.0")] mod prim_f32 {} |