Crate half

A crate that provides support for half-precision 16-bit floating point types.

This crate provides the f16 type, which is an implementation of the IEEE 754-2008 standard binary16 a.k.a “half” floating point type. This 16-bit floating point type is intended for efficient storage where the full range and precision of a larger floating point value is not required. This is especially useful for image storage formats.

This crate also provides a bf16 type, an alternative 16-bit floating point format. The bfloat16 format is a truncated IEEE 754 standard binary32 float that preserves the exponent to allow the same range as f32 but with only 8 bits of precision (instead of 11 bits for f16). See the bf16 type for details.

Because f16 and bf16 are primarily for efficient storage, floating point operations such as addition, multiplication, etc. are not always implemented by hardware. When hardware does not support these operations, this crate emulates them by converting the value to f32 before performing the operation and then back afterward.

Note that conversion from f32/f64 to both f16 and bf16 are lossy operations, and just as converting a f64 to f32 is lossy and does not have Into/From trait implementations, so too do these smaller types not have those trait implementations either. Instead, use from_f32/from_f64 functions for the types in this crate. If you don’t care about lossy conversions and need trait conversions, use the appropriate num-traits traits that are implemented.

This crate also provides a slice module for zero-copy in-place conversions of u16 slices to both f16 and bf16, as well as efficient vectorized conversions of larger buffers of floating point values to and from these half formats.

The crate supports #[no_std] when the std cargo feature is not enabled, so can be used in embedded environments without using the Rust std library. The std feature enables support for the standard library and is enabled by default, see the Cargo Features section below.

A prelude module is provided for easy importing of available utility traits.

Serialization

When the serde feature is enabled, f16 and bf16 will be serialized as a newtype of u16 by default. In binary formats this is ideal, as it will generally use just two bytes for storage. For string formats like JSON, however, this isn’t as useful, and due to design limitations of serde, it’s not possible for the default Serialize implementation to support different serialization for different formats.

Instead, it’s up to the containter type of the floats to control how it is serialized. This can easily be controlled when using the derive macros using #[serde(serialize_with="")] attributes. For both f16 and bf16 a serialize_as_f32 and serialize_as_string are provided for use with this attribute.

Deserialization of both float types supports deserializing from the default serialization, strings, and f32/f64 values, so no additional work is required.

Hardware support

Hardware support for these conversions and arithmetic will be used whenever hardware support is available—either through instrinsics or targeted assembly—although a nightly Rust toolchain may be required for some hardware. When hardware supports it the functions and traits in the slice and vec modules will also use vectorized SIMD intructions for increased efficiency.

The following list details hardware support for floating point types in this crate. When using std cargo feature, runtime CPU target detection will be used. To get the most performance benefits, compile for specific CPU features which avoids the runtime overhead and works in a no_std environment.

Architecture	CPU Target Feature	Notes
x86/x86_64	f16c	This supports conversion to/from f16 only (including vector SIMD) and does not support any bf16 or arithmetic operations.
aarch64	fp16	This supports all operations on f16 only.

Cargo Features

This crate supports a number of optional cargo features. None of these features are enabled by default, even std.

• alloc — Enable use of the alloc crate when not using the std library.

  Among other functions, this enables the vec module, which contains zero-copy conversions for the Vec type. This allows fast conversion between raw Vec<u16> bits and Vec<f16> or Vec<bf16> arrays, and vice versa.

• std — Enable features that depend on the Rust std library. This also enables the alloc feature automatically.

  Enabling the std feature enables runtime CPU feature detection of hardware support. Without this feature detection, harware is only used when compiler target supports them.

• serde — Adds support for the serde crate by implementing Serialize and Deserialize traits for both f16 and bf16.

• num-traits — Adds support for the num-traits crate by implementing ToPrimitive, FromPrimitive, AsPrimitive, Num, Float, FloatCore, and Bounded traits for both f16 and bf16.

• bytemuck — Adds support for the bytemuck crate by implementing Zeroable and Pod traits for both f16 and bf16.

• zerocopy — Adds support for the zerocopy crate by implementing AsBytes and FromBytes traits for both f16 and bf16.

• rand_distr — Adds support for the rand_distr crate by implementing Distribution and other traits for both f16 and bf16.

• rkyv – Enable zero-copy deserializtion with rkyv crate.

Modules

• prelude
  A collection of the most used items and traits in this crate for easy importing.
• sliceNon-target_arch="spirv"
  Contains utility functions and traits to convert between slices of u16 bits and f16 or bf16 numbers.
• vecalloc
  Contains utility functions and traits to convert between vectors of u16 bits and f16 or bf16 vectors.

Structs

• bf16
  A 16-bit floating point type implementing the bfloat16 format.
• f16
  A 16-bit floating point type implementing the IEEE 754-2008 standard binary16 a.k.a “half” format.