portable_simd)Expand description
Portable SIMD module.
This module offers a portable abstraction for SIMD operations that is not bound to any particular hardware architecture.
§What is “portable”?
This module provides a SIMD implementation that is fast and predictable on any target.
§Portable SIMD works on every target
Unlike target-specific SIMD in std::arch, portable SIMD compiles for every target.
In this regard, it is just like “regular” Rust.
§Portable SIMD is consistent between targets
A program using portable SIMD can expect identical behavior on any target.
In most regards, Simd<T, N> can be thought of as a parallelized [T; N] and operates like a sequence of T.
This has one notable exception: a handful of older architectures (e.g. armv7 and powerpc) flush subnormal f32 values to zero.
On these architectures, subnormal f32 input values are replaced with zeros, and any operation producing subnormal f32 values produces zeros instead.
This doesn’t affect most architectures or programs.
§Operations use the best instructions available
Operations provided by this module compile to the best available SIMD instructions.
Portable SIMD is not a low-level vendor library, and operations in portable SIMD do not necessarily map to a single instruction. Instead, they map to a reasonable implementation of the operation for the target.
Consistency between targets is not compromised to use faster or fewer instructions.
In some cases, std::arch will provide a faster function that has slightly different behavior than the std::simd equivalent.
For example, _mm_min_ps1 can be slightly faster than SimdFloat::simd_min, but does not conform to the IEEE standard also used by f32::min.
When necessary, Simd<T, N> can be converted to the types provided by std::arch to make use of target-specific functions.
Many targets simply don’t have SIMD, or don’t support SIMD for a particular element type. In those cases, regular scalar operations are generated instead.
_mm_min_ps(x, y)is equivalent tox.simd_lt(y).select(x, y)↩