* Explain unsafe contracts of core::simd
This permeates the module with remarks on safety for pub methods,
layout of the Simd type, correct use of intrinsics, et cetera.
This is mostly to help others curious about how core::simd works,
including other Rust contributors, `unsafe` library authors,
and eventually ourselves.
Aligns module with rust-lang/library/core, creating an... unusual
architecture that is easier to pull in as a module, as core itself can
have no dependencies (as we haven't built core yet).
Add SimdArray trait and safe gather/scatter API (rust-lang/stdsimd#139)
This PR has four parts, without which it doesn't make a lot of sense:
- The introduction of the SimdArray trait for abstraction over vectors.
- The implementation of private vector-of-pointers types.
- Using these to allow constructing vectors with SimdArray::gather_{or, or_default, select}.
- Using these to allow writing vectors using SimdArray::scatter{,_select}.
This const generic implementation for certain lane sizes represents
a more limited interface than what LLVM's shufflevector instruction
can handle, as normally the length of U can be different from the
length of T, but offers an interface that it is expected to be able
to expand the capabilities of in the future.
