Tracking Issue for Integer Funnel Shifts

[sayantn]

Feature gate: #![feature(funnel_shifts)]

This is a tracking issue for implementation of funnel shifts on integers.

Funnel shifts are essentially a generalization of bitwise rotations. a.funnel_shl(b, n) means concatenate a and b (with a in the most significant half), creating an integer twice as wide, shifting this to the left by n (taken modulo the bit size of a and b), shifting in zeros, and finally extract the most significant half of the wide integer as the result.

The functions will panic if n >= T::BITS, and the wrapping versions consider n modulo T::BITS. The unchecked variant doesn't do any checks (as the name suggests), and so is unsafe.

a.wrapping_funnel_shl(a, n) is meant to be exactly equivalent to a.rotate_left(n).

Public API

• funnel_shl and funnel_shr (and their wrapping and unchecked variants) on the primitive integer types u8, u16, u32, u64 and u128

Steps / History

A SIMD version of these intrinsics were added in #142078

• Implementation
• Final comment period (FCP)¹
• Stabilization PR

Unresolved Questions

• Should we also provide these functions on signed integer types (akin to rotates)?

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩