Tracking Issue for Integer Funnel Shifts

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

impl {u8, u16, u32, u64, u128, usize} {
    pub const fn funnel_shl(self, rhs: Self, shift: u32) -> Self;
    pub const fn funnel_shr(self, rhs: Self, shift: u32) -> Self;

    pub const fn wrapping_funnel_shl(self, rhs: Self, shift: u32) -> Self;
    pub const fn wrapping_funnel_shr(self, rhs: Self, shift: u32) -> Self;

    pub const unsafe fn unchecked_funnel_shl(self, rhs: Self, shift: u32) -> Self;
    pub const unsafe fn unchecked_funnel_shr(self, rhs: Self, shift: u32) -> Self;
}

Steps / History

A SIMD version of these intrinsics were added in #142078

• ACP: https://github.com/rust-lang/libs-team/issues/642
• Implementation
  • #145690
  • https://github.com/rust-lang/rust/pull/154153
• Final comment period (FCP)^1
• Stabilization PR

Unresolved Questions

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

Start by reading the Public API and Steps / History sections, then inspect implementation references #145690 and #154153. No file or test is named. The remaining work is the final comment period and stabilization PR, with the signed-integer question still unresolved.