Bit field macro

Evolution Pitches
1

Currently, making a bit field in Swift is difficult and requires the use of bitwise operations and special knowledge by the programmer. A macro could make this safer and easier.

@BitField
public struct ExampleBitField {
  @BitFieldStored(bits: 2) var i: Int
  @BitFieldStored(bits: 3) var j: Int
  @BitFieldStored(bits: 3) var n: Int
  
  // total bit size of 8
}

The macro could scan the type for each occurrence of a BitFieldStored macro, which would just expand to nothing. From there, it could use the total number of bits to determine an appropriate rawValue for the type.

// expands to:
public struct ExampleBitField {
  var _bits: UInt8

  var i: Int {
    // getter/setter code here
  }

  var j: Int {
    // code here
  }

  var n: Int {
    // code here
  }
}
2 Likes
2

Would an OptionSet do the job instead?

OptionSet

A type that presents a mathematical set interface to a bit set.

protocol OptionSet : RawRepresentable, SetAlgebra

Overview

You use the OptionSet protocol to represent bitset types, where individual bits represent members of a set. Adopting this protocol in your custom types lets you perform set-related operations such as membership tests, unions, and intersections on those types. What’s more, when implemented using specific criteria, adoption of this protocol requires no extra work on your part.

3

This would be very useful when packing/unpacking data types encoded within something like UInts. For example, the IEEE Decimal/Binary number types have multiple fields within a UInt32/64/128 word that normally require masking and shifting to pack/unpack the multi-bit fields. While this messiness is normally hidden within a Double data structure having a bit field facility would make the underlying code easier to understand and perhaps give the compiler more optimization opportunities.

2 Likes
4

Not entirely since this is only a bit-level encoding. It doesn't address multi-bit fields that would be mapped to integers.

5
2 Likes