[swift-evolution] [Swift4][Pitch] Control struct layout with @layout, @offset, @order

[Joe Groff]

> On Aug 17, 2016, at 11:28 AM, Russ Bishop via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I wanted to gauge the interest in supporting explicit struct layout and alignment.
> 
> The general idea would be to support attributes to create packed structs and set alignment. It will be critical for certain kinds of interop and systems programming in pure Swift. I don’t know about you but I want to write a kernel module in Swift someday :) I’m bringing it up because it probably affects ABI stability and resilience and so meets the requirements for Swift 4 phase 1.
> 
> Hopefully this could be a jumping-off point for further discussion. If the core team doesn’t think this affects ABI stability then we can postpone discussing it until Swift 4 phase 1 wraps up.
> 
> 
> @layout(style: auto, alignment: natural)
> 
> enum LayoutStyle {
>     /// Compiler is free to order the fields as it likes
>     case automatic
>     /// Places a struct's members in source order
>     case sequential
>     /// Requires each member to have an @order attribute
>     case ordered
>     /// Requires each member to have an @offset attribute
>     case explicit
> }
> 
> Only structs with certain @layout attributes would be exportable to non-Swift languages (assuming such support is added at some point). A struct defined with ordered or explicit layout and as resilient could possibly avoid indirect access to members since the ABI contract guarantees the location of each member.
> 
> 
> enum Alignment {
>     /// Compiler decides
>     case natural
>     /// Align as if the natural alignment were specified bytes
>     case bytes(Int)
> }
> 
> 
> 
> @order(<int>)
> 
> Specifies the order of the member in memory. Order must start at zero and increase monotonically without gaps. This makes accidental changes to the ordering errors.
> 
> 
> 
> @offset(<int>)
> 
> Specifies the offset from the start of the struct where this member should be placed. The size of the struct becomes the highest offset plus the size of the member with that offset.
> 
> It is an open question whether overlapping alignment should be allowed. If it is allowed I’d propose that for any set of members overlapping all of the members must be entirely contained within the largest overlapping member (I’m thinking of C-style unions when the struct format is imposed by something outside your control).
> 
> 
> 
> Thoughts?

Property behaviors could conceivably get you at least some of the way here. You could do something like this:

protocol ManualLayout {
  associatedtype Storage
  var storage: Storage
}

struct Foo: ManualLayout {
  var storage: [4 x Int32] // give me 16 bytes of 32-bit-aligned storage

  var [offset(0)] foo: Int32
  var [offset(5)] bar: Int32
  var [offset(10)] bas: Int32
}

-Joe