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

Russ Bishop xenadu at gmail.com
Wed Aug 17 13:28:20 CDT 2016

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)


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.


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).



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