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?
Russ
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