[SR-3723] Determine if the layout algorithm for structs should layout fields in a different order than declared

[ematejska]

Previous ID	SR-3723
Radar	rdar://problem/31411220
Original Reporter	@ematejska
Type	Sub-task

Additional Detail from JIRA

Votes	0
Component/s
Labels	Sub-task, AffectsABI
Assignee	None
Priority	Medium

md5: a416a15ebb462bf1f07d78415242bde1

Parent-Task:

• SR-7809 Define layout algorithm for structs

is duplicated by:

• SR-3724 Evaluate a fully declaration-order-agnostic layout algorithm for structs

Issue Description:

The layout algorithm for structs should result in an efficient use of space, possibly by laying out fields in a different order than declared. This needs to be figured out ahead of ABI stability.

[bob-wilson]

@swift-ci create

[dabrahams]

Here's an example that I would like to work:

import Swift
struct X { var a: Builtin.Int120, b: UInt8 }
assert(MemoryLayout<X>.size * 8 == 128)

The alternative, in the general case, is some really dangerous unsafeBitcast’ing.

Note that this works:

struct X { var a: (Int64,Int32,Int16,Int8), b: UInt8 }
assert(MemoryLayout<X>.size * 8 == 128)

[swift-ci]

Comment by Andrew (JIRA)

Another example which would be nice ...

import Swift
struct X { var a : UInt32, b : UInt8  }
struct Y { var a : UInt8,  b : UInt32 }
assert(MemoryLayout<X>.size * 8 == 40)
assert(MemoryLayout<Y>.size * 8 == 40) 

[bob-wilson]

We are not currently planning to make changes to the struct layout algorithm for Swift 5, either to minimize alignment padding or to define a declaration-order-agnostic layout (see SR-3724). Future versions of Swift might still make those changes if the layouts are versioned.