introduce @_manualOwnership performance attribute #81858

kavon · 2025-05-30T01:24:25Z

This attribute forces programmers to acknowledge every copy that is required to happen in the body of the function. only those copies that make sense according to Swift's ownership rules are "required". For example,

func consumingFunc(_ t0: consuming Triangle) {}

@_manualOwnership
func basic_function_call(_ t1: Triangle) {
  consumingFunc(t1)  // error: explicit 'copy' required here
}

The way this is implemented is to flag every implicitly-created copy by SILGen in a function as an error. This keeps SILGen and the Swift language in sync about what copies are actually happening, which motivates fixes for cases where SILGen can be more clever.

This also means that there is no hand-holding to avoid exclusivity violations. Static checking will continue to catch some cases, but for other cases, like class objects, it is up to the programmer to understand where they may need to rewrite code with mutable references. This is where the "manual" comes from. For example,

// unsafe if x === y
swap(&x.a, &y.a)

// a safe rewrite
let t = copy x.a
x.a = copy y.a
y.a = consume t

This attribute forces programmers to acknowledge every copy that is required to happen in the body of the function. only those copies that make sense according to Swift's ownership rules are "required". The way this is implemented is to flag every non-explicit copy in a function as an error. This keeps SILGen and the Swift language in sync about what copies are actually happening, which motivates fixes for cases where SILGen can be more clever. This also means that there is _no_ hand-holding to avoid exclusivity violations. Static checking will continue to catch some cases, but for other cases, like class objects, it is up to the programmer to understand where they may need to rewrite code with mutable references. This is where the "manual" comes from. For example, ```swift // unsafe if x === y swap(&x.a, &y.a) // a safe rewrite let t = copy x.a x.a = copy y.a y.a = consume t ```

kavon mentioned this pull request May 30, 2025

SILGen: avoid double copies with 'copy' expr #81861

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introduce @_manualOwnership performance attribute #81858

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kavon commented May 30, 2025 •

edited

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kavon commented May 30, 2025 •

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