Deployment-gate treatment of protocols without witness tables as never-dependent #75769
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…r-dependent Within Swift 6.0, we expanded an optimization for witness tables that that allowed direct access to the witness table for conformances to any protocol that can never have a witness table, rather than requiring access through `swift_getWitnessTable` that might need to instantiate the witness table. The previous optimization only covered Objective-C protocols, but Swift 6.0 expanded that to marker protocols (such as `Sendable`) as well. However, this constituted an API break when a Swift 6.0 compiler uses a witness table that comes from a library built with an earlier version of Swift, when the protocol inherits from Sendable but the conformance to that protocol otherwise does not require an instantiation function. In such cases, Swift 6.0 would generate code that directly accesses the uninstantiated witness table symbol, which will have NULL entries for any conformance in it that was considered "dependent" by the earlier Swift compiler. Introduce a deployment target check to guard the new optimization. Specifically, when building for a deployment target that predates Swift 6.0, treat conformances to marker protocols as if they might be dependent (so the access patterns go through `swift_getWitnessTable` for potential instantiation on older platforms). For newer deployment targets, use the more efficent direct access pattern. Fixes rdar://133157093.
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aschwaighofer
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Aug 8, 2024
DougGregor
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… targets A change to the way we determined whether a protocol conformance is "dependent" for marker protocols caused an ABI break for Sendable-refining protocols built with pre-6.0 Swift compilers. The fix for this issue (swiftlang#75769) gated the change on deployment target. The deployment target change fixed the original problem, then caused a related issue when a project mixes deployment targets (pre-6.0 and 6.0+) with non-resilient protocols. Rework the logic here to (1) look at the availability of the protocol rather than the deployment target, and (2) always use the more efficient (non-dependent) answer for non-resilient protocols. Fixes rdar://134953989.
DougGregor
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Sep 10, 2024
… targets A change to the way we determined whether a protocol conformance is "dependent" for marker protocols caused an ABI break for Sendable-refining protocols built with pre-6.0 Swift compilers. The fix for this issue (swiftlang#75769) gated the change on deployment target. The deployment target change fixed the original problem, then caused a related issue when a project mixes deployment targets (pre-6.0 and 6.0+) with non-resilient protocols. Exempt non-resilient protocols from this change so we get consistent behavior. Fixes rdar://134953989.
DougGregor
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to DougGregor/swift
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Sep 10, 2024
… targets A change to the way we determined whether a protocol conformance is "dependent" for marker protocols caused an ABI break for Sendable-refining protocols built with pre-6.0 Swift compilers. The fix for this issue (swiftlang#75769) gated the change on deployment target. The deployment target change fixed the original problem, then caused a related issue when a project mixes deployment targets (pre-6.0 and 6.0+) with non-resilient protocols. Exempt non-resilient protocols from this change so we get consistent behavior. Fixes rdar://134953989.
DougGregor
added a commit
that referenced
this pull request
Sep 11, 2024
… targets A change to the way we determined whether a protocol conformance is "dependent" for marker protocols caused an ABI break for Sendable-refining protocols built with pre-6.0 Swift compilers. The fix for this issue (#75769) gated the change on deployment target. The deployment target change fixed the original problem, then caused a related issue when a project mixes deployment targets (pre-6.0 and 6.0+) with non-resilient protocols. Exempt non-resilient protocols from this change so we get consistent behavior. Fixes rdar://134953989.
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Within Swift 6.0, we expanded an optimization for witness tables that that allowed direct access to the witness table for conformances to any protocol that can never have a witness table, rather than requiring access through
swift_getWitnessTablethat might need to instantiate the witness table.The previous optimization only covered Objective-C protocols, but Swift 6.0 expanded that to marker protocols (such as
Sendable) as well.However, this constituted an ABI break when a Swift 6.0 compiler uses a witness table that comes from a library built with an earlier version of Swift, when the protocol inherits from Sendable but the conformance to that protocol otherwise does not require an instantiation function. In such cases, Swift 6.0 would generate code that directly accesses the uninstantiated witness table symbol, which will have NULL entries for any conformance in it that was considered "dependent" by the earlier Swift compiler.
Introduce a deployment target check to guard the new optimization. Specifically, when building for a deployment target that predates Swift 6.0, treat conformances to marker protocols as if they might be dependent (so the access patterns go through
swift_getWitnessTablefor potential instantiation on older platforms). For newer deployment targets, use the more efficent direct access pattern.Fixes rdar://133157093, fixes #75155