Doug Gregor d78143e5f2 Deployment-gate treatment of protocols without witness tables as never-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.

2024-08-07 22:05:56 -07:00

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