A number of tests exercise features only available in Apple OSes that
shipped with Swift 5.0 in the OS; this includes the following versions:
- macOS 10.14.4
- iOS 12.2
- tvOS 12.2
- watchOS 5.2
Previously these tests were restricted to running on macOS only, with
an explicit -target x86_64-apple-macosx10.14.4. To get better test
coverage, add a new %target-stable-abi-triple substitution which
expands to a triple with the correct OS version on all Apple platforms.
On non-Apple platforms, this is the same as %target-variant-triple,
but for now any test that uses this exercises Apple platform features
anyway.
One caveat is that since iOS 12.2 does not have a 32-bit slice, we
have to skip any tests that use -target %target-stable-abi-triple
on this platform. A new swift_stable_abi feature flag can be tested
with 'REQUIRES: swift_stable_abi'. To get maximum test coverage,
I split off a 'stable_abi' version of a few tests that build with both
an old and new deployment target. This allows the old deployment
target case to still be tested on 32-bit iOS.
@_optimize(none) does not prevent LLVM from optimizing the function, and the LLVM
optimizer was able to inline blackHole() and elide the metadata access altogether.
This initialization pattern can only be used if there is a backward deployment
layout (IRGen calls this ClassMetadataStrategy::FixedOrUpdate) or if we are
running on a newer Objective-C runtime that supports class metadata update
hooks (IRGen calls this ClassMetadataStrategy::Update).
If neither condition holds, we must trap here to avoid undefined behavior.
