Andrew Trick 26a62645b4 Fix MandatoryARCOpts tryJoiningCopyValueLiveRangeWithOperand. ...

Fix a miscompile in Debug builds at -Onone.

This optimization ignores uses of owned values that aren't enclosed in
borrow scopes. This is fairly eggregious since project_box
instructions are never borrowed, which means that all local variables
have this problem.

This is a well-known issue that occurs throughout OSSA
optimizations. The reason that we don't see the problem often is that
the optimizations are hidden behind a pile of ad-hoc pattern matching,
so they only kick in for simple cases. This approach to optimization
is great at hiding problems for a long time.

We're attempting to design away this class of problems in the next
release. Until then, it's one miscompile at a time.

Fixes rdar://107420448 (Variable mutation in block isn't reflected
in outer scope: new behavior in swift 5.9)

2023-03-31 20:33:53 -07:00

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