[6.2] Insert end_cow_mutation_addr for lifetime dependent values dependent on mutable addresses #81216
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…on mutable addresses Array/ArraySlice/ContiguousArray have support for mutableSpan property. These types are "optimized COW" types, we use compiler builtins begin_cow_mutation/end_cow_mutation to optimize uniqueness checks. Since mutableSpan is a property and not a coroutine there is no way to schedule the end_cow_mutaton operation at the end of the access. This can lead to miscompiles in rare cases where we can end up using a stale storage buffer after a cow. This PR inserts end_cow_mutation_addr to avoid this issue. Note: We can end up with unnecessary end_cow_mutation_addr. But it is just a barrier to prevent invalid optimizations and has no impact.
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tbkka
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Apr 30, 2025
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Explanation : Array/ArraySlice/ContiguousArray have support for mutableSpan property. These types are "optimized COW" types, we use compiler builtins begin_cow_mutation/end_cow_mutation to optimize uniqueness checks. Since mutableSpan is a property and not a coroutine there is no way to schedule the end_cow_mutaton operation at the end of the access.
This can lead to miscompiles in rare cases where we can end up using a stale storage buffer after a cow.
This PR inserts end_cow_mutation_addr to avoid this issue.
Note: We can end up with unnecessary end_cow_mutation_addr. But it is just a barrier to prevent invalid optimizations
and has no impact.
Scope: Affects ~Escapable types
Risk: Medium. Although the scope is minimal, classifying this as "Medium" since this is a large change.
Main PR: #81043
Reviewers: @atrick @eeckstein
Issue: rdar://146785284