[AllocBoxToStack] Don't destroy in dead-ends. #83907
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SwiftCompilerSources/Sources/Optimizer/FunctionPasses/AllocBoxToStack.swift
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The rewrite was missing the intentional omission of `dealloc_stack`s corresponding to `[dead_end]` `dealloc_box`es. Add the necessary bridging to get to parity with the original. Without this check, `dealloc_box [dead_end]`s are promoted to `dealloc_stack`s but the memory projected out of such `alloc_box`s need not be valid. rdar://159271158
It will be used by both AllocBoxToStack in addition to StackPromotion and for the same reason.
It is valid to leak a value on paths into dead-end regions. Specifically, it is valid to leak an `alloc_box`. Thus, "final releases" in dead-end regions may not destroy the box and consequently may not release its contents. Therefore it's invalid to lower such final releases to `dealloc_stack`s, let alone `destroy_addr`s. The in-general invalidity of that transformation results in miscompiling whenever a box is leaked and its projected address is used after such final releases. Fix this by not treating final releases as boundary markers of the `alloc_box` and not lowering them to `destroy_addr`s and `dealloc_stack`s. rdar://158149082
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It is valid to leak a value on paths into dead-end regions. Specifically, it is valid to leak an
alloc_box. Thus, "final releases" in dead-end regions may not destroy the box and consequently may not release its contents. Therefore it's invalid to lower such final releases todealloc_stacks, let alonedestroy_addrs. The in-general invalidity of that transformation results in miscompiling whenever a box is leaked and its projected address is used after such final releases.Fix this by not treating final releases as boundary markers of the
alloc_boxand not lowering them todestroy_addrs anddealloc_stacks.rdar://158149082