[Concurrency] Nest stack traffic in withValue. #65274
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nate-chandler
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ktoso
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DougGregor
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Because `_taskLocalValuePush` and `_taskLocalValuePop` can result in calls to `swift_task_alloc` and `swift_task_dealloc` respectively, and because the compiler hasn't been taught about that (e.g. `SILInstruction::isAllocatingStack`, `SILInstruction::isDeallocatingStack`, etc), calling them (push and pop) from a function which makes use the stack for dynamically sized allocations can result in violations of stack discipline of the form ``` swift_task_alloc // allocates %ptr_1 copy_value_witness // copies into %ptr_1 swift_task_localValuePush // calls swift_task_alloc and allocates %ptr_2 swift_task_dealloc // deallocates %ptr_1 swift_task_localValuePop // calls swift_task_dealloc and deallocates %ptr_2 ``` Avoid the problem by not allocating dynamically sized stack space in the function which calls `_taskLocalValuePush` and `_taskLocalValuePop`. Split the calls to those functions into `withValueImpl` function which takes its argument `__owned`. Call that function from `withValue`, ensuring that the necessary copy (to account for the fact that withValue takes its argument `__guaranteed` but `_taskLocalValuePush` takes its `__owned`) and associated stack traffic occur in `withValue`. Still, allow `withValueImpl` to be inlined. The stack nesting will be preserved across it. rdar://107275872
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Because
_taskLocalValuePushand_taskLocalValuePopcan result in calls toswift_task_allocandswift_task_deallocrespectively, and because the compiler hasn't been taught about that (e.g.SILInstruction::isAllocatingStack,SILInstruction::isDeallocatingStack, etc), calling them (push and pop) from a function which makes use the stack for dynamically sized allocations can result in violations of stack discipline of the formAvoid the problem by not allocating dynamically sized stack space in the function which calls
_taskLocalValuePushand_taskLocalValuePop. Split the calls to those functions intowithValueImplfunction which takes its argument__owned. Call that function fromwithValue, ensuring that the necessary copy (to account for the fact that withValue takes its argument__guaranteedbut_taskLocalValuePushtakes its__owned) and associated stack traffic occur inwithValue.Still, allow
withValueImplto be inlined. The stack nesting will be preserved across it.rdar://107275872