Disable the definition and use of swift_async_extendedFramePointerFlags on watchOS #39935
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…gs on watchOS The asm definition of `swift_async_extendedFramePointerFlags` prevents the use of bitcode with the back-deployment libraries, so remove the definition and use of this symbol from watchOS binaries entirely. Instead, always force the async frame bit to be set. This trades off backtraces on older OS's for debuggability of newer ones. If it causes problems, it can be disabled via the option `-swift-async-frame-pointer=never`. Fixes rdar://84687579.
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mikeash
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Oct 27, 2021
Introduce a few changes to the logic for watchOS concurrency back-deployment with respect to the async frame pointer. * Only apply the change to watchOS device targets, not simulator targets * Only introduce the override when no specific `-swift-async-frame-pointer=<value>` option is provided on the command line * Only override the default when deploying to watchOS < 8, and * Use "never" for the default rather than "always". This represents a different but safer trade-off than before. Setting the async bit in the frame pointer can cause older APIs (such as backtrace APIs in the OS) to crash when they encounter such frame pointers. So, with this change we never set the bit when back-deploying for watchOS device, to avoid said crashes. The trade-off here is that a back-deployed watchOS app will never have the async frame pointer bit set, so async backtraces will be unavailable even when running on watchOS 8 or newer.
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The asm definition of
swift_async_extendedFramePointerFlagsprevents the use of bitcode with the back-deployment libraries on watchOS, so remove the definition and use of this symbol from watchOS binaries entirely. This affects whether the async bit is set in frame pointers. Setting the async bit in the frame pointer can cause older APIs (such as backtrace APIs in the OS) to crash when they encounter such frame pointers, but we cannot dynamically set it withoutswift_async_extendedFramePointerFlags. So, we never set the bit when back-deploying for watchOS device, to avoid said crashes.The trade-off here is that a back-deployed watchOS app will never have the async frame pointer bit set, so async backtraces will be unavailable even when running such an app on watchOS 8 or newer. Moving the deployment target forward to watchOS 8, or running under the simulator for watchOS 8, will restore async back traces.
One can override this behavior with the option
-swift-async-frame-pointer=always, which statically sets the async bit in the frame pointer. This restores async back traces on watchOS 8, but can lead to the aforementioned crashes on older versions. This approach may be suitable for debugging but is unlikely to be a good idea when deploying.Fixes rdar://84687579.