[6.0] SILOptimizer: Allow inlining of transparent functions in @backDeployed thunks
#76140
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In the standard library shipped in Apple's SDKs and OSes, the implementation of `_stdlib_isOSVersionAtLeast()` has diverged in order to solve some tricky issues related to supporting iOS applications running on macOS. It's now time to bring that change upstream in order to unblock further changes that depend on it. Originally introduced to resolve rdar://83378814.
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tshortli
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…d thunks. In order for availability checks in iOS apps to be evaluated correctly when running on macOS, the application binary must call a copy of `_stdlib_isOSVersionAtLeast_AEIC()` that was emitted into the app, instead of calling the `_stdlib_isOSVersionAtLeast()` function provided by the standard library. This is because the call to the underlying compiler-rt function `__isPlatformVersionAtLeast()` must be given the correct platform identifier argument; if the call is not emitted into the client, then the macOS platform identifier is used and the iOS version number will be mistakenly interpreted as a macOS version number at runtime. The `_stdlib_isOSVersionAtLeast()` function in the standard library is marked `@_transparent` on iOS so that its call to `_stdlib_isOSVersionAtLeast_AEIC()` is always inlined into the client. This works for the code generated by normal `if #available` checks, but for the `@backDeployed` function thunks, the calls to `_stdlib_isOSVersionAtLeast()` were not being inlined and that was causing calls to `@backDeployed` functions to crash in iOS apps running on macOS since their availability checks were being misevaluated. The SIL optimizer has a heuristic which inhibits mandatory inlining in functions that are classified as thunks, in order to save code size. This heuristic needs to be relaxed in `@backDeployed` thunks, so that mandatory inlining of `_stdlib_isOSVersionAtLeast()` can behave as expected. The change should be safe since the only `@_transparent` function a `@backDeployed` thunk is ever expected to call is `_stdlib_isOSVersionAtLeast()`. Resolves rdar://134793410.
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hborla
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Aug 29, 2024
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@_transparentfunctions into@backDeployedthunks so that the codegen for the availability checks in@backDeployedworks as expected when compiling for iOS.@backDeployedfunctions from the_Concurrencylibrary crash when the app runs on macOS, due to the availability condition being misevaluated as checking a macOS version instead of an iOS version.@backDeployedthunks #76135@backDeployedthunks, but any change of this nature carries some risk of unforeseen consequences.