swift

f0017326 - SILOptimizer: Allow inlining of transparent functions in @backDeployed thunks.

SILOptimizer: Allow inlining of transparent functions in @backDeployed 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.