[5.9] Introduce -unavailable-decl-optimization=stub
#65269
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AnthonyLatsis
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🍒 release cherry pick
Part of rdar://107388493
When `-unavailable-decl-optimization=stub` is specified, insert a call to `_diagnoseUnavailableCodeReached()` at the beginning of the function to cause it to trap if executed at run time. Part of rdar://107388493
…tubs. User code should not be diagnosed as "unreachable" by the SIL optimizer when the no-return function that made the code unreachable is a compiler inserted call to `_diagnoseUnavailableCodeReached()`. Part of rdar://107388493
Part of rdar://107388493
Part of rdar://107388493
Part of rdar://107388493
Part of rdar://107388493
…thunk. Part of rdar://107388493.
The logic was defaulting to `arm64` when a binary only contained `amr64_32` and `arm64e` slices.
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nkcsgexi
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May 5, 2023
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This is a mode behind an opt-in flag so it should be safe to be landed on 5.9 |
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Cherry pick of #65557
Implement a new
-unavailable-decl-optimizationmode that replaces the bodies of unavailable functions with calls to a standard library function that invokesfatalError. Unlike-unavailable-decl-optimization=complete, this compilation mode is designed to improve the code size of unavailable code while also preserving ABI compatibility. This is important since there are existing compiled binaries that link the symbols for unavailable functions even though those symbols should be provably unreachable.This optimization is implemented in SILGen by inserting a call to a new standard library function
_diagnoseUnavailableCodeReached()in the prologue of each unavailable function. The existing dead code elimination optimization pass then determines that the remainder of the function body is unreachable and removes all other code, leaving the body of the function as a single call to a no-return function.There are several follow up changes that can be made to maximize the effectiveness of this optimization:
_diagnoseUnavailableCodeReached()and sometimes also includes abrkinstruction after the call. Some work to reduce the machine code for these stubs down to the minimal necessary instructions (probably just a single branch?) may be warranted.Resolves rdar://107388493