[5.9] Introduce -unavailable-decl-optimization
#64810
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Part of rdar://106674022
Avoid SIL lowering for declarations marked unavailable when `-unavailable-decl-optimiation=complete` is specified. Part of rdar://106674022
This ensures SIL lowering will avoid generating code for unavailable global vars when `-unavailable-decl-optimization=complete` is specfied. Part of rdar://106674022
Part of rdar://106674022
Part of rdar://107425181
Part of rdar://107425181
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nkcsgexi
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Mar 31, 2023
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Cherry pick of #64644 and #64808
Introduce the
-unavailable-decl-optimizationfrontend flag which can be used to reduce the size of a compiled binary by removing machine code related to declarations that are marked unavailable with an@availableattribute. Unavailable code is meant to be provably unreachable at runtime for a specific execution context and therefore including machine code for these declarations is wasteful.In this iteration, the flag supports two modes:
none: Does not remove any code related to unavailable declarations. This is the default.complete: Avoids generating any machine code for unavailable declarations. This will be the most aggressive optimization available.The plan is to expand the available modes in the future with at least one more option,
stub, which will cause unavailable functions to be emitted as stubs containing justfatalError(). This option will be needed by owners of resilient libraries to preserve ABI compatibility since existing clients may link against the symbols for unavailable declarations, even if they never execute them.Resolves rdar://106674022 and rdar://107425181