The check for implementationOnly imports was already done for types, but it was missing for functions.
Fixes a crash when implementationOnly-importing a C module.
https://bugs.swift.org/browse/SR-15048
rdar://81701218
If a function uses a type imported as implementationOnly (or similar), it cannot be serialized.
I added a new API in ModuleDecl (canBeUsedForCrossModuleOptimization), which performs this check.
rdar://72864719
* Include small non-generic functions for serializaion
* serialize initializer of global variables: so that global let variables can be constant propagated across modules
rdar://problem/60696510
To avoid duplicate-symbol linker errors. Instead make them alwaysEmitIntoClient.
But only do that for thunks to limit the code size impact. Anyway, it's only important for thunks because thunks are generated at SILGen, i.e. before CrossModuleSerializationSetup.
Other shared functions, e.g. specializations are created after CrossModuleSerializationSetup, so we don't have to deal with them.
This is a first version of cross module optimization (CMO).
The basic idea for CMO is to use the existing library evolution compiler features, but in an automated way. A new SIL module pass "annotates" functions and types with @inlinable and @usableFromInline. This results in functions being serialized into the swiftmodule file and thus available for optimizations in client modules.
The annotation is done with a worklist-algorithm, starting from public functions and continuing with entities which are used from already selected functions. A heuristic performs a preselection on which functions to consider - currently just generic functions are selected.
The serializer then writes annotated functions (including function bodies) into the swiftmodule file of the compiled module. Client modules are able to de-serialize such functions from their imported modules and use them for optimiations, like generic specialization.
The optimization is gated by a new compiler option -cross-module-optimization (also available in the swift driver).
By default this option is off. Without turning the option on, this change is (almost) a NFC.
rdar://problem/22591518
