So far, the swift-frontend decided by itself if CMO can be enabled. This caused problems when used with an old driver, which doesn't consider CMO.
Now, the driver decides when to use default CMO by passing this flag to swift-frontend.
The main point of this change is to make sure that a shared function always has a body: both, in the optimizer pipeline and in the swiftmodule file.
This is important because the compiler always needs to emit code for a shared function. Shared functions cannot be referenced from outside the module.
In several corner cases we missed to maintain this invariant which resulted in unresolved-symbol linker errors.
As side-effect of this change we can drop the shared_external SIL linkage and the IsSerializable flag, which simplifies the serialization and linkage concept.
If we are emitting a TBD file, the TBD file only contains public symbols of this module.
But not public symbols of imported modules which are statically linked to the current binary.
This prevents referencing public symbols from other modules which could (potentially) linked statically.
Unfortunately there is no way to find out if another module is linked statically or dynamically, so we have to be conservative.
Fixes an unresolved-symbol linker error.
rdar://89364148
The "regular" CMO is done with the option `-cross-module-optimization`. It's good for performance but can increase code size.
Now, which this change CMO is also done if the option is not given, but in a very conservative way. Only very small functions are serialized and not additional type metadata is kept alive.
rdar://70082202
* rename the CrossModuleSerializationSetup pass to simply CrossModuleOptimization
* remove the CMO specific serializer pass. Instead run the CrossModuleSerializationSetup pass directly before the standard serializer pass.
* correctly handle shared functions (e.g. specializations)
* refactoring
