* Remove RegisterPreservingCC. It was unused.
* Remove DefaultCC from the runtime. The distinction between C_CC and DefaultCC
was unused and inconsistently applied. Separate C_CC and DefaultCC are
still present in the compiler.
* Remove function pointer indirection from runtime functions except those
that are used by Instruments. The remaining Instruments interface is
expected to change later due to function pointer liability.
* Remove swift_rt_ wrappers. Function pointers are an ABI liability that we
don't want, and there are better ways to get nonlazy binding if we need it.
The fully custom wrappers were only needed for RegisterPreservingCC and
for optimizing the Instruments function pointers.
The generated wrapper simply invokes a corresponding entry point by means of
an indirect call via a global the symbol, which is a function pointer referring
to the implementation of a runtime function.
Using such wrappers allows for invocations of runtime functions from dynamic
libraries without the usual indirections via dynamic linker stubs.
If the calling convention and the current target require a wrapper, it will be
generated. Each wrapper gets a hidden linkage and is marked as ODR, so that
a linker can merge all wrappers with the same name.
This functionality could be re-used by e.g. LLVMPasses, which currently create LLVM IR declarations of runtime entry points on their own.
To make the function re-usable, slightly change the API of the function:
- use llvm::Module instead of IRGenModule.
- use llvm::ArrayRef instead of std::initializer_list, which allows the clients of this API to dynamically form the lists of return types and arguments.
