> So perhaps as a temporary workaround, we could tweak setup.py to set
> -O0 for building ctypes with clang?

That would make the tests pass, but it still won't fix the fundamental 
issue.  clang and GCC have a difference in opinion as to when function
parameters and return values should be sign extended.  libffi follows
the GCC model.

I am talking to the LLVM folks right now to see why things are 
implemented this way.  Whether LLVM or libffi should be fixed depends
on the outcome of that discussion.

Due to this ABI difference compiling _ctypes with -O0 for clang will not
fix the problem because any ctypes code that interacts with C code that
was compiled by clang with optimizations has the potential to break just
like our test cases.