Rather than relying on the stack being 0'ed out always, do it manually.
The stack generally happened to be zero, and because the compiler
realizes that the tests are dealing with chars truncates the read value.
However, the top 3 nibbles of the value are undefined and may be
non-zero. The indirection level caused a null-pointer dereference.
Explicitly scribbling on the stack during the allocation causes test
failures without the original zexting behaviour.
The closure function (invoked as closure->fun in ffi_closure_XXX_inner)
will only populate the actual number of bytes for the true return type,
which may be a character. This leaves garbage on the stack when the
assembly closure function (i.e. ffi_closure_XXX) reads the return value
off of the stack into r0 as a 4-byte value. ffi_closure_XXX always
leaves room for at least 4 bytes here, so we can safely set them to 0.
Otherwise, if there is garbage in any of these bytes, these end up in r0
and in the returned value as well.
Saleem Abdulrasool