It appears empirically to me that there are issues with the Xcode (I am currently using 8) toolchain when it comes to constructor attribute priorities.
- __attribute__((constructor(99))) doesn't generate a compile time warning but should since 0-100 is supposed to be off limits to mortals.
- constructor attribute priorities only work inside the same module. It appears I cannot use them to sort constructor calls across linked modules in a final product. :-(
Cf things like http://lists.llvm.org/pipermail/cfe-dev/2013-September/031733.html