https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49391
--- Comment #3 from Richard Earnshaw <rearnsha at gcc dot gnu.org> --- The v7 edition of the Arm ARM states that r13 is UNPREDICTABLE in pretty much all of the T2 variants. I think that might have been removed in v8, but we're still pretty much suck with the fact that v7 chips aren't required to support r13
