https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68273
--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Thu, 21 Jan 2016, rguenth at gcc dot gnu.org wrote: > With C11 aligning a type is no longer a GCC extension(?), for previous > C versions either GCC shouldn't change the ABI or document the ABI > extension. C11 doesn't allow under-aligning a type and doesn't allow changing alignment of parameters. "An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or a function, or a parameter, or an object declared with the register storage-class specifier.". (You're meant to be able to have structures with over-aligned elements, which of course could be passed as function arguments - each structure type would still have a fixed alignment - though in fact the syntax fails to allow for this; see DR#444.)
