https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77728
--- Comment #19 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Indeed, with the above plus: #include <stdarg.h> int fn1 (int a, int b, int c, int d, int e, int f, int g, int h, int i, int j, int k, int l, int m, B<0> n, ...) { va_list ap; va_start (ap, n); int x = va_arg (ap, int); va_end (ap); return x; } int fn2 (int a, int b, int c, int d, int e, int f, int g, int h, int i, int j, int k, int l, int m, B<1> n, ...) { va_list ap; va_start (ap, n); int x = va_arg (ap, int); va_end (ap); return x; } there is ABI change for fn1 but not fn2. But we already get a warning with your patch on that: warning: Parameter passing changed for argument in function_arg_boundary so perhaps no need to emit it for the second time.