https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78436
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> --- The over-sized bitfield isn't really needed for this, making it more severe: struct S { long int : 23; long int a : 24; long int b : 10; long int c : 24; signed char d : 8; } s; __attribute__((noinline, noclone)) void foo () { s.d = 0; s.c = -1193165L; } int main () { foo (); if (s.d != 0) __builtin_abort (); return 0; } Indeed, I think the bug is in encode_tree_to_bitpos, still debugging where exactly.