https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120781
--- Comment #1 from Siddhesh Poyarekar <siddhesh at gcc dot gnu.org> --- Looks like check_sizes is not inlined when you defined DYN? I haven't built the latest tree recently, but the only way I can get __bos to give me a valid value (i.e. without -DDYN) is by forcibly inlining check_sizes.