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g_type_ensure has a large comment saying _get_type shouldn't be marked with G_GNUC_CONST because it has side-effects on the first run, but it's OK if you consistently call g_type_ensure first. Unfortunately, this isn't accurate. GCC trunk (observed at r17-1725-ge259a182e1f100) now analyses points-to information more effectively and propagates its findings, so it sees the function modifies some global state, concludes that can't be possible (because of the const attribute), and deletes the stores. pure unfortunately is unsuitable for *_get_type too as it has a side-effect (writes to global memory). Mesa had a similar issue which I fixed recently [0]. glib and glibmm likely want the (not-yet-implemented) 'idempotent' attribute instead [1]. In fact, this exact problem is what motivated that request! :) ebassi mentions an alternative to the current setup as well, using atomics [2]. This problem has gone back many years but the compiler has finally fully outsmarted us here, so give in. [0] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/42361 [1] https://gcc.gnu.org/PR32911 [2] https://gitlab.gnome.org/GNOME/glib/-/merge_requests/4249#note_2583110 Fixes: https://gitlab.gnome.org/GNOME/glib/-/work_items/3984 Signed-off-by: Sam James <sam@gentoo.org>