My best guess would be to prevent the 'useless instruction' from being optimized out, but an or with 0 is still useless, and I don't see what optimizer lies bet

My best guess would be to prevent the 'useless instruction' from being optimized out, but an or with 0 is still useless, and I don't see what optimizer lies between this GCC feature and the final binary. Maybe the segfault only occurs on a write?