I'm not entirely convinced that the ninth case is necessarily an miscompilation. My understanding of the standard is that C99 6.7.3 p6 and C11 6.7.3 p7 allows f

I'm not entirely convinced that the ninth case is necessarily an miscompilation. My understanding of the standard is that C99 6.7.3 p6 and C11 6.7.3 p7 allows for exactly this behavior. One can argue (as the author does in the linked paper) that the last sentence is about what it means hardware-wise, but completely optimizing away statements that have no effect on the abstract machine level ('x;', 'x = x;', ...) is something that seems not only permitted but even reasonable. But it does not make much sense to actually want to force a read for side-effect in portable code. For a MMIO register of some MCU/SoC the code is not going to be portable in any meaningful sense anyway, and for things like portable OS drivers for weird hardware (like VGA...) you have to use some kind of OS-provided macro that does the right thing wrt. caching and barriers anyway.