The argument that it was once a bug of the C standard makes absolutely no sense. It is not allowed by C11. C99 does not talk about that because C99 does not off

The argument that it was once a bug of the C standard makes absolutely no sense. It is not allowed by C11. C99 does not talk about that because C99 does not officially support multithreading, but that has not prevented largely C99 compliant compilers from supporting multithreading, while this kind of "optimization" intrinsically prevents multithreading. So the situation is just that, at one point, "optimizations" have been added by people who did not know what the where doing, and tried to hide, as usual for modern compiler authors, behind the literal wording of the C standard to justify their incompetence and the risks they force on the world.