Compilers have always been making guesses about what the most likely code path is behind the scenes, but it still needs to behave correctly in the case where it was wrong (that will just be the less-optimal code path). All these attributes are doing is helping the compiler know instead of guess what the hot path is. if there is any way to confuse the compiler into giving undefined behavior with hints like this, that's a compiler bug. (not saying compiler bugs don't exist, but are you aware of a specific bug like this)?