I'm not sure how "it can go the other way around too" -- in that case (assigning to a uint8_t local variable), it seems like that particular optimisation is just not being applied.
Interestingly, if the local variable is "volatile uint8_t", the optimisation is applied. Perhaps with an uint8_t local variable and size_t return value, an earlier optimisation removes the cast to uint8_t, because it only has an effect when undefined behaviour has been triggered? It would certainly be interesting to investigate further.
In general I agree that being more explicit is better if you really care about performance. It would be great if languages provided more ways to specify this kind of thing. I tried using __builtin_expect to trigger this optimisation too, but no dice.
Interestingly, if the local variable is "volatile uint8_t", the optimisation is applied. Perhaps with an uint8_t local variable and size_t return value, an earlier optimisation removes the cast to uint8_t, because it only has an effect when undefined behaviour has been triggered? It would certainly be interesting to investigate further.
In general I agree that being more explicit is better if you really care about performance. It would be great if languages provided more ways to specify this kind of thing. I tried using __builtin_expect to trigger this optimisation too, but no dice.
Anyway, thanks for the interesting article.