Comment by lelanthran
4 hours ago
I'm not saying that the result of the dereference be known, I'm saying that the instructions to do the dereference be always emitted.
Right now, if a dereference results in UB, the compiler may omit it entirely.
4 hours ago
I'm not saying that the result of the dereference be known, I'm saying that the instructions to do the dereference be always emitted.
Right now, if a dereference results in UB, the compiler may omit it entirely.
I think I would defer to someone more of a language lawyer than we, but I'm not sure what you're describing can be expressed in the C abstract machine. If a pointer is invalid, not pointing to an object, then I'm not sure it means anything to "read from there".
I know what you mean, but I'm just not sure you're describing something that fits what C "is". We program C to the abstract machine specified in the standard (5.1.2), and the compiler's job is to translate that into something with identical behavior on particular hardware. Piercing the layers down to actual hardware or assembly isn't really done.
Even "volatile" just says (basically) "touching this object has side effects". It implies no double-loading, speculative store, etc, but doesn't say "don't emit assembly instructions to load this unless the program logic path takes the route where the C program does load it".
The standard is not using ancient language when it refers to "objects with static storage duration" instead of "heap" or ".data segment". It is the true class of objects in the abstract machine.
Wouldn't that make a compiler that emitted bounds checks violate the standard, since it would not be emitting the actual memory operations if you deref out of bounds?
No, because it's UB so there is no standard.