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Comment by thomashabets2

6 hours ago

Are you talking about creating a pointer (more than one item) past an array, or dereferencing that pointer? Both are currently UB.

For the former, I kinda get it. It may need to be there for cases like with segmented address space where p+10 could actually be a value less than p, for the eventually generated assembly. Maybe it should be fine to create such a pointer, but have it be "indeterminate value" or whatever, if you try to compare that pointer to anything? I don't know enough about compiler internals to say one way or the other.

Dereferencing, though, can only be UB. There may not be a "value" behind that address. There may be a motor that's been I/O mapped, or a self destruct button.

4 comments

thomashabets2

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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.

  • thomashabets2  3 hours ago

    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.

  • charleslmunger  4 hours ago

    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?