4 ms·

Yeah, there's a certain level of hermeneutics involved in arguing about undefined behavior, it's a great source of unending entertainment because no one can pro

Yeah, there's a certain level of hermeneutics involved in arguing about undefined behavior, it's a great source of unending entertainment because no one can prove you wrong _definitely_. I've long had daydreams of a C or C++ implementation that went out of its way to dynamically track all those ephemeral distinctions described in the standard but not commonly made concrete in compiler diagnostics or emitted code. Maybe this implementation would take some liberties with regards to the prescribed space/time complexity of certain operations, but that'd be okay, since it's pedagogical and not for production use. Wouldn't it be amazing to have every bit of undefined behavior at runtime (except for some of the more esoteric ones, I guess) result in diagnostic messages outlining the bad state that the participating objects are in and how they got there? Like, at runtime, keep track of the dynamic type stored at each byte of memory, and then have debug logging output as the runtime evaluates the strict aliasing rules for each memory access until it finds a clause that makes it valid. Then we mere mortals can actually have arguments about these kinds of things and forward "falsifiable" theories! I feel like setting this up in some sort of C interpreter can't be fundamentally nearly as hard as the inner workings of current-day optimizing compilers, so I keep thinking that someone must have done this already...

int_19h 8y ago
That's interesting - I had such thoughts as well, way back after my first experience writing significant quantities of C++ (03 back then) in the industry. And yes, I think this would require an interpreter, and some kind of shadow memory as you describe (that keeps track of type and lifetime metadata associated with bytes). In fact, I think it would even have to be multi-level segmented memory, with segment per each object (including subobjects!), to fully enforce rules such as out-of-bounds array access, and comparing pointers to different objects. It could be an interesting exercise. And if written in a verifiable language, the result could, in theory, be declared the specification for the memory model.

int_19h 8y ago

That's interesting - I had such thoughts as well, way back after my first experience writing significant quantities of C++ (03 back then) in the industry. And yes, I think this would require an interpreter, and some kind of shadow memory as you describe (that keeps track of type and lifetime metadata associated with bytes). In fact, I think it would even have to be multi-level segmented memory, with segment per each object (including subobjects!), to fully enforce rules such as out-of-bounds array access, and comparing pointers to different objects. It could be an interesting exercise. And if written in a verifiable language, the result could, in theory, be declared the specification for the memory model.