im not an c++ expert, but it looks like the unreachable macro of rust.
In rust it allows optimizing the generated binary to not care about special paths.
If an integer can only be 1-4, then it doesn't make sense to dedicate an code path for 0 or 5+ that will never be called.
in an explicit for loop its easy to know for the compiler, but when this information crosses boundaries like function calls then it can make it easier and can reduce the binary size.
tyushk•10m ago
unreachable! in Rust is panic! with a different name [1]. The unsafe unreachable_unchecked [2] is C++ std::unreachable.
The solution is to avoid UB. std::unreachable() is just an easy-to-invoke way of hitting UB, mentioned for the sake of the discussion. They did later take your advice “Let’s say we change std::unreachable() to int* ptr = nullptr; *ptr = 99; in the previous example.”. They still got improper codegen out of the compiler.
Though, it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()" at that point in the code, as that would better convey how the optimizer passes will see things after dozens of frontend and middle-end passes have run before it.
pestatije•23m ago
UB - undefined behaviour
charleslmunger•15m ago
This is sort of true, except that behavior can be undefined in the standard but defined in your implementation. One of the best examples is -fwrapv, which makes signed integer overflow not UB. And Fil-C has no undefined behavior that I'm aware of. But most implementations target the standard and "define" what the standard doesn't to be "whatever I feel like, in service of implementation simplicity and performance". What matters is what your implementation guarantees, which should be a superset of what the standard does.
johnea•54m ago
stop50•34m ago
tyushk•10m ago
[1] https://doc.rust-lang.org/std/macro.unreachable.html [2] https://doc.rust-lang.org/std/hint/fn.unreachable_unchecked....
mpyne•31m ago
Though, it would be better if programmers read deliberate instances of UB as if they literally said "std::unreachable()" at that point in the code, as that would better convey how the optimizer passes will see things after dozens of frontend and middle-end passes have run before it.