Comment by tardedmeme
3 hours ago
I told someone at a conference that UB actually means "implementation-defined, no documentation required". He started to refute me and then stopped.
3 hours ago
I told someone at a conference that UB actually means "implementation-defined, no documentation required". He started to refute me and then stopped.
That isn't true, for UB the compiler is allowed to assume the UB can never happen. For example if you dereference a pointer and only after check if it is NULL, the compiler can remove the NULL check, since it is clearly impossible (nevermind that you might be on a microcontroller where NULL is a valid address).
The fallout of this are quite large! If behaviour is implementation defined the compiler has to stick to one consistent behaviour. No such need for UB, you can get different behaviour bu changing unrelated code, by changing between debug and release or just because of what garbage happened to be on the stack.
Since the compiler is allowed to assume the UB doesn't happen it will also sometimes look like the compiler miscompiled your code elsewhere, but what actually happened was some inlining followed by extrapolating "this can never happen".
UB is often surprising: I have seen unaligned loads crash on x86 due to it bring UB in C (even though x86 is generally fine with it). But once a newer compiler decided that it was fine to vectorise that code (since it clearly aligned) the CPU was no longer happy with it.
Except that UB doesn't mean that. UB means "the developer must never write this".
Both are wrong. It means "this standard does not constrain the behaviour of code that does this".
It's entirely legal for implementations to have predictable behaviour, documented or not, for code that is undefined by the standard. In their quest for maxxing benchmark performance they generally choose not to, but there's really nothing in any standard that stops you from making an implementation that prioritises safety.
Every implementation so far has predictable behavior in all cases. Sometimes the rules for predicting it are very obscure. But it's all fully defined within the compiler's binary code. And none of them link to nasal portals.