It also seems to completely fail to realise that the code has to be unreachable (unless UB occurs). If I add this code (with f still visible) :

int g(bool cond, int a, int b)
{
if (cond)
return f(a, b);
return 12;
}

LLVM is incapable of optimizing this to return 12; (removing the conditional), instead inlining the broken f and jumping to it if cond is true. If I change the body of f to nothing, it manages to do so properly (and the codegen for f emits nothing as expected).

PS: In my second comment, I am not saying that behaviour should be defined where cond == true. I am saying that the code is not properly optimized based on the fact that f cannot be called without invoking UB.

Also, here is a Godbolt comparison with GCC (which is capable of optimizing this better than LLVM) : https://godbolt.org/z/Kbs1f1

Extra note: This doesn't occur when compiling the code as C (https://godbolt.org/z/n5d65h), so this bug seems even odder now.

https://reviews.llvm.org/D87149

Changing to IR bug - the sooner we can remove dead code, the better (potentially, there's also some clang front-end part of this to account for the C/C++ difference).

To echo here what was aid in a review [0], we should have a pass early on removing all instructions preceding an unreachable if the are known to transfer control.

For the example https://godbolt.org/z/Kbs1f1 , the Attributor knows f is noreturn and replaces the return in g with unreachable. That results in the "good" code: https://godbolt.org/z/6j36x8

That said, we might want to have a more localized optimization doing this, and finally land the default willreturn and nounwind annotations for intrinsics.

[0] https://reviews.llvm.org/D87149#2256885

Should be fixed with more instcombine zapping:
https://reviews.llvm.org/rGb22910daab95

Depending on target, we may do better than nothing on UB code as shown in the description. For example, on x86, I see a 'ud2' instruction, so that should crash and make the programmer aware of their buggy code even if they ignore the compile-time warning:
47416.cpp:5:1: warning: non-void function does not return a value [-Wreturn-type]