If you call free((void *)-1) in C code, it will very likely crash, but it will crash in libc, not your program. Is that a bug in libc?

I think even deriving the argument might invoke undefined behaviour here, but I'm not certain.

If I understand correctly, it's undefined behaviour to do this:

    int *ptr = (int*)42;

As, in order to avoid undefined behaviour, ptr should only be assigned a valid address of an int, or the 'address' one past the end of an array of int, or NULL (logical zero).

It's possible things are different when the type is void*, I'm not certain.

Make it free(free) if you want it to be a valid pointer.

No, because the defined contract for free() is that you pass it a pointer that was previously returned from malloc(), that you don't try to free() it twice, etc.

That's documented undefined behavior.

Is it documented undefined behavior to feed bad IR into LLVM?

Of course. If there's an input checking mode, and you disable the checks, then you're guaranteeing that you won't supply invalid input.

LLVM makes trade off on enabling/disabling certain checks, including assertions and IR verification, primarily to keep compilation time acceptable in release build. The idea is that we catch as many bugs as possible in debug version of LLVM such that release version can run fast.

LLVM makes use of assertions to validate things like this, but many users of LLVM, including rustc, turn them off for performance reasons.

It is.

Terse affirmative is ambiguous.

If you feed LLVM bad IR, all bets are off. LLVM's assertions and IR verifier are impressively comprehensive, but not a guarantee.

For example, LLVM has a pointer type and a void type, but you may not make a pointer-to-void type, see https://llvm.org/docs/LangRef.html#pointer-type . If you do call `Type::getVoidTy(C)->getPointerTo()` then LLVM will hit an assertion only in a build with assertions enabled. Without assertions LLVM may silently execute UB.

As are all tautologies;)