[SR-15137] Null Pointer Dereference in SILFunction.front()

[swift-ci]

Previous ID	SR-15137
Radar	None
Original Reporter	justinfargnoli (JIRA User)
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	jonathanalvares9009 (JIRA)
Priority	Medium

md5: b4666bf8c44aac3523ff8b945b555370

Issue Description:

If `SILFunction.front()` link is called on a SILFunction that contains zero basic blocks, then `*begin()` will be a null pointer dereference.

[typesanitizer]

This is in line with how the C++ standard library's front and back methods on vector work. Is there a specific case where you have hit this null pointer dereference in practice?

[swift-ci]

Comment by Justin Fargnoli (JIRA)

I ran into this bug when writing cloneModule for my Alive2 for SIL GSoC project. The `SILCloner` infrastructure that `cloneModule` uses would trigger this bug for ceratin `SILModule`s.

My project mentor, CodaFi (JIRA User), recommended that we add an `assert` here instead of invoking undefined behavior.

Do we want to follow the C++ standard library's example in this case? If we do, then we should edit the `SILCloner` code that invoked the null pointer dereference.

[typesanitizer]

I think adding an assert is okay, we do have assertions in other places for similar checks.

// Type.h
  TypeBase *operator->() const {
    assert(Ptr && "Cannot dereference a null Type!");
    return Ptr;
  }

An assert will still be compiled away in release mode though (such as in shipping builds).