RFR: 8347426: Invalid value used for enum Cell in iTypeFlow::StateVector::meet_exception

[Dean Long]

On Tue, 25 Feb 2025 10:11:54 GMT, Marc Chevalier <duke at openjdk.org> wrote:

> As guess on the JBS ticket, we have a UB when `_outer->max_locals() == 0`, because then we try to do `(Cell)(-1)` which is out of range since Cell's range is [0, `INT_MAX`].
> 
> The obvious fix that is
> 
> Cell limit = local(_outer->max_locals());
> for (Cell c = start_cell(); c < limit; c = next_cell(c)) {
> 
> since `local` asserts its argument to be in [0, `outer->max_locals()`). Of course
> 
> Cell limit = (Cell)(_outer->max_locals());
> 
> would work, but it seems to break (the very light) abstraction.
> 
> I've also added an assert to transform the UB into a clear failure.
> 
> This fix makes the UB warning go away on Mac with arm64.
> 
> Thanks,
> Marc

This seems fine.  One alternative would be to introduce a new helper:

    Cell        local_limit_cell() const {
      return (Cell)(outer()->max_locals());
    }

similar to the existing `limit_cell`.  A 2nd alternative would be something like:

  for (int i = 0; i < _outer->max_locals(); ++i)) {
    Cell c = local(i);
    [....]

-------------

Marked as reviewed by dlong (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23772#pullrequestreview-2642815317