grt/CUGR: discarded unionWith in MazeRoute::getSteinerTree, and the high() - 1 bound in constructDetours - intended?

[shivarammysore]

Two small questions about src/grt/src/cugr/src (CUGR, global_route -use_cugr). Both lines are the same on current master and in CUHK's original CUGR 2.0 sources, so they may well be deliberate. I wanted to check the intent.

1. MazeRoute::getSteinerTree, "Remove redundant tree nodes"

if (child->getFixedLayers().isValid()) {
  if (node->getFixedLayers().isValid()) {
    node->getFixedLayers().unionWith(child->getFixedLayers());
  } else {
    node->setFixedLayers(child->getFixedLayers());
  }
}

IntervalT::unionWith is const and returns the union, so this call's result is discarded: when a co-located child that carries pin layers is folded into a parent that already has layers, the parent keeps only its own interval. Is the parent meant to take the union here (node->setFixedLayers(node->getFixedLayers().unionWith(...)))? For what it's worth, assigning the union changes no guide on the grt regression scripts that use -use_cugr, so it looks latent either way.

2. PatternRoute::constructDetours, the shift-interval expansion

while (shift_interval.low() - 1 >= 0
       && get_total_stem_length(stems, shift_interval.low() - 1) - original_length
              <= max_length_increase) {
  shift_interval.addToLow(-1);
}
while (shift_interval.high() + 1 < grid_graph_->getSize(1 - direction)
       && get_total_stem_length(stems, shift_interval.high() - 1) - original_length
              <= max_length_increase) {
  shift_interval.addToHigh(1);
}

The downward loop guards and evaluates the stem length at low() - 1. The upward loop guards high() + 1 but evaluates it at high() - 1. Is high() - 1 intended, or was high() + 1 meant? If it's the latter, changing it does change results: the routes of cugr_adjustment3 and ndr_rrr_cugr differ, so their goldens would move.

Thanks, and happy to send a PR for either if that's useful.

[shivarammysore]

Moved to issues, one per question: #11508 (the discarded unionWith in MazeRoute::getSteinerTree) and #11509 (the high() - 1 in constructDetours). Closing this in favour of those.