dpl: remove abacus

[gudeh]

Summary

Experiments show no gain in using it. It looks quite bugged.

Type of Change

• Refactoring

Impact

No-op, abacus was disabled by default.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

Related Issues

This should follow PR #10226

[gemini-code-assist]

Code Review

This pull request removes the Abacus pre-pass from the NegotiationLegalizer, making the NegotiationLegalizer the default engine while keeping the legacy diamond search available via the new -use_diamond_legalizer flag. It also simplifies the initial snapping logic to a 4-direction linear search and adds reporting for illegal nodes upon convergence failure. The review feedback highlights critical safety issues, including potential null-pointer dereferences when calling debug_observer_->startPlacement and when accessing network_ in getIllegalNodes(), as well as a suggestion to mark isUseNegotiationLegalizer() as const for const-correctness.

[gemini-code-assist]

Moving debug_observer_->startPlacement before initFromDb() introduces a potential null-pointer dereference. db_->getChip() or db_->getChip()->getBlock() can be nullptr (which is explicitly checked and guarded against inside initFromDb()). Calling startPlacement before these guards run can cause a crash. It should be guarded with null checks.

Suggested change

	if (debug_observer_) {
	debug_observer_->startPlacement(db_->getChip()->getBlock());
	}
	if (debug_observer_ && db_->getChip() && db_->getChip()->getBlock()) {
	debug_observer_->startPlacement(db_->getChip()->getBlock());
	}

[gemini-code-assist]

The network_ pointer can be nullptr in certain contexts, as treated by other functions in this class. Calling network_->getNode without a null check will cause a crash if network_ is null. A guard should be added at the beginning of the function.

std::vector<Node*> NegotiationLegalizer::getIllegalNodes() const
{
  std::vector<Node*> illegal;
  if (!network_) {
    return illegal;
  }
  for (int i = 0; i < static_cast<int>(cells_.size()); ++i) {
    if (!cells_[i].fixed && !isCellLegal(i)) {
      if (Node* node = network_->getNode(cells_[i].db_inst)) {
        illegal.push_back(node);
      }
    }
  }
  return illegal;
}

[gemini-code-assist]

The member function isUseNegotiationLegalizer does not modify any class state and should be marked const to adhere to const-correctness principles.

Suggested change

	bool isUseNegotiationLegalizer() { return !use_diamond_legalizer_; }
	bool isUseNegotiationLegalizer() const { return !use_diamond_legalizer_; }