gpl: accumulate nonPlaceArea before multiplying density

[LucasYuki]

Summary

Accumulate the non-place area before multiplying the density to reduce rounding errors.
This PR should generate the same results as #11085.

Type of Change

• Bug fix

Impact

This slightly changes some test results.

Verification

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

Related Issues

#11085

[gemini-code-assist]

Code Review

This pull request refactors BinGrid::updateBinsNonPlaceArea in nesterovBase.cpp to accumulate raw non-place areas in a temporary vector before scaling and adding them to the bins, and updates getOverlapArea to return int64_t instead of float. The review feedback suggests wrapping this new accumulation logic in a check to ensure pb_->nonPlaceInsts() is not empty, which avoids unnecessary memory allocation and loop overhead when there are no non-placeable instances.

[gemini-code-assist]

If pb_->nonPlaceInsts() is empty, allocating nonPlaceAreaRaw with the size of bins_ and then iterating over all bins is an unnecessary $O(\text{bins_})$ overhead. Wrapping this logic in a check to ensure pb_->nonPlaceInsts() is not empty avoids this allocation and loop entirely, keeping the complexity at $O(1)$ when there are no non-placeable instances.

  if (!pb_->nonPlaceInsts().empty()) {
    std::vector<int64_t> nonPlaceAreaRaw(bins_.size(), 0);
    for (auto& inst : pb_->nonPlaceInsts()) {
      std::pair<int, int> pairX = getMinMaxIdxX(inst);
      std::pair<int, int> pairY = getMinMaxIdxY(inst);
      for (int y = pairY.first; y < pairY.second; y++) {
        for (int x = pairX.first; x < pairX.second; x++) {
          Bin& bin = bins_[y * binCntX_ + x];
          nonPlaceAreaRaw[y * binCntX_ + x]
              += getOverlapArea(&bin, inst, dbu_per_micron);
        }
      }
    }
    for (size_t i = 0; i < bins_.size(); ++i) {
      if (nonPlaceAreaRaw[i] == 0) {
        continue;
      }
      bins_[i].addNonPlaceArea(
          static_cast<int64_t>(nonPlaceAreaRaw[i] * bins_[i].getTargetDensity()));
    }
  }