grt: dedup duplicate pin-via guides when reloading routes_ from ODB (#10474)

[saurav-fermions]

Summary

Global routing produced different final results when repair_antennas started from ODB-reconstructed guides versus a warm in-memory state, because the globalRoutingToBox -> ODB guide -> boxToGlobalRouting round-trip was not an identity. This de-duplicates the double-written pin-via guides so reloading routes_ from ODB is deterministic.

Type of Change

• Bug fix

Impact

Antenna-repair global routing is reproducible regardless of warm/cold start.

Verification

• Local build succeeds.
• Relevant tests pass (rebuilt from source): ctest -R '^grt\.' 131/131.
• Code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

Related Issues

Fixes #10474

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Developed with SAIGE, Fermions' autonomous RTL/EDA debugging agent; root-caused, tested, and signed off by the submitter (@saurav-fermions).

[gemini-code-assist]

Code Review

This pull request introduces a dedupViaSegments method to remove duplicate via segments created when reconstructing routes from ODB guides, preventing redundant via GSegments and their layer-swapped inverses. The review feedback suggests optimizing this method by replacing std::set with std::vector and std::find to avoid heap allocation overhead and improve cache locality, as the number of vias per net is typically small.

[gemini-code-assist]

Using std::set inside a function called for every net in the design can introduce significant performance overhead due to heap allocations for each unique via. Since the number of vias per net is typically very small, using a std::vector with std::find is much more efficient because it avoids node allocation overhead and benefits from cache locality.

  std::vector<std::tuple<int, int, int, int>> seen_vias;  // x, y, lo_layer, hi
  size_t write = 0;
  for (size_t read = 0; read < route.size(); read++) {
    const GSegment& seg = route[read];
    if (seg.isVia() && seg.init_layer != seg.final_layer) {
      const int lo = std::min(seg.init_layer, seg.final_layer);
      const int hi = std::max(seg.init_layer, seg.final_layer);
      const std::tuple<int, int, int, int> key{seg.init_x, seg.init_y, lo, hi};
      if (std::find(seen_vias.begin(), seen_vias.end(), key) != seen_vias.end()) {
        // Duplicate via (or its inverse) already kept; skip it.
        continue;
      }
      seen_vias.push_back(key);
    }
    if (write != read) {
      route[write] = route[read];
    }
    write++;
  }
  route.resize(write);

[eder-matheus]

It would be nice to have a specific unit test that exercises the specific problem. This test could compare the guides generated post a fresh GRT run with the guides generated after loading them from ODB.