BFS Atomic Counter Task

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[dsengupta0628]

Some initial feedback (more to come):

• No tests or benchmarks. A change to the core BFS traversal should include regression tests (timing correctness) and performance benchmarks (runtime comparison on representative designs).
• No PR description. The motivation, design rationale, and expected improvements are not documented.
• There are issues with both OpenSTA/OpenROAD regressions. Suggest to please merge with latest OpenROAD/OpenSTA master and rerun regressions

[dsengupta0628]

The most critical issue here is in-degree count/decrement mismatch (topological ordering violation)

computeInDegrees() deduplicates per unique successor vertex:

  std::set<Vertex*> counted_successors;
  // ...
  if (counted_successors.insert(to_vertex).second) {
      in_degrees_[to_vertex->objectIdx()].fetch_add(1, ...);
  }

But enqueueAdjacentVertices() decrements per edge (deduped via processed_edges_):

  inserted = processed_edges_.insert(edge).second;  // per edge, not per vertex
  if (inserted) {
      int old_deg = in_degrees_[to_vertex->objectIdx()].fetch_sub(1, ...);
      if (old_deg == 1) { /* process vertex */ }
  }

When multiple edges connect the same two vertices (e.g., different timing arc sets with different when conditions), the in-degree is incremented once but decremented multiple times. Concrete example:

• Vertex A has edges e1, e2 to C. Vertex B has edge e3 to C.
• computeInDegrees: C gets in-degree = 2 (A counted once, B once)
• enqueueAdjacentVertices(A):
  • e1: decrement C: 2→1, old_deg=2, not ready
  • e2: decrement C: 1→0, old_deg=1, process C
• But B hasn't been visited yet — C is processed before all predecessors are done.

This violates topological ordering. C's delay calculation uses stale/uninitialized data from B's contribution. Fix: either remove the counted_successors dedup in computeInDegrees (count per edge), or dedup per target vertex in enqueueAdjacentVertices.

--One possible fix would be----
Count per-edge in computeInDegrees (remove the dedup), and remove the counted_successors set entirely in computeInDegrees, so that in-degree counts every edge passing the predicate, matching the per-edge decrement in enqueueAdjacentVertices.

[dsengupta0628]

One comment regarding the change- this change bypasses the incremental delay tolerance.

Old behavior
With the old BfsFwdIterator, if a vertex's output slews didn't change, enqueueAdjacentVertices was not called — successors were never enqueued, and the entire downstream cone was skipped. This is the key incremental optimization: a small change that doesn't affect slews stops
propagating immediately.

New behavior
In the new enqueueAdjacentVertices, when a successor's in-degree reaches 0, , it dispatches this lambda:

  dispatch_queue_->dispatch([this, to_vertex](size_t tid) {
      current_thread_id = tid;
      visitors_[tid]->visit(to_vertex);     // → findVertexDelay (may or may not propagate)
      visit_count_->fetch_add(1, ...);
      enqueueAdjacentVertices(to_vertex);   // always propagates
  });

Two calls to enqueueAdjacentVertices(to_vertex) happen:

1. Conditionally from findVertexDelay inside the visitor (only if slews changed)
2. Unconditionally from the lambda itself

If slews didn't change, call 1 is skipped. But call 2 runs anyway, decrements all successors' in-degrees, and dispatches any that become ready. The processed_edges_ set is empty for those edges (since call 1 was skipped), so they all get processed.

This cascades - every successor is visited, and every successor unconditionally propagates to its successors, and so on through the entire downstream cone.

But with new in-degree increment change, you probably need this. If a vertex C has 2 edges from A and B, and say A's slews changed but B's didn't:

• With the old BFS: A propagates, enqueues C. B doesn't propagate. C is still visited (reached via A). Works fine — any single predecessor can trigger a visit.
• With in-degree counting: A decrements C (2→1). If B doesn't decrement (because its slews didn't change), C stays at in-degree 1 forever — it's never processed, even though A's change means C needs recomputation.

So the unconditional propagation is necessary to make the in-degree mechanism work. But it means every vertex in the downstream cone is visited during incremental updates, even when slew changes die out early. For a small ECO on a large design, the old code might recompute tens
of vertices; the new code recomputes the entire fanout cone.

If the parallel speedup outweighs the incremental efficiency loss, it's a valid tradeoff. Do you see this making a dent in runtime?
Or you can use in-degree BFS only for non-incremental (first pass), fall back to the old BfsFwdIterator for incremental updates where selective propagation matters most?

[dsengupta0628]

One comment regarding the change- this change bypasses the incremental delay tolerance.

Old behavior With the old BfsFwdIterator, if a vertex's output slews didn't change, enqueueAdjacentVertices was not called — successors were never enqueued, and the entire downstream cone was skipped. This is the key incremental optimization: a small change that doesn't affect slews stops propagating immediately.

New behavior In the new enqueueAdjacentVertices, when a successor's in-degree reaches 0, , it dispatches this lambda:

  dispatch_queue_->dispatch([this, to_vertex](size_t tid) {
      current_thread_id = tid;
      visitors_[tid]->visit(to_vertex);     // → findVertexDelay (may or may not propagate)
      visit_count_->fetch_add(1, ...);
      enqueueAdjacentVertices(to_vertex);   // always propagates
  });

Two calls to enqueueAdjacentVertices(to_vertex) happen:

1. Conditionally from findVertexDelay inside the visitor (only if slews changed)
2. Unconditionally from the lambda itself

If slews didn't change, call 1 is skipped. But call 2 runs anyway, decrements all successors' in-degrees, and dispatches any that become ready. The processed_edges_ set is empty for those edges (since call 1 was skipped), so they all get processed.

This cascades - every successor is visited, and every successor unconditionally propagates to its successors, and so on through the entire downstream cone.

But with new in-degree increment change, you probably need this. If a vertex C has 2 edges from A and B, and say A's slews changed but B's didn't:

• With the old BFS: A propagates, enqueues C. B doesn't propagate. C is still visited (reached via A). Works fine — any single predecessor can trigger a visit.
• With in-degree counting: A decrements C (2→1). If B doesn't decrement (because its slews didn't change), C stays at in-degree 1 forever — it's never processed, even though A's change means C needs recomputation.

So the unconditional propagation is necessary to make the in-degree mechanism work. But it means every vertex in the downstream cone is visited during incremental updates, even when slew changes die out early. For a small ECO on a large design, the old code might recompute tens of vertices; the new code recomputes the entire fanout cone.

If the parallel speedup outweighs the incremental efficiency loss, it's a valid tradeoff. Do you see this making a dent in runtime? Or you can use in-degree BFS only for non-incremental (first pass), fall back to the old BfsFwdIterator for incremental updates where selective propagation matters most?

Also note that the unconditional call was necessary because the in-degree mechanism requires every vertex to decrement its successors (otherwise successors are stuck). But it means the incremental optimization (stop propagating when slews don't change) is defeated. Now, processed_edges_ was added to prevent the two calls from double-decrementing the same edge.
If you remove the unconditional call then processed_edges_ + its mutex become unnecessary. No mutex, no std::set, no O(log n) tree operations. Just an atomic decrement per edge — the same cost as incrementing in computeInDegrees.