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## Summary GB200 support work: introduces point-to-point send/receive in the MSCCL++ DSL and extends the executor for split-NVL-domain topologies where some ranks are NVL-connected within a node and other ranks must communicate across the network. ### DSL - New `SendRecv` collective with separate input/output buffers (`python/mscclpp/language/collectives.py`). - New multi-node sendrecv DSL example (`python/mscclpp/language/tests/multi_node/send_recv.py`) with `--split_mask` (group size − 1) and `--instances` CLI options. Documents the channel-ordering trick that keeps signal tags cross-matched between paired peers when `prev == next`. - `BaseBuffer.__getitem__` now accepts slices with `None` start/stop (e.g., `buf[:]`). ### Executor - One connection (unique QP) per channel entry instead of one per peer. Required for HostNoAtomic IB mode where each QP can forward signals to a single semaphore. Uses per-peer tag counters so paired ranks agree on tag ordering regardless of the order peers appear in each rank's `connected_to` list. - MEMORY channels now unconditionally use `Transport::CudaIpc`; only PORT channels can use IB. Matches the invariant already enforced by `getTransportFlags`. - `ExecutionContext::connections` is now a `vector<Connection>` indexed by channel order (was `unordered_map<int, Connection>` keyed by peer). Removes redundant semaphore fields from `ExecutionContext`. - TODO: explicit NVL-domain check in `useIB` --------- Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: Changho Hwang <changhohwang@microsoft.com>