[Ziminli]

Summary

Complete the MCCL backend operation surface that is already supported by the NCCL backend. MetaX and Moore builds can now use AllGather, ReduceScatter, Send, and Recv through MCCL while reusing the existing common CCL implementations.

Changes

• MCCL API Support and Registeration
  • Add direct McclApi wrappers for mcclAllGather, mcclReduceScatter, mcclSend, and mcclRecv in src/backends/ccl/mccl/api.h.
  • Add MCCL specializations for AllGather, ReduceScatter, Send, and Recv under src/backends/ccl/mccl/impl/.
  • Reuse CclAllGatherImpl, CclReduceScatterImpl, CclSendImpl, and CclRecvImpl so datatype mapping, communicator validation, stream handling, and error propagation remain centralized.

Platform and Backend Affected

Platform

• CPU
• NVIDIA GPU
• Iluvatar GPU
• MetaX GPU
• Moore Threads GPU
• Cambricon MLU
• HYGON DCU
• T-Head PPU
• Ascend NPU

Backend

• OpenMPI
• MPICH
• NCCL/RCCL
• MCCL
• CNCL
• HCCL

Performance Impact

• No performance impact
• Performance improved
• Performance regression possible

N/A.

Known Issues & Future Work

N/A.

Test Results

Component	MetaX	Moore
GPU	8 × C550, 64 GiB each	8 × MTT S5000, 80 GiB each
Driver	3.3.12	3.3.6-server
SDK	MACA 3.3.0.15	MUSA 4.3.6
MCCL	2.16.5	2.11.4
Open MPI	4.1.6	5.0.6
GCC	13.3.0	11.4.0
OS	Ubuntu 24.04.1	Ubuntu 22.04.5

Test Involved Platform

• CPU
• NVIDIA GPU
• Iluvatar GPU
• MetaX GPU
• Moore Threads GPU
• Cambricon MLU
• HYGON DCU
• T-Head PPU
• Ascend NPU

Test Involved Backend

• OpenMPI
• MPICH
• NCCL/RCCL
• MCCL
• CNCL
• HCCL

MetaX:
MCCL + OMPI:
ccl_mpi_hybrid_all_gather.log
ccl_mpi_hybrid_all_reduce.log
ccl_mpi_hybrid_send_recv.log

MCCL:
ccl_all_gather.log
ccl_all_reduce.log
ccl_reduce_scatter.log
ccl_send_recv.log

Moore Threads:
MCCL + OMPI:
ccl_mpi_hybrid_all_gather.log
ccl_mpi_hybrid_all_reduce.log
ccl_mpi_hybrid_send_recv.log

MCCL:
ccl_all_gather.log
ccl_all_reduce.log
ccl_reduce_scatter.log
ccl_send_recv.log

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Checklist

Every contributor must verify every item below before requesting
review. Tick each box only after the check has actually been performed —
do not tick speculatively. If an item truly does not apply, replace the
checkbox with N/A and briefly explain why in an inline comment.

Title, Branch, and Commits

• PR title follows Conventional Commits (e.g. feat: …, fix(nccl): …).
• Branch name follows <type>/xxx-yyyy-zzzz where <type> matches the PR title's Conventional Commits type and words are joined with hyphens (see CONTRIBUTING.md §Branches).
• Each commit message follows Conventional Commits.
• Small PR is a single squashable commit; or, for a large PR, every commit is meaningful, well-formed, and independently reviewable (see CONTRIBUTING.md §Pull Requests).
• No stray merge commits from master — the branch is rebased cleanly on top of the current master.
• No fixup! / squash! / wip commits remain.

Scope and Design

• Changes are minimal — no unrelated modifications were introduced (CONTRIBUTING.md §Code/General).
• No dead code, commented-out blocks, debug prints, printf/std::cout/print(...) left behind, or TODO without an owner and issue link.
• No unrelated formatting churn that would obscure the diff.
• N/A- Public API changes (if any) are intentional, documented, and reflected in affected callers/tests.

General Code Hygiene

• The code is self-explanatory; comments were added only where the intent or rationale is non-obvious (CONTRIBUTING.md §Code/General).
• Every modified or added file ends with a single trailing newline (CONTRIBUTING.md §Code/General).
• No trailing whitespace, inconsistent indentation, or mixed formatting styles remain.
• Identifiers referenced in comments or error messages are wrapped in Markdown backticks (e.g. the `AllReduce` implementation) (CONTRIBUTING.md §Code/General).
• All comments and error messages are in English (CONTRIBUTING.md §Code/General).
• Comments and error messages are complete sentences — capitalized first letter, terminal punctuation — unless the language/framework convention says otherwise (CONTRIBUTING.md §Code/General; §Python).

C++ Specific (if C++ files changed)

• Code follows the Google C++ Style Guide strictly.
• clang-format (version 16, per .github/workflows/clang-format.yml) has been run against all modified applicable files; the diff is clean.
• No exceptions are thrown. Error paths use assert with messages that include at least __FILE__, __LINE__, and __func__ (CONTRIBUTING.md §C++).
• Error and warning message wording follows the LLVM Coding Standards (CONTRIBUTING.md §C++).
• N/A- Constructor initializer list order matches member declaration order (CONTRIBUTING.md §C++).
• Exactly one blank line between classes, between classes and functions, and between functions (CONTRIBUTING.md §C++).
• Exactly one blank line between members (functions and variables) within a class (CONTRIBUTING.md §C++).
• Exactly one blank line before and after the contents of a namespace (CONTRIBUTING.md §C++).

Python Specific (if Python files changed)

N/A. No Python files were changed.

Testing

• All applicable example programs have been built and tested successfully on at least one supported heterogeneous cluster setup.

Build, CI, and Tooling

• N/A- New backends or devices have been added to auto-detection in CMakeLists.txt under if(AUTO_DETECT_DEVICES) or to if(AUTO_DETECT_BACKENDS) if applicable.
• Both CI workflows (clang-format.yml, ruff.yml) are green locally (or expected to be green on CI).

Documentation

• N/A- README.md, CONTRIBUTING.md, or inline docs updated when behavior, build flags, or developer workflow changed.
• N/A- Any user-visible breaking change is called out explicitly under "Summary" and in the commit/PR title with a ! or BREAKING CHANGE: footer.

Security and Safety

• No secrets, access tokens, internal URLs, customer data, or personal hardware identifiers have been committed.
• N/A- Third-party code is license-compatible and attributed.
• No unsafe pointer arithmetic, uninitialized reads, or missing bounds checks were introduced.