refactor: split host-only members out of CUDA translation units

[ramakrishnap-nv]

1 of 4 toward a CUDA-free client library (#1802, #1803, #1804 stack on this one).

Why

Talking to a remote cuopt_grpc_server currently requires the full GPU stack. pip install cuopt pulls cudf, cupy-cuda13x[ctk], rmm, pylibraft, numba-cuda, scipy, pandas and libcuopt (which itself pulls cuda-toolkit) — GB-scale, onto a machine whose only job is to serialize protobuf over a socket.

The motivating consumer is the MCP server in #1701: it imports exactly Client, TlsConfig, DataModel, Read, SolverSettings — no Solve — yet installs all of the above.

That coupling is mostly accidental. The gRPC client sources are already GPU-free; what ties them to CUDA is that the host-side implementations they need are compiled into .cu translation units, so anything wanting them must link the CUDA library. This PR only separates those, so #1804 can place them in a cuopt_client library whose NEEDED list has no CUDA, rmm or raft.

What

Several classes are mostly host code but live entirely in .cu files, so anything needing their host-side members has to link the CUDA library. This separates them.

File	Size	CUDA-touching lines
math_optimization/solver_settings.cu → .cpp + _gpu.cu	713	5
mip_heuristics/solver_settings.cu → .cu + .cpp	58	3
pdlp/solution_conversion.cu → + solution_conversion_cpu.cpp	225	23

math_optimization/solver_settings.cu is the clearest case — 713 lines of parameter handling with 5 lines that touch a stream.

The rule each split follows

Host code moves to the .cpp; members taking an rmm::cuda_stream_view or returning a device_uvector stay in the .cu; every member moved out of the original TU is instantiated explicitly, because template class in the .cpp can only emit members whose definitions it can still see.

Two traps this pattern sets — both hit during development

1. A moved member with no explicit instantiation silently disappears. An earlier revision of this PR moved the 19-argument solver_settings_t::set_pdlp_warm_start_data into solver_settings_gpu.cu but instantiated only its five neighbours. The symbol vanished from libcuopt.so. It is the overload the Cython layer binds to, so every conda-python-tests config, docs-build and wheel-tests-cuopt-server failed while every C++ job passed. There is no compile or link error locally — the C++ build does not use that overload.

The check that catches this class of bug in one shot:

nm -D --defined-only libcuopt.so | awk '{print $3}' | sort -u > new.txt
comm -23 main.txt new.txt | c++filt     # anything here is a lost export

2. Guarded instantiations can compile to nothing. The instantiations sit behind MIP_INSTANTIATE_* / PDLP_INSTANTIATE_*, so each new file must include mip_heuristics/mip_constants.hpp. Without it the guards evaluate false and the TU compiles to zero symbols — no error, just a link failure much later. nm --defined-only on the object is how you spot it.

Risk

Moderate, not low — see above. The moved definitions are byte-identical and no build targets change here, so behaviour is unaffected; the risk is entirely in symbol emission, which the exported-symbol diff now covers.

Testing

• Full build + all 126 test binaries: 0 errors
• Exported symbols diffed against main: no losses
• ctest: 119/125. The 6 failures are missing downloaded datasets (ci/test_cpp.sh fetches them; I did not locally) — unmodified main fails the identical six in a clean worktree.

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[ramakrishnap-nv]

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[ramakrishnap-nv]

/ok to test

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• cpp/tests/linear_programming/unit_tests/solver_settings_test.cu

🚧 Files skipped from review as they are similar to previous changes (1)

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📝 Walkthrough

Walkthrough

Changes

The PR separates host-only and CUDA-specific solver settings and solution conversion implementations. It updates CMake source lists, adds the adaptive barrier regularization parameter, and extracts testable remote callback eligibility logic.

CPU and GPU solver settings

Layer / File(s)	Summary
Host and device solver settings cpp/src/math_optimization/..., cpp/src/mip_heuristics/..., cpp/tests/linear_programming/unit_tests/solver_settings_test.cu	Host-side MIP settings methods move to solver_settings.cpp. CUDA-facing PDLP and MIP methods move to solver_settings_gpu.cu. Tests cover both numeric specializations, initial solutions, warm-start data, callbacks, and tolerances.
CPU solution conversion cpp/src/pdlp/..., cpp/tests/linear_programming/unit_tests/solution_interface_test.cu	CPU LP and MIP conversion methods move to solution_conversion_cpu.cpp. Tests validate solution vectors, return metadata, and warm-start data.
Remote callback eligibility cpp/src/grpc/client/..., cpp/tests/linear_programming/grpc/grpc_client_test.cpp	Semi-continuous callback detection moves into a namespace-level helper. Tests cover callback and variable-type combinations.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🟡 Moderate · up to c9200

The host/device split changes where template specializations are emitted, and an unconditional test reference can cause supported configurations to fail at link time when that specialization is disabled. The affected reference should be guarded or explicitly accepted before merging.

Suggested reviewers: tmckayus, rg20, chris-maes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 12.90% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 31 functions across 8 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely summarizes the main change: moving host-only members out of CUDA translation units to support a CUDA-free client library.
Description check	✅ Passed	The description directly explains the CUDA-free client-library objective, the translation-unit splits, symbol-instantiation risks, and validation results. It is fully related to the changeset.

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Inline comments:
In `@cpp/src/grpc/client/solve_remote.cpp`:
- Line 154: Add gtest coverage under cpp/src/tests for the callback-disabling
logic in the solve-remote path, using host variable-type cases with and without
var_t::SEMI_CONTINUOUS and asserting callbacks are cleared only when the
semi-continuous type is present.

In `@cpp/src/math_optimization/solver_settings_gpu.cu`:
- Around line 45-84: Add explicit instantiations for solver_settings_t<int,
float>::set_pdlp_warm_start_data and solver_settings_t<int,
double>::set_pdlp_warm_start_data in the corresponding MIP_INSTANTIATE_FLOAT and
MIP_INSTANTIATE_DOUBLE blocks, alongside the other explicitly instantiated moved
members.
- Around line 28-105: Add gtest coverage under cpp/src/tests for exported float
and double solver_settings_t specializations. In
cpp/src/math_optimization/solver_settings_gpu.cu lines 28-105, exercise initial
primal/dual solution, warm-start APIs, and every explicitly instantiated member
to verify linkage. In cpp/src/mip_heuristics/solver_settings.cpp lines 26-47,
test callback registration, user-data propagation, callback retrieval, and
tolerance retrieval.

In `@cpp/src/pdlp/solution_conversion_cpu.cpp`:
- Around line 27-110: Add GoogleTest coverage under cpp/src/tests for the
exported int,double methods cpu_lp_solution_t::to_cpu_linear_programming_ret_t
and cpu_mip_solution_t::to_cpu_mip_ret_t. Test LP conversion both with empty and
populated pdlp_warm_start_data_, asserting every returned solution, diagnostic,
and iteration field; test MIP conversion asserting every field of the returned
mip_ret_t, including status, errors, objectives, timing, violations, and counts.

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📒 Files selected for processing (10)

• cpp/src/grpc/client/solve_remote.cpp
• cpp/src/math_optimization/CMakeLists.txt
• cpp/src/math_optimization/solver_settings.cpp
• cpp/src/math_optimization/solver_settings_gpu.cu
• cpp/src/mip_heuristics/CMakeLists.txt
• cpp/src/mip_heuristics/solver_settings.cpp
• cpp/src/mip_heuristics/solver_settings.cu
• cpp/src/pdlp/CMakeLists.txt
• cpp/src/pdlp/solution_conversion.cu
• cpp/src/pdlp/solution_conversion_cpu.cpp

💤 Files with no reviewable changes (3)

• cpp/src/math_optimization/solver_settings.cpp
• cpp/src/mip_heuristics/solver_settings.cu
• cpp/src/pdlp/solution_conversion.cu

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[ramakrishnap-nv]

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[ramakrishnap-nv]

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[ramakrishnap-nv]

@rg20 @tmckayus @chris-maes — ready for another look. Both review points are addressed and the branch is merged up to current main (5045ca7d).

Since the last review:

should_disable_semi_continuous_callbacks in the public header	moved to solve_remote_impl.hpp (mirrors the existing cython_grpc_client_impl.hpp) and renamed should_disable_unsupported
"What's the rationale for this refactoring?"	answered in the thread above, and a ## Why section added to the PR description so it is not buried in comments

On the placement point — it was worse than "too specific". grpc_client.hpp is the client's public interface, and that predicate is an implementation detail of solve_remote.cpp that I had only lifted there so a unit test could reach it. Test convenience leaking into a public header. The doc comment now also records that semi-continuous + MIP callbacks is the only rule today, and that further rules belong inside the predicate rather than as new branches at the call site.

Short version of the rationale, for anyone joining here: this is 1 of 4 toward a CUDA-free cuopt_client library, so a remote client (the MCP server in #1701 is the concrete consumer) can talk to cuopt_grpc_server without installing cudf/cupy/rmm/pylibraft. That MCP server imports only Client, TlsConfig, DataModel, Read, SolverSettings — no Solve — yet pulls the whole CUDA stack today. This PR is a pure prerequisite: every moved definition is byte-identical and everything still builds into libcuopt exactly as before.

Worth knowing while reviewing: the risk in this PR is not behaviour, it is symbol emission. A member moved out of a translation unit with no matching explicit instantiation silently disappears — no compile or link error locally, because the C++ build does not exercise the Cython-facing overloads. That bit this PR once (set_pdlp_warm_start_data, which took down every Python job while all C++ jobs passed). The check that catches the whole class in one shot:

nm -D --defined-only libcuopt.so | awk '{print $3}' | sort -u > new.txt
comm -23 main.txt new.txt | c++filt     # anything listed is a lost export

I run this against main before each push; it is currently clean. Whether it belongs in CI is worth your opinion — I have kept it out of this PR to avoid widening the diff, but five separate missing-instantiation bugs happened across this stack and only one was caught by anything other than a human or CI.

Status: merged to main (5045ca7d), build + all 126 test binaries clean, CI green so far (18 pass / 8 pending / 0 fail). #1802–#1804 stack on this one and will each need the same merge treatment once it lands.

[ramakrishnap-nv]

@rg20 May I get your review on this PR

[rg20]

You should actually pass in the problem and settings as arguments here

[ramakrishnap-nv]

Done in 7de9056 — it now takes both objects directly:

template <typename i_t, typename f_t>
bool should_disable_unsupported(const cpu_optimization_problem_t<i_t, f_t>& problem,
                                const mip_solver_settings_t<i_t, f_t>& settings);

Call site drops the two pre-extraction lines, and get_variable_types_host() (a full vector copy) now happens inside the predicate after the callback check, so it is skipped when no callbacks are registered. Tests rebuilt around real problem/settings objects going through set_mip_callback(); all 5 pass.

[ramakrishnap-nv]

@rg20 Done — should_disable_unsupported now takes the problem and settings directly:

template <typename i_t, typename f_t>
bool should_disable_unsupported(const cpu_optimization_problem_t<i_t, f_t>& problem,
                                const mip_solver_settings_t<i_t, f_t>& settings);

This is what makes the generalized name earn itself — a new rule can consult anything either object exposes without changing the signature, threading another argument through, or adding a branch at the call site. It also tightened the call site:

-  auto mip_callbacks   = settings.get_mip_callbacks();
-  const auto var_types = cpu_problem.get_variable_types_host();
-  if (should_disable_unsupported(var_types, !mip_callbacks.empty())) {
+  auto mip_callbacks = settings.get_mip_callbacks();
+  if (should_disable_unsupported(cpu_problem, settings)) {

Minor side benefit: get_variable_types_host() copies the whole vector, and it now happens inside the predicate after the callback check — so it is skipped entirely when no callbacks are registered, which is the common case.

The tests now build real cpu_optimization_problem_t / mip_solver_settings_t objects and register a callback through set_mip_callback(), rather than passing a bare vector and a bool. That exercises the real path instead of a stand-in. All 5 cases pass.

One note for reviewers: making it a template means it needs an explicit instantiation in solve_remote.cpp, since the test's TU only sees the declaration. That is the same footgun described in the PR body — a missing instantiation here would be a link error in the test rather than a silent drop, so it fails loudly, but it is worth knowing why that line is there.

[ramakrishnap-nv]

/ok to test

[rg20]

Thanks for the changes!

[nguidotti]

/merge