Expose solver statistics as scalar solution attributes - #1715
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The C API reports the outcome of a solve - status, objective, primal and dual solution, MIP gap, solution bound, solve time - but none of the diagnostics the C++ solution interfaces already carry. Of the eleven fields tracked in #1202 (primal and dual residual, gap, iteration count, solved-by, presolve time, node count, simplex iterations, and the three violation magnitudes), zero are reachable today. Python sees all of them by binding to the C++ structs through Cython, so the C ABI is a second-class path and every non-Python binding hits the same wall. Add cuOptGetSolutionIntAttribute and cuOptGetSolutionFloatAttribute, reading the CUOPT_SOLUTION_ATTR_* selectors in constants.h. Attributes rather than one getter per statistic, for three reasons: it matches how problem data is already read; a future statistic becomes a new constant instead of a new exported symbol, so existing callers need no relink; and bindings that generate from constants.h pick new statistics up with no hand-written code. Selectors live in their own numeric range so a problem selector passed to a solution accessor, or the reverse, is rejected rather than silently read. LP selectors require an LP solution and MIP selectors require a MIP solution, since the two come from different solvers; CUOPT_ATTR_IS_MIP on the originating problem says which set applies. The data is read straight off lp_solution_interface_t and mip_solution_interface_t, so this is exposure only - no computation and no solve-time cost. Tests cover both solvers and the ways a caller can get it wrong: a float selector through the integer accessor and the reverse, the other solver's selectors, unknown selectors, and null arguments. Float outputs are seeded with NaN rather than a numeric sentinel, so an accessor that never writes is caught rather than mistaken for a real result. Split out of #1524 so the C API can be reviewed on its own. Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>
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📝 WalkthroughWalkthroughThe C API now exposes scalar LP and MIP solution statistics through typed getters. New selector constants identify supported attributes. Implementations validate solution types and arguments. Tests cover valid retrieval and error cases. ChangesSolution attribute access
Estimated code review effort: 3 (Moderate) | ~20 minutes Mergeability Score: ⚪ Minimal · up to This PR exposes solver statistics through new scalar C API attributes without changing solve behavior; no actionable merge-blocking risk remains beyond normal checks and review. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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In `@cpp/tests/linear_programming/c_api_tests/c_api_tests.cpp`:
- Around line 1145-1158: Update the post-creation solution attribute checks in
the surrounding test to avoid ASSERT_EQ exiting before
cuOptDestroySolution(&solution) runs. Replace the affected ASSERT_EQ checks,
including the float and integer attribute loops and the primal residual check,
with non-aborting expectations while preserving the existing validation messages
and cleanup flow.
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cpp/include/cuopt/mathematical_optimization/constants.hcpp/include/cuopt/mathematical_optimization/cuopt_c.hcpp/src/pdlp/cuopt_c.cppcpp/tests/linear_programming/c_api_tests/c_api_tests.cpp
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CI Test Summary✅ All 31 test job(s) passed. |
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| /* | ||
| * Solver statistics are read as scalar attributes, using the CUOPT_SOLUTION_ATTR_* selectors in | ||
| * constants.h. New statistics can then be added as constants rather than as new exported |
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The comment should describe what the code does, not what alternative designs could have been.
| * constants.h. New statistics can then be added as constants rather than as new exported | ||
| * functions, which keeps the ABI stable for existing callers. | ||
| * | ||
| * LP and MIP statistics come from different solvers, so an LP selector requires an LP solution |
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Do we ever present "LP" and "MIP" as different solvers in the C API? It sounds like we're leaking an implementation detail. How about simply saying that not all attributes are present for all solutions, depending on the problem class?
| cuopt_int_t cuOptGetReducedCosts(cuOptSolution solution, cuopt_float_t* reduced_cost_ptr); | ||
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| /* -------------------------------------------------------------------------- */ | ||
| /* Solution attributes */ |
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We're introducing a new concept of a solution attribute without a clearly documented distinction on what makes them different from parameters.
Documentation, per mlubin: - Drop the rationale about adding statistics as constants rather than exported functions. That is an argument for the design, which belongs in the pull request, not in a public header describing what the accessors do. - Stop describing LP and MIP as different solvers. That leaks an implementation detail and is not vocabulary the C API uses anywhere else. Availability is now stated in terms of the class of problem that produced the solution. - Define what a solution attribute is and how it differs from a parameter, which the header introduced without saying: a parameter is an input, set on a cuOptSolverSettings before solving; an attribute is an output, read from a solved solution or from a problem. Test cleanup, per CodeRabbit: the attribute checks use ASSERT, which returns early on failure, so the explicit cuOptDestroySolution at the end of each test was skipped exactly when a test failed, leaking the solution into the rest of the binary. A scoped guard now destroys it on every exit path, which keeps the fail-fast assertions rather than weakening them to EXPECT. Verified: 69/69 C_API_TEST cases pass. Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>
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Thanks @mlubin — all three taken, in Rationale in the header. Removed. The point about statistics being addable as constants rather than exported functions is an argument for the design; it belongs in the PR description, which is where it now lives. The block describes what the accessors do. "Different solvers." Agreed that it leaks an implementation detail — and checking, "solver" was not vocabulary this header used anywhere else for this purpose; the phrase appeared exactly once, in my own text. Reworded as you suggested: not every attribute applies to every solution, depending on the class of problem that produced it. Same change applied to the selector block in Attribute vs parameter. Fair — the header introduced the concept without defining it. Now stated explicitly:
The existing problem-attributes block does not define the term either, so if this wording works I am happy to fold a shared definition into one place in a follow-up rather than have two blocks each half-explaining it. @coderabbitai the leak is real and is fixed. The checks use 69/69 |
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Documentation wording and the scoped solution guard in the C API tests, kept identical to the branch behind NVIDIA#1715 so this stack stays consistent until that PR merges and these files drop out of this diff. Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>
| * range from the problem selectors so a problem selector passed to a solution accessor, or the | ||
| * reverse, is rejected rather than silently read. |
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| * range from the problem selectors so a problem selector passed to a solution accessor, or the | |
| * reverse, is rejected rather than silently read. | |
| * range from the problem selectors. |
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| bool is_lp_solution_attribute(cuopt_int_t attribute) | ||
| { | ||
| return attribute >= CUOPT_SOLUTION_ATTR_LP_PRIMAL_RESIDUAL && |
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This approach doesn't leave any room for us to define new LP solution attributes in the future. Maybe we don't really need is_lp_solution_attribute and can instead define macros like:
PROCESS_MIP_INT_ATTR(CUOPT_SOLUTION_ATTR_MIP_NUM_NODES, get_num_nodes)
which translates to:
case CUOPT_SOLUTION_ATTR_MIP_NUM_NODES:
auto* mip = as_mip_solution(solution);
if (mip == nullptr) { return CUOPT_INVALID_ARGUMENT; }
*value_out = static_cast<cuopt_int_t>(mip->get_num_nodes());
return CUOPT_SUCCESS;
is_lp_solution_attribute decided whether a selector was LP or MIP by testing whether its value fell between CUOPT_SOLUTION_ATTR_LP_PRIMAL_RESIDUAL and CUOPT_SOLUTION_ATTR_LP_SOLVED_BY. That made the numeric values load-bearing: a new LP attribute would either have to be inserted inside that range, renumbering the constants after it and breaking anyone compiled against them, or be appended after the MIP block where the range test would classify it as MIP. Replace it with the per-case macros mlubin suggested. Each case names the kind of solution it reads, so selector values carry no meaning beyond identity and new selectors can be appended anywhere. Behaviour is unchanged. A selector that names the other kind of solution still returns CUOPT_INVALID_ARGUMENT, now because the corresponding accessor yields nullptr rather than because of a range test, and a selector of the wrong value type falls through to default. Both remain covered by the existing tests. Also trims the selector comment in constants.h, per review. Verified: 69/69 C_API_TEST cases pass. Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>
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Both taken, in Selector range. You're right that the range test made the numeric values load-bearing — a new LP attribute would have had to be inserted inside 300–304, renumbering everything after it and breaking anyone already compiled against those constants, or be appended after the MIP block where
CUOPT_READ_MIP_ATTRIBUTE(CUOPT_SOLUTION_ATTR_MIP_NUM_NODES, get_num_nodes, cuopt_int_t)expanding to a case that resolves the solution kind, rejects a mismatch, and reads the field. Each case now states which kind it reads, so selector values carry no meaning beyond identity and new ones can be appended anywhere in the block. Behaviour is unchanged, which the existing tests confirm: a selector naming the other kind of solution still returns Comment. Applied your suggestion verbatim. 69/69 |
Keeps the cpp/ files in this stack identical to the branch behind NVIDIA#1715. Signed-off-by: Ramakrishna Prabhu <ramakrishnap@nvidia.com>
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/merge |
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Description
Adds two C API accessors so solver statistics are reachable from the C ABI, and closes #1202.
Split out of #1524 at @mlubin's request, so the C API can be reviewed without the Java diff attached. The Java bindings are the first consumer but nothing here refers to them.
The gap
The C API reports the outcome of a solve —
cuOptGetTerminationStatus,cuOptGetObjectiveValue,cuOptGetMIPGap,cuOptGetSolutionBound,cuOptGetSolveTime, and the primal/dual/reduced-cost arrays — but none of the diagnostics the C++ solution interfaces already carry. Of the eleven fields listed in #1202, zero are reachable today.Python sidesteps this by binding to the C++ structs through Cython, so the C ABI is a second-class path and every non-Python binding hits the same wall.
Concretely, what these unlock:
CUOPT_METHOD_CONCURRENTthe caller otherwise cannot tell PDLP from dual simplex.Why attributes rather than one getter per statistic
#1202 proposed eleven individual getters. This uses the attribute model instead, following review feedback from @chris-maes:
cuOptGetProblemIntAttributeand friends);constants.h— as the Java bindings already do forCuOptConstants.java— pick up new statistics with no hand-written code at all.Structured data stays on dedicated functions, which is the existing convention:
cuOptGetConstraintMatrixis a function because CSR is three parallel arrays, and the same will apply to the quadratic constraint rows in #1703. The rule is scalars and homogeneous arrays as attributes, ragged or multi-output as functions.Safety
Solution selectors are numbered in their own range (300+), so a problem selector passed to a solution accessor, or the reverse, is rejected rather than silently read. LP selectors require an LP solution and MIP selectors require a MIP solution, since the two come from different solvers;
CUOPT_ATTR_IS_MIPon the originating problem says which set applies.The values are read straight off
lp_solution_interface_t/mip_solution_interface_t, so this is exposure only — no computation, no solve-time cost.Tests
c_api.lp_solution_attributesandc_api.mip_solution_attributescover both solvers and the ways a caller can get it wrong: a float selector through the integer accessor and the reverse, the other solver's selectors, unknown selectors, and null arguments.Float outputs are seeded with
NaNrather than a numeric sentinel, since the solver cannot legitimately produceNaN— so an accessor that never writes its output is caught, where a numeric sentinel would be indistinguishable from a real result.Verified locally: 69/69
C_API_TESTcases pass with the full LP/MIP/QP dataset.Checklist