Fix find dense columns out of bounds

[hlinsen]

This PR fixes a bug observed on miplib on H100 during concurrent root solve.
The number of nnz in columns can be equal to m.

Summary by CodeRabbit

• Bug Fixes
  • Enhanced internal safety checks to prevent potential overflow conditions in barrier algorithm computations.

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[coderabbitai]

📝 Walkthrough

Walkthrough

The change increases histogram array sizes by one slot (columns: m+1, rows: n+1) in the dual simplex CUDA barrier code and adds runtime assertions to prevent histogram bin index overflow. No public APIs or exported entities were changed.

Changes

Cohort / File(s)	Summary
Histogram Array Sizing & Overflow Guards cpp/src/dual_simplex/barrier.cu	Increased histogram array allocation by one slot for column and row histograms and added runtime assertions to guard against out-of-bounds increments when updating histogram bins.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly describes the main bug fix: preventing out-of-bounds access in the find dense columns logic when nnz equals m.

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✨ Finishing touches

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@cpp/src/dual_simplex/barrier.cu`:
- Around line 1147-1153: The loop that builds histogram_row accesses
histogram_row[row_nz[k]] before validating the index; move the cuopt_assert that
checks row_nz[k] <= n to immediately before the histogram_row[row_nz[k]]++ line
(in the same loop that iterates k) so the bound is validated prior to the array
access; keep updating max_row_nz and incrementing histogram_row only after the
assertion.

[chris-maes]

LGTM. Thanks for the fix @hlinsen . Apologies for my buggy code.

[hlinsen]

/ok to test 640a33f

[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

cpp/src/dual_simplex/barrier.cu (1)

1147-1164: Fix histogram_row loop bounds to avoid OOB when m > n.
histogram_row is sized n + 1, but subsequent loops use k < m. If m > n, those loops can access past the end of histogram_row.

🐛 Proposed fix

`#ifdef` HISTOGRAM
-    for (i_t k = 0; k < m; k++) {
+    for (i_t k = 0; k <= n; k++) {
       if (histogram_row[k] > 0) { settings.log.printf("%6d %6d\n", k, histogram_row[k]); }
     }
`#endif`

     n_dense_rows = 0;
-    for (i_t k = 0; k < m; k++) {
+    for (i_t k = 0; k <= n; k++) {
       if (histogram_row[k] > .1 * n) { n_dense_rows++; }
     }

[hlinsen]

/merge