Support empty segments in segmented bitmask reductions

[vuule]

Description

segmented_offset_bitmask_binop seeded each segment's reduction by loading the segment's first mask, sources[segment_start], before knowing whether the segment contains any masks. An empty segment therefore had no defined result and read a mask it does not own; for a trailing empty segment, that read is past the end of the mask array.

With this PR, seeding is done with the identity of the binary operator. An empty segment then means the identity, which for bitwise AND is an all-valid mask with a null count of zero; this is now documented on the public segmented_bitmask_and overloads.

The kernel also indexed segment_offsets and destinations before the segment_id >= num_segments guard, which the excess warps of the last block do whenever the segment count is not a multiple of the warps per block; those loads now happen after the guard.

Segment offsets are additionally validated as non-decreasing and within the mask array, since a decreasing pair leaves a segment's bounds meaningless. The bitmask benchmark's segment size generator was producing exactly that: it truncated draws from an unbounded normal distribution into size_type, so a small expected_masks_per_segment gave negative sizes and an illegal memory access. It now clamps at zero, keeping empty segments.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📥 Commits

Reviewing files that changed from the base of the PR and between dbf991d and 76d938b.

📒 Files selected for processing (3)

• cpp/benchmarks/bitmask/bitmask_and.cpp
• cpp/include/cudf/detail/null_mask.cuh
• cpp/tests/bitmask/bitmask_tests.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved segmented bitmask operations to correctly handle empty segments, including leading, trailing, and interior gaps.
  • Added validation for invalid segment offsets, such as decreasing or out-of-range values.
  • Ensured empty segments produce the correct identity mask and null counts.

• Tests

  • Added coverage for empty segments and invalid offset handling.

Walkthrough

Changes

The segmented bitmask API now supports identity values, empty segments, and offset validation. The benchmark generates empty segments safely, and tests cover identity behavior and invalid offsets.

Segmented bitmask operations

Layer / File(s)	Summary
Identity-aware kernel and validation cpp/include/cudf/detail/null_mask.cuh	Segmented bitmask functions accept an identity value, validate non-decreasing in-bounds offsets, and initialize empty destinations with the identity.
Empty-segment benchmark generation cpp/benchmarks/bitmask/bitmask_and.cpp	Generated segment sizes are clamped to zero, with headers added for the generation and scan operations.
Empty-segment and offset validation tests cpp/tests/bitmask/bitmask_tests.cpp	Tests cover leading, interior, and trailing empty segments, identity masks, null counts, decreasing offsets, and out-of-range offsets.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 76d93

The change is localized to segmented bitmask reduction behavior and its tests and benchmark inputs; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: bdice

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.
Description check	✅ Passed	The description clearly explains support for empty segments, identity-based reduction, offset validation, and benchmark fixes.
Title check	✅ Passed	The title clearly and concisely summarizes the primary change: support for empty segments in segmented bitmask reductions.

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[vuule]

exit before reading from segment_offsets

[vuule]

/ok to test 400bf84