Fix compute-sanitizer racecheck hazards in DELTA_BYTE_ARRAY decode path

[pramodsatya]

Description

Fixes compute-sanitizer racecheck hazards in the Parquet DELTA_BYTE_ARRAY decode path.

The prefix and suffix length decoders are traversed concurrently on separate warps but dispatched through a runtime-selected delta_binary_decoder pointer (db = mgr == 0 ? &prefixes : &suffixes). Since the target is not known at compile time, the compiler speculatively loads both sub-objects on every db-> access, so one warp reads the other's decoder state while it is being written, which racecheck reports as a cross-warp hazard.

Dispatches to each decoder through a compile-time-constant object per warp instead, matching the main decode loop. This is semantically identical and only changes codegen.

Closes #23900

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 e3fd258 and 515fd04.

📒 Files selected for processing (2)

• cpp/src/io/parquet/page_delta_decode.cu
• cpp/src/io/parquet/page_string_decode.cu

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📝 Walkthrough

Summary by CodeRabbit

• Performance
  • Improved Parquet string decoding efficiency by assigning prefix and suffix processing to dedicated GPU warps.
  • Preserved existing byte-size and maximum-length calculations while reducing runtime dispatch overhead.

Walkthrough

Changes

DELTA_BYTE_ARRAY decoding

Layer / File(s)	Summary
Warp-specific skip dispatch cpp/src/io/parquet/page_delta_decode.cu	skip directly decodes prefixes on warp 0 and suffixes on warp 1.
Warp-specific size traversal cpp/src/io/parquet/page_string_decode.cu	totalDeltaByteArraySize uses shared traversal logic and explicitly assigns prefix and suffix processing to separate warps.

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

Merge Risk: ⚪ Minimal · up to 515fd

The PR changes decoder dispatch to use compile-time-resolved prefix and suffix paths while preserving decode behavior, reducing reported racecheck hazards without introducing a supported merge-blocking risk. No actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: igorpeshansky, pmattione-nvidia, mhaseeb123

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: fixing compute-sanitizer racecheck hazards in the Parquet DELTA_BYTE_ARRAY decode path.
Description check	✅ Passed	The description directly explains the racecheck hazard, the runtime-selected decoder pointer, the compile-time dispatch fix, and the linked issue.
Linked Issues check	✅ Passed	The changes address issue #23900 by replacing runtime-selected prefix and suffix decoder access with explicit warp-specific dispatch in both affected traversal paths. This targets the reported cross-w…
Out of Scope Changes check	✅ Passed	The changes are limited to Parquet DELTA_BYTE_ARRAY decoder dispatch logic and directly support the racecheck fix described in issue #23900. No unrelated changes are present.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files.

Full details: Linked Issues check

Explanation

The changes address issue #23900 by replacing runtime-selected prefix and suffix decoder access with explicit warp-specific dispatch in both affected traversal paths. This targets the reported cross-warp racecheck hazards.

✨ Finishing Touches 💡 1

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[davidwendt]

/ok to test 515fd04

[davidwendt]

/merge