[Backport to 3.4] Fix scanning out-of-bounds items in OpenMP scan (#9759)

[bernhardmgruber]

(cherry picked from commit a0fd4f2)

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 9f34bc56-75a2-4b15-bc36-975b9a5f6add

📥 Commits

Reviewing files that changed from the base of the PR and between 305c95b and 4137a0d.

📒 Files selected for processing (1)

• thrust/thrust/system/omp/detail/scan.h

---

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved parallel scan behavior across varying input sizes.
  • Enhanced work distribution to ensure all portions of the input are processed consistently.
  • Preserved existing inclusive and exclusive scan behavior.

• Performance

  • Improved efficiency by using only the number of processing threads needed for each scan.

Walkthrough

Changes

The OMP scan now computes block_size and active_threads once, sizes block sums accordingly, and uses the active thread count for block reduction and per-block scanning.

OMP scan partitioning

Layer / File(s)	Summary
Active block decomposition and scanning thrust/thrust/system/omp/detail/scan.h	scan_impl derives active block bounds, sizes block_sums, and runs Steps 1 and 3 with active_threads; inclusive and exclusive scan branches remain supported.

Possibly related PRs

• NVIDIA/cccl#9759: Updates the same OMP scan partitioning and removes redundant range guards.

Suggested labels: backport branch/3.4.x

Suggested reviewers: elstehle, miscco, davebayer, jacobfaib, wmaxey

---

_{Comment @coderabbitai help to get the list of available commands.}

[bernhardmgruber]

pre.commit-ci autofix

[bernhardmgruber]

pre-commit.ci autofix

[bernhardmgruber]

/ok to test 017691f

[github-actions]

🥳 CI Workflow Results

🟩 Finished in 1h 14m: Pass: 100%/115 | Total: 19h 08m | Max: 45m 24s | Hits: 99%/187842

See results here.