Fix scanning out-of-bounds items in OpenMP scan

[bernhardmgruber]

Fixes: #9725

[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: a73f8b85-9818-47e1-bedb-d562bc431c8a

📥 Commits

Reviewing files that changed from the base of the PR and between d5fc08b and 3078353.

📒 Files selected for processing (1)

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

🚧 Files skipped from review as they are similar to previous changes (1)

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

---

📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes
  • Improved parallel scan reliability by allocating and running work based on the actual number of active threads needed for the input size.
  • Simplified per-thread bounds handling while preserving the existing inclusive/exclusive scan semantics, including the special-case behavior for the no-initial inclusive scan.

Walkthrough

OpenMP scan now derives active_threads from the input size, uses it to size block_sums, and applies it to both parallel regions while removing the prior start < n guards.

Changes

OMP scan fix

Layer / File(s)	Summary
Block-sum reduction sizing thrust/thrust/system/omp/detail/scan.h	block_sums is sized to active_threads, and the block reduction region runs with num_threads(active_threads) after removing the start < n guard.
Per-block scan with prefix offsets thrust/thrust/system/omp/detail/scan.h	Step 3 runs with num_threads(active_threads) and drops the start < n guard while keeping the existing inclusive/exclusive and no-init prefix behavior.

Assessment against linked issues

Objective	Addressed	Explanation
Fix assertion failure in thrust.cpp.omp.test.scan due to invalid data reads [#9725]	✅

Out-of-scope changes

---

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

[coderabbitai]

🧹 Nitpick comments (1)

thrust/thrust/system/omp/detail/scan.h (1)

115-115: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

suggestion: first_elem is never mutated; mark it const. As per coding guidelines, "All variables that are not modified must be declared const".

-    accum_t first_elem = *(first + start);
+    const accum_t first_elem = *(first + start);

Source: Coding guidelines

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: cff36059-3fd5-4e46-b97b-ceb100b7d0e9

📥 Commits

Reviewing files that changed from the base of the PR and between f51b10a and d5fc08b.

📒 Files selected for processing (1)

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

[Jacobfaib]

Sorry fat-fingered approval before adding comment. Could you add a test that exercises the bug?

[bernhardmgruber]

Sorry fat-fingered approval before adding comment. Could you add a test that exercises the bug?

The existing test covers the bug if run with a CPU of at least 128 threads :)

[github-actions]

🥳 CI Workflow Results

🟩 Finished in 23h 46m: Pass: 100%/119 | Total: 1d 02h | Max: 1h 08m | Hits: 69%/252077

See results here.