perf(spectrum): software-prefetch SA entries in the boundary-search and forward/tabled probe loops

[nh13]

PR #8 of the v0.2 stack (carries 6c5abb3, with d77b83b folded in). Base is feat/v0.2-benches (#20).

Software-prefetches SA entries in the boundary-search probe loop, extended to the forward + tabled binary searches (d77b83b folded). Pure perf — byte-identical (oracle proptests unchanged). Clean cherry-pick onto #7.

Green-pass: build / clippy -D warnings / nightly fmt / cargo test --workspace all ✅ (162 lib + all FFI; only Plan-3 deferrals ignored).

Summary by CodeRabbit

• Chores
  • Optimized internal memory access patterns to improve lookup performance latency through prefetching enhancements.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 29d54953-abcd-4bf7-b2bd-50f0991cd37c

📥 Commits

Reviewing files that changed from the base of the PR and between fdedec3 and 16f99ff.

📒 Files selected for processing (3)

• prmi/src/index/mod.rs
• prmi/src/index/spectrum.rs
• prmi/src/sidecar/sa_file.rs

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📝 Walkthrough

Walkthrough

This PR adds software prefetch hints for suffix array cache optimization in binary search operations. A new SaFileReader::prefetch() method issues CPU-specific prefetch instructions, wrapped through LearnedIndex::prefetch_sa(), and integrated into forward spectrum and boundary-finding search loops via a prefetch_bsearch() helper to overlap DRAM reads with ongoing key comparisons.

Changes

SA Prefetching Cache Optimization

Layer / File(s)	Summary
Low-level SA file prefetch implementation prmi/src/sidecar/sa_file.rs	SaFileReader::prefetch(i) with inline(always), bounds checking, byte-offset calculation from file header, and architecture-specific prefetch directives on x86_64 and aarch64.
LearnedIndex prefetch wrapper prmi/src/index/mod.rs	LearnedIndex::prefetch_sa(i) inlined pub(crate) method delegates to underlying SA reader prefetch for use by binary search loops.
Binary search loop prefetch integration prmi/src/index/spectrum.rs	New prefetch_bsearch(lo, hi) helper computes and prefetches both next potential child midpoints; wired into forward spectrum lower/upper-bound loops, tabled forward deep-band search, and find_boundary iteration to overlap next SA entry loads with current key comparisons.

🎯 2 (Simple) | ⏱️ ~12 minutes

🐰 Cache whispers sweet as clover—
Binary search hops the bounds,
Prefetch tips before you leap,
Memories bloom on CPU rounds, 🌾✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding software prefetching of SA entries in binary-search loops (boundary-search, forward, and tabled probe loops).
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
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.

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✨ Finishing Touches

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• Create stacked PR
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