perf: software prefetch bitmask filter K=8 steps ahead in iterate_to_…

[cave-g-f]

…fixed_point

Reduces contain_filtered_label self-cost from 49.2% to 14.3% by hiding DRAM latency on the random-access node bitmask load. Measured on DANNMemoryTable with 21M-node index, 189 filter labels, 12 threads: QPS 7,705 -> 12,435 (+61.4%), P99 3012us -> 1656us (-45%).

Changes:

• filter_match_proxy: add prefetch_bitmask(id) virtual method that issues _mm_prefetch on the node's bitmask address. No-op for the integer_label path.
• iterate_to_fixed_point: pre-prefetch first K=8 neighbors' bitmasks before the loop, then sliding-window prefetch K steps ahead inside the loop. Guarded by use_filter to avoid overhead on unfiltered queries. Applied to both dynamic and static graph paths.
• Use NeighborList::data() for indexed access (no operator[]).
• Also brings AVX2 buffer padding from downstream d9583a32:
  • bitmask_filter_match ctor: query_bitmask_buf resized to at least 4 words for safe 256-bit loads.
  • build_bitmask_index: node bitmask buffer padded with 4 extra uint64 words at end for safe AVX2 reads on the last node.

The prefetch instruction itself accounts for 11% self-cost in the post- optimization profile, which is the expected steady-state cost of hiding the DRAM stall on 44 billion filter checks (4.6M queries x 200 search list x ~48 out-edges).

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

Reference Issues/PRs

What does this implement/fix? Briefly explain your changes.

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