lgj-abi: hop_cached_vs_gather probe — the M1b tile pays off on hop two - #79
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Measures lgj_hop's selection three ways at 65 536 rows / 32 facets, every arm bit-identical before timing: the shipped per-hop recompute (64 contiguous eq_u32 passes + ternlog + scatter), a cached sel_f = class_f ∧ struct_f built once per store generation (256 KiB), and the scalar row-walk gather as the random-access baseline. Cached: 15.5 / 28 / 78 / 141 / 797 µs across rand7 / rand655 / classid / hop2 / all, against recompute 756-1395 µs — 48x to 1.8x — with the 850 µs build breaking even at 1.0-1.4 hops on every frontier. Above ~1 % the remaining cost is the scatter, O(frontier). The gather column is the access-shape baseline, not a candidate; the next arms (row-granular zmm / per-facet xmm decision inside the walk) are named in the module doc and not built here. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
📝 WalkthroughWalkthroughAdds a Rust benchmark for recomputed masks, cached masks, and AoS gather traversal. It validates identical outputs and reports median timings, speedups, cache construction time, and cache break-even points. ChangesGraph Hop Benchmark
Priority: ⬇️ Low Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Other Sequence Diagram(s)sequenceDiagram
participant Benchmark
participant HopStrategies
participant OutputBuffers
Benchmark->>HopStrategies: Execute recompute, cached, and gather hops
HopStrategies->>OutputBuffers: Emit destination bitsets
Benchmark->>OutputBuffers: Compare outputs and measure timings
Suggested reviewers: Merge Risk: 🔵 Low · up to The benchmark is usable, but its published break-even documentation should be corrected before relying on the reported results. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 1 files. (1 skipped: 1 unsupported.)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
A rabbit hops through masks so bright Comment |
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| let n_dst = kernels::simd_popcount(&out_r); | ||
| assert!(n_dst > 0, "{name}: vacuous hop"); | ||
| let t_r = time_us(|| { | ||
| hop_recompute( |
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In the configured release/LTO build, each timed call returns () and none of the mutated output buffers are read after these measurements; applying black_box only to the inputs does not make the output stores observable. LLVM can therefore eliminate out.fill, the scatter writes, and computations used only to produce them, making the reported timings and break-even conclusion unreliable. Consume or black-box each output inside its timed closure.
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Correct — the closures returned () and nothing read the outputs afterwards. Each timed closure now black_boxes its own output buffer (&out_r / &out_c / &out_g). Re-measured twice with the stores observed: every number held (cache build 843–930 µs; cached 14.4–15.0 / 31.8–33.0 / 96–100 / 159–196 / 903–1 056 µs across the five frontiers; break-even 0.9–1.3 hops), so the stores were not being eliminated before, but the measurement now proves it rather than assuming it. Table in the module doc re-banked as the observed ranges.
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Actionable comments posted: 1
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Inline comments:
In `@native/lgj-abi/examples/hop_cached_vs_gather.rs`:
- Line 35: Update both scatter cost-model statements in the example
documentation to describe scatter as O(N / 64 + selected_bits) per facet,
reflecting its scan of every word in selected before visiting set bits; replace
the O(frontier) wording at both referenced locations without changing the
implementation.
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Codex P1 on #79: the timed closures returned () and the output buffers were never read afterwards, so LLVM could drop the scatter stores and the computations feeding them. Each closure now black_boxes its own output. Re-measured twice: every number held (cache build 843-930 µs; cached 14.4-15.0 / 31.8-33.0 / 96-100 / 159-196 / 903-1056 µs; break-even 0.9-1.3 hops). Table re-banked as observed ranges. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
…t, not O(frontier) CodeRabbit on #79: scatter walks every word of selected before visiting set bits — 1 024 words per facet at 65 536 rows regardless of frontier size. Doc and board entry corrected; implementation unchanged. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
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🤖 Prompt for all review comments with AI agents
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Inline comments:
In `@native/lgj-abi/examples/hop_cached_vs_gather.rs`:
- Line 35: Update the break-even range in the documentation comment near the
cached-versus-gather example from 1.0–1.4 to 0.9–1.3 so it matches the displayed
rows and hop2 values; leave the table and surrounding prose unchanged.
After applying the fix, consider running `coderabbit review --agent` for local
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Fix all unresolved CodeRabbit comments on this PR:
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Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
…e table CodeRabbit on #79 (posted as it merged): the re-measured rows span 0.9–1.3 hops; the sentence above the table still carried the first run's 1.0–1.4. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
The entry was reconstructed after the merge, not written at open; the inventory's own header promises entries at merge time from #4 onward, so presenting it as contemporaneous would conceal the lapse. Storno-style note added above the entry; the entry itself is unchanged. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DCfrD5y19cvFc4AoyydXYv
…) + break-even prose fix board: PR #79 arc entry (post-merge)
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What
native/lgj-abi/examples/hop_cached_vs_gather.rs—lgj_hop's selection three ways at 65 536 rows / 32 facets, every arm bit-identical before timing:eq_u32passes +ternlog<AND3>+ scatter per hop)sel_f = class_f ∧ struct_fbuilt once per store generation (the M1b tile, 256 KiB), per hop onemask_andper facet + scatterMeasured (Xeon 2.10 GHz, v4, median of 7)
Cache build: 850 µs.
The tile pays for itself on the second hop at every frontier; above ~1 % the remaining cost is the scatter, O(frontier).
Next (named, not built)
Inside the random-access scatter walk, use the op that matches the access granularity — one zmm
mask_cmpeq_epi32per 4 facets deciding all 32 facets of a visited row in 8 loads, or one xmm compare per facet — instead of the scalar 64-load loop. Armsgather_xmm/gather_zmm_row.No ABI change, no Java change, example only.
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