lgj-abi: hop gathers instead of sweeping — the predicted crossover does not exist

[AdaWorldAPI]

Closes ISS-LGJ-HOP-SWEEPS-FULL-POPULATION. The issue named a gather as the next rung and said its merit depended on a density crossover — sparse frontiers favouring a gather, dense ones favouring the sweep's sequential vectorised access — and that this had to be measured rather than assumed.

Measured. There is no crossover.

60 configurations, gather wins all 60

examples/hop_gather_vs_sweep.rs runs both shapes over five populations × twelve densities, asserting byte-identical output at every configuration plus an anti-vacuity guard against an empty hop (which would let both shapes "agree" on nothing and time two empty loops).

n_rows	0.01 %	1 %	25 %	100 %
1 024	361×	113×	7.3×	1.73×
4 096	1 295×	143×	5.9×	1.75×
65 536	2 666×	117×	2.5×	1.78×
262 144	2 754×	90×	2.6×	1.73×

Why it holds even at 100 % density — the part the prediction got wrong

A sweep materialises an n-element per-row intermediate that each row reads exactly once, so its cost is never amortised. At full density it does everything the gather does plus allocate, zero, write and re-read n u32s. It is strictly more work at every density, not merely at sparse ones.

The prediction reasoned about access pattern and missed that one shape simply does more.

The better outcome: no gate at all

A real crossover would have required a threshold, a dispatch, and the two-sided evidence a heuristic gate demands. Its absence makes the change unconditional — simpler than what was planned for.

A scalar gather beats a vectorised ndarray::simd sweep

Same lesson the object-model ladder taught: the win is in not doing the work, not in the vector width. No SIMD primitive is called in the hop now and none is missing — simd_rowstore_facet_match remains the kernel behind lgj_row_facet_match, simd_rowstore_classid_mask behind lgj_op_eq_classid. Neither is orphaned (checked, not assumed).

End to end through Component G

native_hop	1 % / 4096	1 % / 65536	25 % / 4096	25 % / 65536
original (32 sweeps)	479.0 µs	24 798.3 µs	521.2 µs	23 633.9 µs
one-pass (#39)	374.7 µs	7 120.3 µs	375.8 µs	8 076.9 µs
gather	1.9 µs	34.4 µs	61.0 µs	3 534.1 µs
vs original	246×	720×	8.5×	6.7×

The ordering the component was built to test has inverted. Native is now fastest at every configuration, 2.0×–13× ahead of the best scalar arm — having begun this arc slowest at every configuration by 2.6×–165×.

The two instruments are independent and agree: the Rust probe measures the sweep at 6 754–8 118 µs at 1 %/65 536 against JMH's 7 120 µs.

Honesty on the JMH re-run: it was noisier than the previous one — classidScan at 1 %/4096 reports 25.6 ± 61.9 µs, an error bar larger than the score — so the scalar absolutes in that table are weak. Native's own errors are tight (1.9 ± 0.3, 34.4 ± 8.1), and a 720× change is far outside any noise this container produces.

Also fixes a doc I staled one PR ago

lgj_hop's "Kernel composition" block still described the pre-#39 design — one full-width classid sweep per facet, a scratch buffer "REUSED across every participating facet". #39 changed the code and left the prose. Rewritten to describe the gather, with both superseded shapes named so the arc stays legible.

The overflow guard is kept as an explicit check rather than dropped or hidden behind an _n binding: the gather no longer needs n, but row indices are still cast to usize, so a store whose row count doesn't fit must still be refused there.

Still open, named rather than hidden

The one shape that could favour a precomputed per-row mask is reuse — memoising it across many hops on the same (store, classid). That's a caching design with its own invalidation questions, and deliberately not this function's.

Gates

lgj-abi 134/134; AllTests 304, GraphHopTest 66 (incl. G3 at its unchanged allocation floor), TradesParity 12, TradesAllocation 3, BricksAuth 62 = 447 Java checks; clippy -D warnings and fmt clean.

No ABI change, no signature change, no minor bump — an internal shape change behind an unchanged contract.

Raw probe output banked at .claude/board/hop-gather-vs-sweep-crossover.txt.

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