simd: chained gather — a chain of key reads folded into one pass

[AdaWorldAPI]

What

Adds mask_gather_chain_u32 and mask_gather_chain_u32_under, plus ChainHop and ChainTest. They are re-exported from ndarray::simd.

Bit i is set when the key index[i] leads through every hop. At each hop, the row it reaches must be in range, live in that hop's plane, and pass that hop's equality tests (optionally negated). That hop's next lane then gives the key into the following hop.

The targets' predicates are evaluated at the row each key reaches, so no mask over any hop's table is built. This is lance-graph's chained-hop option (a), start[fk^k(i)]: chained hops fold into one pass with one terminal, and nothing is materialized between hops.

• The gated form reads index only at rows where the gate bit is set. Gate tail bits past n are ignored.
• out is fully overwritten.
• The loop is scalar for the same reason as mask_gather_u32: each address is a value the previous hop read.
• One hop with no tests is exactly mask_gather_u32_under (pinned by a test).

The shape is validated at entry and panics on:

• an empty chain;
• a missing next on a non-last hop;
• a next on the last hop;
• a lane shorter than rows.

Tests

• Unit tests:
  • a row-loop oracle over n ∈ {0, 1, 63, 64, 65, 130, 1000}, with dirty gate tails and an anti-vacuity check;
  • the one-hop identity;
  • four should_panic shape guards.
• Parity crate: cases 0xD02 and 0xD03 (two hops, out-of-range keys at both depths, a negated test, a dirty gate). Green on v4 and on v3.
• Disable runs, all red then green after git checkout. Each patch anchor was asserted.
  • Dropping the liveness check fails 2 tests.
  • Ignoring negate fails the row-loop test, and parity reports 0xD02.
  • Not following next fails the row-loop test.
  • Removing the gate-tail clamp fails 2 tests.
• cargo clippy --lib --tests -D warnings and cargo fmt --check are clean.

Next

Consumers, in this order:

1. a mask-risc chained Gather op with a predicate at each depth;
2. Quack lowering;
3. the glove/composer multi-hop out() as one program.

🤖 Generated with Claude Code

https://claude.ai/code/session_011BMJnDBAfzcuQh5HeT39Rh

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-12T21:26:50.025752Z	ef36b96	Draft marked ready

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