← Lab
neonselect
confirmedC++ · NEON / Apple M4Systems & data structuresJuly 6, 2026
the question
What does the sampler's selection step -- argmax and top-k over the vocabulary -- cost, and how much does NEON help?
what came out
A branchless NEON argmax is ~8x the scalar scan (29->3.7us at 32k vocab), top-k by partial selection is 17-62x cheaper than a full sort, and the ratio grows with vocabulary size. This is the kernel behind why top-k is near-free. Standalone C++ ARM NEON; all three pre-registered predictions held.
method & receipts
- Result: confirmed
- Pre-registered — the prediction was written down and committed before the run.
- Reproducible — one script re-runs the whole thing from scratch.
- Independently reviewed — an adversarial review pass, kept in the repo.
- Machine-readable claims — every reported number, in a checkable file.
- Tested — a correctness/benchmark suite ships alongside the code.
→ read the code and re-run it
github.com/v-code01/neonselect