← Lab
neonrmsnorm
confirmedC++ · NEON / Apple M4Systems & data structuresJuly 9, 2026
the question
Where exactly does the speedup in a hand-vectorized RMSNorm come from?
what came out
Entirely the sum-of-squares reduction's 4-wide vector accumulator -- asm confirms the compiler won't reassociate an FP reduction at either precision, so NEON beats a float-accumulator scalar 2.5-3x. ~2x overall; rsqrt is free via 2 Newton steps. Independently verified.
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/neonrmsnorm