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fastinvsqrt

null resultC++ · NEON / Apple M4Systems & data structuresJuly 10, 2026

the question

Is Quake's fast inverse square root (0x5f3759df) still worth it?

what came out

Obsolete -- but not because it's slow. It ties hardware vrsqrte on speed (asm shows the register round-trip is on the trick's side); vrsqrte is simply a strictly better seed, exactly one Newton step ahead. Superseded on accuracy, not speed.

method & receipts

  • Result: null result — an honest negative, reported as it came out.
  • Pre-registered the prediction was written down and committed before the run.

→ read the code and re-run it

github.com/v-code01/fastinvsqrt