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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 Superseded on accuracy, not speed.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