[libre-riscv-dev] [Bug 44] IEEE754 FPU inverse-sqrt

bugzilla-daemon at libre-riscv.org bugzilla-daemon at libre-riscv.org
Mon Jun 24 14:56:18 BST 2019


--- Comment #30 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #29)
> writing this down here so I don't forget:
> rsqrt can be calculated by a binary search for 1 by adding powers of 2 (such
> as 0.5, 0.25, 0.125) to x in the equation 1 = a * x^2 where a is the input
> to rsqrt, x starts at 1.0

 ooo it's a variant on that sqrt algorithm, the one that moves values
 from a to b in the factorisation of a^2 + 2ab + b^2 [a(a + 2ab) + b^2].


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