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

[bugzilla-daemon at libre-riscv.org]

http://bugs.libre-riscv.org/show_bug.cgi?id=44

--- Comment #6 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #4)
> different approach:
> 
> https://rei.iteso.mx/bitstream/handle/11117/5844/Design+and+Implementation+of+Reciprocal+Square+Root+Units+on+Digital+ASIC+Technology+For+Low+Power+Embedded+Applications.pdf
there was a server error when I tried to access this

> https://github.com/tomverbeure/math/blob/master/src/main/scala/math/FpxxRSqrt.scala

as far as I can tell, this is only a lookup table.

> both these, as best i can tell, use a polynomial lookup table which requires
> only 1 iteration of newton-raphson to give full accuracy.

I think it would be wise to avoid newton-raphson since it needs several
multiplications and will occupy the multiplier for quite a few cycles.

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