[libre-riscv-dev] [isa-dev] Re: FP transcendentals (trigonometry, root/exp/log) proposal
MitchAlsup
MitchAlsup at aol.com
Thu Aug 8 16:58:16 BST 2019
We are talking about all of this without a point of reference.
Here is what I do know about correctly rounded transcendentals::
My technology for performing transcendentals in an FMAC unit performs a
power series polynomial calculation.
I can achieve 14 cycle LN2, EXP2 and 19 cycle SIN, COS faithfully rounded
with coefficient tables which are (essentially) the same size as the
FDIV/FSQRT seed tables for Newton-Raphson (or Goldschmidt) iterations. FDIV
will end up at 17 cycles and FSQRT at 23 cycles. This is exactly what
Opteron FDIV/FSQRT performance was (oh so onog ago).
If you impose the correctly rounded requirement::
a) the size of the coefficient tables grows by 3.5× and
b) the number of cycles to compute grows by 1.8×
c) the power to compute grows by 2.5×
For a gain of accuracy of about 0.005 ULP
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