[Libre-soc-bugs] [Bug 782] add galois field bitmanip instructions

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=782

--- Comment #29 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
here is gcd:

  59 def xgcd(a, b):
  60     """return (g, x, y) such that a*x + b*y = g = gcd(a, b)"""
  61     x0, x1, y0, y1 = 0, 1, 1, 0
  62     while a != 0:
  63         (q, a), b = divmodGF2(b, a), a
  64         y0, y1 = y1, y0 ^ multGF2(q , y1)
  65         x0, x1 = x1, x0 ^ multGF2(q , x1)
  66     return b, x0, y0

it works, but the complexity is high (3 higher functions
in a loop, and 3 return results) so is perfect for doing
in assembler, with divmodgf and multgf as opcodes.

not recommending putting this one in as instruction, nor
gf_invert which uses gcd

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