gf_reference/decode_reducing_polynomial.py

   1 from nmigen_gf.reference.state import ST
   2
   3
   4 def decode_reducing_polynomial():
   5     """returns the decoded reducing polynomial as an integer.
   6         Note: the returned integer is `XLEN + 1` bits wide.
   7     """
   8     v = ST.GFBREDPOLY & ((1 << ST.XLEN) - 1)  # mask to XLEN bits
   9     if v == 0 or v == 2:  # GF(2)
  10         return 0b10  # degree = 1, poly = x
  11     if (v & 1) == 0:
  12         # all reducing polynomials of degree > 1 must have the LSB set,
  13         # because they must be irreducible polynomials (meaning they
  14         # can't be factored), if the LSB was clear, then they would
  15         # have `x` as a factor. Therefore, we can reuse the LSB clear
  16         # to instead mean the polynomial has degree XLEN.
  17         v |= 1 << ST.XLEN
  18         v |= 1  # LSB must be set
  19     return v