src/nmutil/clz.py

   1 from nmigen import Module, Signal, Elaboratable, Cat, Repl
   2 import math
   3 """ This module is much more efficient than PriorityEncoder
   4     although it is functionally identical.
   5     see https://bugs.libre-soc.org/show_bug.cgi?id=326
   6
   7     This work is funded through NLnet under Grant 2019-02-012
   8
   9     License: LGPLv3+
  10
  11 """
  12
  13 class CLZ(Elaboratable):
  14     def __init__(self, width):
  15         self.width = width
  16         self.sig_in = Signal(width, reset_less=True)
  17         out_width = math.ceil(math.log2(width+1))
  18         self.lz = Signal(out_width)
  19
  20     def generate_pairs(self, m):
  21         comb = m.d.comb
  22         pairs = []
  23         for i in range(0, self.width, 2):
  24             if i+1 >= self.width:
  25                 pair = Signal(1, name="cnt_1_%d" % (i/2))
  26                 comb += pair.eq(~self.sig_in[i])
  27                 pairs.append((pair, 1))
  28             else:
  29                 pair = Signal(2, name="pair%d" % i)
  30                 comb += pair.eq(self.sig_in[i:i+2])
  31
  32                 pair_cnt = Signal(2, name="cnt_1_%d" % (i/2))
  33                 with m.Switch(pair):
  34                     with m.Case(0):
  35                         comb += pair_cnt.eq(2)
  36                     with m.Case(1):
  37                         comb += pair_cnt.eq(1)
  38                     with m.Default():
  39                         comb += pair_cnt.eq(0)
  40                 pairs.append((pair_cnt, 2))  # append pair, max_value
  41         return pairs
  42
  43     def combine_pairs(self, m, iteration, pairs):
  44         comb = m.d.comb
  45         length = len(pairs)
  46         ret = []
  47         for i in range(0, length, 2):
  48             if i+1 >= length:
  49                 right, mv = pairs[i]
  50                 width = right.width
  51                 new_pair = Signal(width, name="cnt_%d_%d" % (iteration, i))
  52                 comb += new_pair.eq(Cat(right, 0))
  53                 ret.append((new_pair, mv))
  54             else:
  55                 left, lv = pairs[i+1]
  56                 right, rv = pairs[i]
  57                 width = right.width + 1
  58                 new_pair = Signal(width, name="cnt_%d_%d" % (iteration, i))
  59                 if rv == lv:
  60                     with m.If(left[-1] == 1):
  61                         with m.If(right[-1] == 1):
  62                             comb += new_pair.eq(Cat(Repl(0, width-1), 1))
  63                         with m.Else():
  64                             comb += new_pair.eq(Cat(right[0:-1], 0b01))
  65                     with m.Else():
  66                         comb += new_pair.eq(Cat(left, 0))
  67                 else:
  68                     with m.If(left == lv):
  69                         comb += new_pair.eq(right + left)
  70                     with m.Else():
  71                         comb += new_pair.eq(left)
  72
  73                 ret.append((new_pair, lv+rv))
  74         return ret
  75
  76     def elaborate(self, platform):
  77         m = Module()
  78         comb = m.d.comb
  79
  80         pairs = self.generate_pairs(m)
  81         i = 2
  82         while len(pairs) > 1:
  83             pairs = self.combine_pairs(m, i, pairs)
  84             i += 1
  85
  86         comb += self.lz.eq(pairs[0][0])
  87
  88         return m
  89