src/nmutil/clz.py

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