src/nmutil/grev.py

   1 # SPDX-License-Identifier: LGPL-3-or-later
   2 # Copyright Jacob Lifshay ADD EMAIL ADDRESS ADD YEAR - FOLLOW STANDARD PRACTICE
   3 # Copyright (C) 2021 Luke Kenneth Casson Leighton <lkcl@lkcl.net>
   4
   5 # Funded by NLnet Assure Programme 2021-02-052, https://nlnet.nl/assure part
   6 # of Horizon 2020 EU Programme 957073.
   7
   8
   9 """Generalized bit-reverse. See `GRev` for docs. - no: move the
  10 module docstring here, to describe the Grev concept.
  11 * module docs tell you "about the concept and anything generally useful to know"
  12 * class docs are for "how to actually use the class".
  13 """
  14
  15 from nmigen.hdl.ast import Signal, Mux, Cat
  16 from nmigen.hdl.ast import Assert
  17 from nmigen.hdl.dsl import Module
  18 from nmigen.hdl.ir import Elaboratable
  19 from nmigen.cli import rtlil
  20
  21
  22 def grev(inval, chunk_sizes, log2_width):
  23     """XXX start comments here with no space
  24     Python reference implementation of generalized bit-reverse.
  25     See `GRev` for documentation.
  26     """
  27     # mask inputs into range
  28     inval &= 2 ** 2 ** log2_width - 1
  29     chunk_sizes &= 2 ** log2_width - 1
  30     # core algorithm:
  31     retval = 0
  32     for i in range(2 ** log2_width):
  33         # don't use `if` so this can be used with nmigen values
  34         bit = (inval & (1 << i)) != 0
  35         retval |= bit << (i ^ chunk_sizes)
  36     return retval
  37
  38
  39 class GRev(Elaboratable):
  40     """ <--no space here>Generalized bit-reverse.
  41
  42     https://bugs.libre-soc.org/show_bug.cgi?id=755
  43
  44     XXX this is documentation about Grev (the concept) which should be in
  45     the docstring.  the class string is reserved for describing how to
  46     *use* the class (describe its inputs and outputs)
  47
  48     A generalized bit-reverse - also known as a butterfly network - is where
  49     every output bit is the input bit at index `output_bit_index XOR
  50     chunk_sizes` where `chunk_sizes` is the control input.
  51
  52     This is useful because many bit/byte reverse operations can be created by
  53     setting `chunk_sizes` to different values. Some examples for a 64-bit
  54     `grev` operation:
  55     * `0b111111` -- reverse all bits in the 64-bit word
  56     * `0b111000` -- reverse bytes in the 64-bit word
  57     * `0b011000` -- reverse bytes in each 32-bit word independently
  58     * `0b110000` -- reverse order of 16-bit words
  59
  60     This is implemented by using a series of `log2_width` 2:1 muxes, exactly
  61     as in a butterfly network: https://en.wikipedia.org/wiki/Butterfly_network
  62
  63     The 2:1 muxes are arranged to calculate successive `grev`-ed values where
  64     each intermediate value's corresponding `chunk_sizes` is progressively
  65     changed from all zeros to the input `chunk_sizes` by adding one bit at a
  66     time from the LSB to MSB.  (XXX i don't understand this at all!)
  67
  68     :reverse_order: if True the butterfly steps are performed
  69                     at offsets of 2^N ... 8 4 2.
  70                     if False, the order is 2 4 8 ... 2^N
  71     """
  72
  73     def __init__(self, log2_width, reverse_order=False):
  74         self.reverse_order = reverse_order    # reverses the order of steps
  75         self.log2_width = log2_width
  76         self.width = 1 << log2_width
  77         self.input = Signal(self.width)       # XXX mark this as an input
  78         # XXX is this an input or output?
  79         self.chunk_sizes = Signal(log2_width)
  80         self.output = Signal(self.width)      # XXX mark this as the output
  81
  82     def elaborate(self, platform):
  83         m = Module()
  84         comb = m.d.comb
  85
  86         # accumulate list of internal signals, exposed only for unit testing.
  87         # contains the input, intermediary steps, and the output.
  88         self._steps = [self.input]
  89
  90         # TODO: no. "see class doc comment for algorithm docs." <-- document
  91         #           *in* the code, not "see another location elsewhere"
  92         #           (unless it is a repeated text/concept of course, like
  93         #            with BitwiseLut, and that's because the API is identical)
  94         #           "see elsewhere" entirely defeats the object of the exercise.
  95         #           jumping back and forth (page-up, page-down)
  96         #           between the text and the code splits attention.
  97         #           the purpose of comments is to be able to understand
  98         #           (in plain english) the code *at* the point of seeing it
  99         #           it should contain "the thoughts going through your head"
 100         #
 101         #           demonstrated below (with a rewrite)
 102
 103         step_i = self.input  # start with input as the first step
 104
 105         # create (reversed?) list of steps
 106         steps = list(range(self.log2_width))
 107         if self.reverse_order:
 108             steps.reverse()
 109
 110         for i in steps:
 111             # each chunk is a power-2 jump.
 112             chunk_size = 1 << i
 113             # prepare a list of XOR-swapped bits of this layer/step
 114             butterfly = [step_i[j ^ chunk_size] for j in range(self.width)]
 115             # create muxes here: 1 bit of chunk_sizes decides swap/no-swap
 116             step_o = Signal(self.width, name="step%d" % chunk_size)
 117             comb += step_o.eq(Mux(self.chunk_sizes[i],
 118                                   Cat(*butterfly), step_i))
 119             # output becomes input to next layer
 120             step_i = step_o
 121             self._steps.append(step_o)  # record steps for test purposes (only)
 122
 123         # last layer is also the output
 124         comb += self.output.eq(step_o)
 125
 126         if platform != 'formal':
 127             return m
 128
 129         # formal test comparing directly against the (simpler) version
 130         m.d.comb += Assert(self.output == grev(self.input,
 131                                                self.chunk_sizes,
 132                                                self.log2_width))
 133         for i, step in enumerate(self._steps):
 134             cur_chunk_sizes = self.chunk_sizes & (2 ** i - 1)
 135             step_expected = grev(self.input, cur_chunk_sizes, self.log2_width)
 136             m.d.comb += Assert(step == step_expected)
 137
 138         return m
 139
 140     def ports(self):
 141         return [self.input, self.chunk_sizes, self.output]
 142
 143
 144 # useful to see what is going on: use yosys "read_ilang test_grev.il; show top"
 145 if __name__ == '__main__':
 146     dut = GRev(3)
 147     vl = rtlil.convert(dut, ports=dut.ports())
 148     with open("test_grev.il", "w") as f:
 149         f.write(vl)