src/ieee754/fpdiv/div2.py

   1 """IEEE Floating Point Divider
   2
   3 Relevant bugreport: http://bugs.libre-riscv.org/show_bug.cgi?id=99
   4 """
   5
   6 from nmigen import Module, Signal, Elaboratable, Cat
   7 from nmigen.cli import main, verilog
   8
   9 from ieee754.fpcommon.fpbase import FPState
  10 from ieee754.fpcommon.postcalc import FPAddStage1Data
  11 from ieee754.div_rem_sqrt_rsqrt.div_pipe import DivPipeOutputData
  12
  13
  14 class FPDivStage2Mod(FPState, Elaboratable):
  15     """ Second stage of div: preparation for normalisation.
  16     """
  17
  18     def __init__(self, pspec):
  19         self.pspec = pspec
  20         self.i = self.ispec()
  21         self.o = self.ospec()
  22
  23     def ispec(self):
  24         return DivPipeOutputData(self.pspec) # Q/Rem in...
  25
  26     def ospec(self):
  27         # XXX REQUIRED.  MUST NOT BE CHANGED.  this is the format
  28         # required for ongoing processing (normalisation, correction etc.)
  29         return FPAddStage1Data(self.pspec) # out to post-process
  30
  31     def process(self, i):
  32         return self.o
  33
  34     def setup(self, m, i):
  35         """ links module to inputs and outputs
  36         """
  37         m.submodules.div1 = self
  38         m.d.comb += self.i.eq(i)
  39
  40     def elaborate(self, platform):
  41         m = Module()
  42
  43         # copies sign and exponent and mantissa (mantissa to be overridden
  44         # below)
  45         m.d.comb += self.o.z.eq(self.i.z)
  46
  47         # TODO: this is "phase 3" of divide (the very end of the pipeline)
  48         # takes the Q and R data (whatever) and performs
  49         # last-stage guard/round/sticky and copies mantissa into z.
  50         # post-processing stages take care of things from that point.
  51
  52         # NOTE: this phase does NOT do ACTUAL DIV processing, it ONLY
  53         # does "conversion" *out* of the Q/REM last stage
  54
  55         # NOTE: see FPDivStage0Mod comment.  the quotient is assumed
  56         # to be in the range 0.499999-recurring to 1.999998.  normalisation
  57         # will take care of that, *however*, it *might* be necessary to
  58         # subtract 1 from the exponent and have one extra bit in the
  59         # mantissa to compensate.  this is pretty much exactly what's
  60         # done in FPMUL, due to 0.5-0.9999 * 0.5-0.9999 also producing
  61         # values within the range 0.5 to 1.999998
  62
  63         with m.If(~self.i.out_do_z):
  64             mw = self.o.z.m_width
  65             # TODO: compensate for answer being in range 0.49999 to 1.99998
  66             pl = len(self.i.quotient_root) + len(self.i.remainder)
  67             pt = Signal(pl, reset_less=True)
  68             m.d.comb += pt.eq(Cat(self.i.remainder, self.i.quotient_root))
  69             p = Signal(pl, reset_less=True)
  70             with m.If(self.i.quotient_root[-1]):
  71                 m.d.comb += p.eq(pt)
  72             with m.Else():
  73                 # get 1 bit of extra accuracy if the mantissa top bit is zero
  74                 m.d.comb += p.eq(pt << 1)
  75                 m.d.comb += self.o.z.e.eq(self.i.z.e-1)
  76
  77             # TODO: use p here instead of quotient_root, direct.
  78             # XXX what to do about remainder? shift that as well?
  79             # hmm, how about concatenate remainder and quotient...
  80             m.d.comb += [
  81                 self.o.z.m.eq(p[-mw-2:]),
  82                 self.o.of.m0.eq(p[-mw-2]), # copy of LSB
  83                 self.o.of.guard.eq(p[-mw-1]),
  84                 self.o.of.round_bit.eq(p[-mw]),
  85                 self.o.of.sticky.eq(p[:-mw].bool())
  86             ]
  87
  88         m.d.comb += self.o.out_do_z.eq(self.i.out_do_z)
  89         m.d.comb += self.o.oz.eq(self.i.oz)
  90         m.d.comb += self.o.ctx.eq(self.i.ctx)
  91
  92         return m
  93
  94
  95 class FPDivStage2(FPState):
  96
  97     def __init__(self, pspec):
  98         FPState.__init__(self, "divider_1")
  99         self.mod = FPDivStage2Mod(pspec)
 100         self.out_z = FPNumBaseRecord(pspec, False)
 101         self.out_of = Overflow()
 102         self.norm_stb = Signal()
 103
 104     def setup(self, m, i):
 105         """ links module to inputs and outputs
 106         """
 107         self.mod.setup(m, i)
 108
 109         m.d.sync += self.norm_stb.eq(0) # sets to zero when not in div1 state
 110
 111         m.d.sync += self.out_of.eq(self.mod.out_of)
 112         m.d.sync += self.out_z.eq(self.mod.out_z)
 113         m.d.sync += self.norm_stb.eq(1)
 114
 115     def action(self, m):
 116         m.next = "normalise_1"
 117