migen/fhdl/bitcontainer.py

   1 from migen.fhdl import structure as f
   2
   3
   4 __all__ = ["log2_int", "bits_for", "value_bits_sign"]
   5
   6
   7 def log2_int(n, need_pow2=True):
   8     l = 1
   9     r = 0
  10     while l < n:
  11         l *= 2
  12         r += 1
  13     if need_pow2 and l != n:
  14         raise ValueError("Not a power of 2")
  15     return r
  16
  17
  18 def bits_for(n, require_sign_bit=False):
  19     if n > 0:
  20         r = log2_int(n + 1, False)
  21     else:
  22         require_sign_bit = True
  23         r = log2_int(-n, False)
  24     if require_sign_bit:
  25         r += 1
  26     return r
  27
  28
  29 def value_bits_sign(v):
  30     """Bit length and signedness of a value.
  31
  32     Parameters
  33     ----------
  34     v : Value
  35
  36     Returns
  37     -------
  38     int, bool
  39         Number of bits required to store `v` or available in `v`, followed by
  40         whether `v` has a sign bit (included in the bit count).
  41
  42     Examples
  43     --------
  44     >>> value_bits_sign(f.Signal(8))
  45     8, False
  46     >>> value_bits_sign(C(0xaa))
  47     8, False
  48     """
  49     if isinstance(v, (f.Constant, f.Signal)):
  50         return v.nbits, v.signed
  51     elif isinstance(v, (f.ClockSignal, f.ResetSignal)):
  52         return 1, False
  53     elif isinstance(v, f._Operator):
  54         obs = list(map(value_bits_sign, v.operands))
  55         if v.op == "+" or v.op == "-":
  56             if not obs[0][1] and not obs[1][1]:
  57                 # both operands unsigned
  58                 return max(obs[0][0], obs[1][0]) + 1, False
  59             elif obs[0][1] and obs[1][1]:
  60                 # both operands signed
  61                 return max(obs[0][0], obs[1][0]) + 1, True
  62             elif not obs[0][1] and obs[1][1]:
  63                 # first operand unsigned (add sign bit), second operand signed
  64                 return max(obs[0][0] + 1, obs[1][0]) + 1, True
  65             else:
  66                 # first signed, second operand unsigned (add sign bit)
  67                 return max(obs[0][0], obs[1][0] + 1) + 1, True
  68         elif v.op == "*":
  69             if not obs[0][1] and not obs[1][1]:
  70                 # both operands unsigned
  71                 return obs[0][0] + obs[1][0], False
  72             elif obs[0][1] and obs[1][1]:
  73                 # both operands signed
  74                 return obs[0][0] + obs[1][0] - 1, True
  75             else:
  76                 # one operand signed, the other unsigned (add sign bit)
  77                 return obs[0][0] + obs[1][0] + 1 - 1, True
  78         elif v.op == "<<<":
  79             if obs[1][1]:
  80                 extra = 2**(obs[1][0] - 1) - 1
  81             else:
  82                 extra = 2**obs[1][0] - 1
  83             return obs[0][0] + extra, obs[0][1]
  84         elif v.op == ">>>":
  85             if obs[1][1]:
  86                 extra = 2**(obs[1][0] - 1)
  87             else:
  88                 extra = 0
  89             return obs[0][0] + extra, obs[0][1]
  90         elif v.op == "&" or v.op == "^" or v.op == "|":
  91             if not obs[0][1] and not obs[1][1]:
  92                 # both operands unsigned
  93                 return max(obs[0][0], obs[1][0]), False
  94             elif obs[0][1] and obs[1][1]:
  95                 # both operands signed
  96                 return max(obs[0][0], obs[1][0]), True
  97             elif not obs[0][1] and obs[1][1]:
  98                 # first operand unsigned (add sign bit), second operand signed
  99                 return max(obs[0][0] + 1, obs[1][0]), True
 100             else:
 101                 # first signed, second operand unsigned (add sign bit)
 102                 return max(obs[0][0], obs[1][0] + 1), True
 103         elif v.op == "<" or v.op == "<=" or v.op == "==" or v.op == "!=" \
 104           or v.op == ">" or v.op == ">=":
 105               return 1, False
 106         elif v.op == "~":
 107             return obs[0]
 108         else:
 109             raise TypeError
 110     elif isinstance(v, f._Slice):
 111         return v.stop - v.start, value_bits_sign(v.value)[1]
 112     elif isinstance(v, f.Cat):
 113         return sum(value_bits_sign(sv)[0] for sv in v.l), False
 114     elif isinstance(v, f.Replicate):
 115         return (value_bits_sign(v.v)[0])*v.n, False
 116     elif isinstance(v, f._ArrayProxy):
 117         bsc = list(map(value_bits_sign, v.choices))
 118         return max(bs[0] for bs in bsc), any(bs[1] for bs in bsc)
 119     else:
 120         raise TypeError("Can not calculate bit length of {} {}".format(
 121             type(v), v))