extending improvements to twos complement handling to Const class

Author: timsherwood

pyrtl/helperfuncs.py

@@ -514,7 +514,7 @@ def _convert_int(val, bitwidth=None, signed=False):
 
 def _convert_verilog_str(val, bitwidth=None, signed=False):
     if signed:
-        raise PyrtlError('error, signed verilog-style string constants not supported currently')
+        raise PyrtlError('error, "signed" option with verilog-style string constants not supported')
 
     bases = {'b': 2, 'o': 8, 'd': 10, 'h': 16, 'x': 16}
     passed_bitwidth = bitwidth
@@ -550,6 +550,10 @@ def _convert_verilog_str(val, bitwidth=None, signed=False):
                          ' (if bitwidth=None is used, pyrtl will determine the bitwidth from the'
                          ' verilog-style constant specification)')
 
+    if num >> bitwidth != 0:
+        raise PyrtlError('specified bitwidth %d for verilog constant insufficient to store value %d'
+                         % (bitwidth, num))
+
     return ValueBitwidthTuple(num, bitwidth)
 
 

pyrtl/wire.py

@@ -578,26 +578,37 @@ class Const(WireVector):
 
     _code = 'C'
 
-    def __init__(self, val, bitwidth=None, block=None):
+    def __init__(self, val, bitwidth=None, signed=False, block=None):
         """ Construct a constant implementation at initialization
 
-        :param int or str val: The value for the const wirevector
+        :param int, bool, or str val: The value for the const wirevector
+        :param int: the desired bitwidth of the resulting const
+        :param signed: specify if bits should be used for twos complement
         :return: a wirevector object representing a const wire
 
         Descriptions for all parameters not listed above can be found at
         py:method:: WireVector.__init__()
+
+        For details of how constants are converted fron int, bool, and
+        strings (for verilog constants), see documentation for the
+        helper function infer_val_and_bitwidth.  Please note that a
+        a constant generated with signed=True is still just a raw bitvector
+        and all arthimetic on it is unsigned by default.  The signed=True
+        argument is only used for proper inference of WireVector size and certain
+        bitwidth sanity checks assuming a two's complement representation of
+        the constants.
         """
         self._validate_bitwidth(bitwidth)
         from .helperfuncs import infer_val_and_bitwidth
-        num, bitwidth = infer_val_and_bitwidth(val, bitwidth)
+        num, bitwidth = infer_val_and_bitwidth(val, bitwidth, signed)
 
         if num < 0:
             raise PyrtlInternalError(
                 'Const somehow evaluating to negative integer after checks')
         if (num >> bitwidth) != 0:
-            raise PyrtlError(
-                'error constant "%s" cannot fit in the specified %d bits'
-                % (str(num), bitwidth))
+            raise PyrtlInternalError(
+                'constant %d returned by infer_val_and_bitwidth somehow not fitting in %d bits'
+                % (num, bitwidth))
 
         name = _constIndexer.make_valid_string() + '_' + str(val)
 

tests/test_wire.py

@@ -235,11 +235,23 @@ def test_integers(self):
         self.check_const(1, 1, 1)
         self.check_const(5, 5, 3)
         self.check_const(1, 1, 5, bitwidth=5)
+        self.check_const(0, 0b0, 1)
+        self.check_const(0, 0b0, 1, signed=True)
+        self.check_const(1, 0b01, 2, signed=True)
+        self.check_const(2, 0b010, 3, signed=True)
+        self.check_const(3, 0b011, 3, signed=True)
+        self.check_const(4, 0b0100, 4, signed=True)
+        self.check_const(5, 0b0101, 4, signed=True)
 
     def test_neg_integers(self):
         self.check_const(-1, 0b11111, 5, bitwidth=5)
         self.check_const(-2, 0b110, 3, bitwidth=3)
         self.check_const(-5, 0b1011, 4, bitwidth=4)
+        self.check_const(-1, 0b1, 1, signed=True)
+        self.check_const(-2, 0b10, 2, signed=True)
+        self.check_const(-3, 0b101, 3, signed=True)
+        self.check_const(-4, 0b100, 3, signed=True)
+        self.check_const(-5, 0b1011, 4, signed=True)
 
     def test_too_big(self):
         self.assert_bad_const(5, 2)