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hdl.ast: prohibit shifts by signed value.

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These are not desirable in a HDL, and currently elaborate to broken
RTLIL (after YosysHQ/yosys#1551); prohibit them completely, like
we already do for division and modulo.

Fixes #302.
This commit is contained in:
whitequark 2020-02-01 23:04:25 +00:00
parent cce6b8687b
commit 49758a3a0c
4 changed files with 32 additions and 10 deletions
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14
nmigen/hdl/ast.py
View file

@ -172,14 +172,28 @@ class Value(metaclass=ABCMeta):
self.__check_divisor()
return Operator("//", [other, self])
def __check_shamt(self):
width, signed = self.shape()
if signed:
# Neither Python nor HDLs implement shifts by negative values; prohibit any shifts
# by a signed value to make sure the shift amount can always be interpreted as
# an unsigned value.
raise NotImplementedError("Shift by a signed value is not supported")
def __lshift__(self, other):
other = Value.cast(other)
other.__check_shamt()
return Operator("<<", [self, other])
def __rlshift__(self, other):
self.__check_shamt()
return Operator("<<", [other, self])
def __rshift__(self, other):
other = Value.cast(other)
other.__check_shamt()
return Operator(">>", [self, other])
def __rrshift__(self, other):
self.__check_shamt()
return Operator(">>", [other, self])
def __and__(self, other):
return Operator("&", [self, other])
def __rand__(self, other):