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back.rtlil: avoid exponential behavior when legalizing Part().

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Fixes #259.
This commit is contained in:
whitequark 2019-10-26 02:01:53 +00:00
parent ffd10e3042
commit 8b05b28f5a
1 changed files with 8 additions and 1 deletions
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9
nmigen/back/rtlil.py
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@ -622,12 +622,19 @@ class _LHSValueCompiler(_ValueCompiler):
def on_Part(self, value):
offset = self.s.expand(value.offset)
if isinstance(offset, ast.Const):
if offset.value == len(value.value):
dummy_wire = self.s.rtlil.wire(value.width)
return dummy_wire
return self(ast.Slice(value.value,
offset.value * value.stride,
offset.value * value.stride + value.width))
else:
# Only so many possible parts. The amount of branches is exponential; if value.offset
# is large (e.g. 32-bit wide), trying to naively legalize it is likely to exhaust
# system resources.
max_branches = len(value.value) // value.stride + 1
raise LegalizeValue(value.offset,
range((1 << len(value.offset)) // value.stride),
range((1 << len(value.offset)) // value.stride)[:max_branches],
value.src_loc)
def on_Repl(self, value):