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hdl.ast: simplify {bit,word}_select with constant offset.
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We don't have any other convenient shortcut for x[off*w:(off+1)*w],
but using word_select to extract a single static range would result
in severe bloat of emitted code through expansion to dead branches.
Recognize and simplify this pattern.
whitequark committed Oct 26, 2019
1 parent 61e6267 commit 51269ad
Showing 2 changed files with 21 additions and 0 deletions.
6 changes: 6 additions & 0 deletions nmigen/hdl/ast.py
Original file line number Diff line number Diff line change
@@ -304,6 +304,9 @@ def bit_select(self, offset, width):
Part, out
Selected part of the ``Value``
"""
offset = Value.cast(offset)
if type(offset) is Const and isinstance(width, int):
return self[offset.value:offset.value + width]
return Part(self, offset, width, stride=1, src_loc_at=1)

def word_select(self, offset, width):
@@ -324,6 +327,9 @@ def word_select(self, offset, width):
Part, out
Selected part of the ``Value``
"""
offset = Value.cast(offset)
if type(offset) is Const and isinstance(width, int):
return self[offset.value * width:(offset.value + 1) * width]
return Part(self, offset, width, stride=width, src_loc_at=1)

def matches(self, *patterns):
15 changes: 15 additions & 0 deletions nmigen/test/test_hdl_ast.py
Original file line number Diff line number Diff line change
@@ -533,15 +533,23 @@ def setUp(self):

def test_shape(self):
s1 = self.c.bit_select(self.s, 2)
self.assertIsInstance(s1, Part)
self.assertEqual(s1.shape(), unsigned(2))
self.assertIsInstance(s1.shape(), Shape)
s2 = self.c.bit_select(self.s, 0)
self.assertIsInstance(s2, Part)
self.assertEqual(s2.shape(), unsigned(0))

def test_stride(self):
s1 = self.c.bit_select(self.s, 2)
self.assertIsInstance(s1, Part)
self.assertEqual(s1.stride, 1)

def test_const(self):
s1 = self.c.bit_select(1, 2)
self.assertIsInstance(s1, Slice)
self.assertRepr(s1, """(slice (const 8'd0) 1:3)""")

def test_width_wrong(self):
with self.assertRaises(TypeError):
self.c.bit_select(self.s, -1)
@@ -558,13 +566,20 @@ def setUp(self):

def test_shape(self):
s1 = self.c.word_select(self.s, 2)
self.assertIsInstance(s1, Part)
self.assertEqual(s1.shape(), unsigned(2))
self.assertIsInstance(s1.shape(), Shape)

def test_stride(self):
s1 = self.c.word_select(self.s, 2)
self.assertIsInstance(s1, Part)
self.assertEqual(s1.stride, 2)

def test_const(self):
s1 = self.c.word_select(1, 2)
self.assertIsInstance(s1, Slice)
self.assertRepr(s1, """(slice (const 8'd0) 2:4)""")

def test_width_wrong(self):
with self.assertRaises(TypeError):
self.c.word_select(self.s, 0)

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