From: whitequark <cz@m-labs.hk>
Date: Sat, 26 Jan 2019 00:54:02 +0000 (+0000)
Subject: hdl.ast: fix shape calculation for *.
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=4e5bbed8f35411fcd854616a886ab2459677dbd9;p=nmigen.git

hdl.ast: fix shape calculation for *.

This was carried over from Migen, and is wrong there too.
Counterexample: 1'sd-1 * 4'sd-4 = 4'sd-4 (but should be 5'sd4).
---

diff --git a/nmigen/hdl/ast.py b/nmigen/hdl/ast.py
index 6e2155e..79b6ed7 100644
--- a/nmigen/hdl/ast.py
+++ b/nmigen/hdl/ast.py
@@ -333,14 +333,7 @@ class Operator(Value):
                 bits, sign = self._bitwise_binary_shape(*op_shapes)
                 return bits + 1, sign
             if self.op == "*":
-                if not a_sign and not b_sign:
-                    # both operands unsigned
-                    return a_bits + b_bits, False
-                if a_sign and b_sign:
-                    # both operands signed
-                    return a_bits + b_bits - 1, True
-                # one operand signed, the other unsigned (add sign bit)
-                return a_bits + b_bits + 1 - 1, True
+                return a_bits + b_bits, a_sign or b_sign
             if self.op == "%":
                 return a_bits, a_sign
             if self.op in ("<", "<=", "==", "!=", ">", ">=", "b"):
diff --git a/nmigen/test/test_hdl_ast.py b/nmigen/test/test_hdl_ast.py
index d1c5bb5..9f20d17 100644
--- a/nmigen/test/test_hdl_ast.py
+++ b/nmigen/test/test_hdl_ast.py
@@ -137,7 +137,7 @@ class OperatorTestCase(FHDLTestCase):
         self.assertEqual(repr(v1), "(* (const 4'd0) (const 6'd0))")
         self.assertEqual(v1.shape(), (10, False))
         v2 = Const(0, (4, True)) * Const(0, (6, True))
-        self.assertEqual(v2.shape(), (9, True))
+        self.assertEqual(v2.shape(), (10, True))
         v3 = Const(0, (4, True)) * Const(0, (4, False))
         self.assertEqual(v3.shape(), (8, True))
         v5 = 10 * Const(0, 4)