Verilator bug in handling $signed in an unsigned expression

CLOSED: fixed in Verilator GIT 14fcfd8

The following module should output 0 for a = 2'b11 and b = 3'b111. But Verilator 3.856 outputs 1 instead.

module issue_002(a, b, y);
  input [1:0] a;
  input [2:0] b;
  output [0:0] y;
  assign y = $signed(a) == b;

Analysis: The argument of $signed is self determined. So even though the comparison is a 3 bit operator, $signed(a) returns the two bit value 2'bs11. This is then extended to 3 bits, but because b is unsigned this is not a sign extension but a zero padding. Thus the expression is 3'b011 == 3'b111, which is false.

Crosscheck: Vivado 2013.4, XST 14.7, Isim 14.7 and Modelsim 10.1d implement this correctly.

2014-04-03 Reported as Issue #729
2014-04-06 Fixed in GIT commit 14fcfd8

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