Quartus Verilog bug in handling $signed in an unsigned expression

OPEN: last verified in Quartus 17.0

The following module should output constant 0, but quartus 13.1 generates a module that outputs constant 1 instead.

module issue_002(y);
  wire [1:0] a = 2'b  11;
  wire [2:0] b = 3'b 111;
  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.

In my tests I have synthesized the module with:

quartus_map issue_002 --source=issue_002.v --family="Cyclone III"
quartus_fit issue_002
quartus_eda issue_002 --formal_verification --tool=conformal

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

2014-01-23 Reported via Altera mySupport (SR #11025071)
2014-03-22 Bugfix for Quartus II v14.1 prospected by Altera Support
2014-09-28 Verified that bug is still present in Quartus II v14.0
2015-05-15 Still broken in Quartus II 15.0
2017-07-01 Still broken in Quartus II 17.0

← Back to VlogHammer Project Page