#901 iverilog miscompile

[Steel1.0]

Attached test shows a miscompile in the test. To confirm the error I checked the output on other simulators. ModelSim and GPL Cver confirm that iverilog has a miscompile. I tried other versions of iverilog and in all cases (include git trunk) I saw the same error.

test.v is a test for miscompile.
To compile it I simply used command 'iverilog test.v'. I believe that the execution of assignment ''mem_p_addr0 <= in_data; //bug" is not correct. Picture wave.jpg shows that error.
File data.txt contains input data. File expected-result.txt contains correct output for the test.
File test-description.c describes the behavior of the state-machine in test_module. It will be helpful if you would like to understand the control flow of the state-machine in order to fix bug.

[Steel1.0]

Archive with test and additional files

[Martin Whitaker]

This is a timing race in your testbench. In your fifo_out module you have:

always @(posedge clk)
begin
if (read) begin
res = $fscanf(stream, "%x\n", dataout);
...

where dataout is directly connected to in_data. So if the always block that reads the data from the file executes before the always block that samples in_data, the sampled value will be the newly read value, otherwise it will be the previously read value.

The reason you see mem_p_addr0 pick up every second value of in_data is that the Icarus scheduler uses a last-in first-out queue for threads waiting on an event. Most other simulators execute the threads in a fixed order (normally the order they appear in the source file).

You can fix your testbench like this:

reg [31:0] temp;

always @(posedge clk)
begin
if (read) begin
res = $fscanf(stream, "%x\n", temp);
data_out <= temp;
...

I'm marking this report as invalid, but will leave it open for a little while in case you have any further questions.

[Steel1.0]

Thank you! I think I mistaken because I poorly understand the behavior of scanf function. I hope that my report didn't take you a lot of time!

[Martin Whitaker]

That's OK. This was very similar to another recent bug report, so I recognised the symptoms and it didn't take long to find the cause. I'll close this report now.