XSim hangs on power operations with large exponents

[cliffordwolf]

The following module should return 6'b110011, but instead it just hangs xsim 2013.4:

module issue_021(y);
  output [5:0] y;
  assign y = 6'd3 ** 123456789;
  initial #10 $display("%b", y);
endmodule

XSim 2013.4 seems to try to evaluate this expression by performing 123456789 multiplications instead of using a proper power-modulus algorithm.

In my tests I have run this module with:

xvlog issue_021.v
xelab -R work.issue_021

Crosscheck: Modelsim 10.1d can simulate this module in an instant and Vivado 2013.4 can synthesize it correctly. Isim 14.7 and XST 14.7 seem to suffer from a similar problem.

[athandr]

In case the Vivado Tool does synthesize it well and not the ISE, then i guess that is something to do with the way XST handles the code. Which device are you targetting. In case you are targetting the 7 series device, it is better to go for Vivado.

[cliffordwolf]

Hi Anirudh,

thanks for your reply. Usually I annotate the my bug reports with "Note: This is a bug report. I don't need support.". I've forgotten it this time.

Regarding XST: I posted this as an XSIM bug report only because my XST bug reports only got "we don't fix bugs in XST anymore" replies. But as you brought up XST: This clearly is a problem with high level synthesis and will happen for any target device.

In the context of this bug reports I don't really target any specific device, I'm systematically identifying bugs in a couple of tools and so far have identified and reported 19 bugs, including 10 bugs in Xilinx tools (2 in XST, not counting this one, 4 in vivado and 4 in xsim):

http://www.clifford.at/yosys/vloghammer.html

My hope is that this bug reports will help fixing this bugs before anyone has to spend hours (or even days) identifying those bugs from production code that "mysteriously" does not work with one of those tools. Chances are that this happened already, but people did not file bug reports.

[debrajr]

This is interesting. Would escalate the same. Thanks for bring up the faults in our notice.

[debrajr]

Thanks for posting. Sorry this one got missed last time. I would file bug report for this one and provide you the CR number.