ISE has issues with $clog2

[alexforencich]

ISE has issues with code containing $clog2. For 6-series, it is not allowed in localparam, only parameter. For parts older than 6-series, it is not supported at all. Perhaps sv2v can define a clog2 macro and substitute that for uses of $clog2, perhaps this can be configurable via a command line option or similar.

[zachjs]

$clog2 was added to Verilog in IEEE 1364-2005, released nearly 19 years in April 2006. There's certainly no reason to support it in some constant expressions but not others. Would Xilinx/AMD be receptive to a bug report in this case?

Perhaps we could have a flag to replace all $clog2 with the following constant function. Can you try this substitution in your code and confirm whether this actually works in ISE? (I don't know what other restrictions it might impose.)

function automatic integer clog2;
    input integer inp;
    clog2 = 0;
    for (inp = inp - 1; inp > 0; inp = inp >> 1)
        clog2 = clog2 + 1;
endfunction

[alexforencich]

I wish, but ISE is currently no longer supported, so we're stuck with what we've got. I'll look into trying that out and report back. Hopefully that works, a macro is also a potential option, but would be decidedly less clean.

[zachjs]

@alexforencich Did you have any luck with the $clog2 substitute above?