Original Redmine Comment
Author Name: Wilson Snyder (@wsnyder)
Original Date: 2019-04-12T20:35:52Z

Verilator currently requires interface (or cell) references must be statically unrolled. This is currently a fairly fundamental assumption that is unlikely to be improved in the near term (so not leaving this bug open).

To work around it move your for loop to become a generate loop outside the always.

And thanks for your good bug report, that makes it a lot easier to help.

     for (genvar i = 0; i < 2; i++)
     begin
         always_comb
         begin
             buffer_a[i] = in_intfs[i].a;
             buffer_b[i] = in_intfs[i].b;
         end
     end

Original Redmine Comment
Author Name: Junyi Xie
Original Date: 2019-04-12T20:54:53Z

This is reasonable.
Thanks for the quick reply by the way!

Original Redmine Comment
Author Name: Junyi Xie
Original Date: 2019-04-15T14:26:34Z

Hi Wilson,
We come across a Xilinx page that suggests having for loop inside always_comb helps to save runtime.

https://www.xilinx.com/support/answers/55302.html

This might be interesting for your investigation.

We will for now use the style you recommended though.

Thanks!

Hi,

I guess I have a similar problem.

I want to select the next action based on a array of interfaces.

always_comb
begin
    // default values
    req_buffer_in.receiver  = IDLE; // do nothing
    req_buffer_in.client_id = '0;
    
    // check higest prio requests
    for( integer i = 0; i < NB_CLIENTS; i++) begin
        if (cfg_addr_stream[i].valid) begin 
            req_buffer_in.receiver  = CFG;
            req_buffer_in.client_id = i;
        end
    end

NB_CLIENTS is a parameter in this design. Therefore, I would expect that this for-loop can be easily unrolled an the interface with the highest number is selected.

Unfortunately, I get the error message:

%Error: hwac_manager_obi.sv:99:29: Expecting expression to be constant, but variable isn't const: 'i'
                                 : ... In instance hwac_top.i_hwac_manager.i_hwac_manager_obi
   99 |         if (cfg_addr_stream[i].valid) begin 
      |                             ^
%Error: hwac_manager_obi.sv:99:28: Could not expand constant selection inside dotted reference: 'i'
                                 : ... In instance hwac_top.i_hwac_manager.i_hwac_manager_obi
   99 |         if (cfg_addr_stream[i].valid) begin 
      |                            ^

The first message points with ^ to the i, while the second message points to the [ bracket.

I don't see why unrolling does not work here?

Regards,

You didn't indicate what cfg_addr_stream is. Regardless this would probably work

always_comb begin
  // default values
  req_buffer_in.receiver = IDLE; // do nothing
  req_buffer_in.client_id = '0;
end

for( integer i = 0; i < NB_CLIENTS; i++) begin
  always_comb begin
    if (cfg_addr_stream[i].valid) begin 
        req_buffer_in.receiver  = CFG;
        req_buffer_in.client_id = i;
    end
end

Note, in the future please avoiding posting to old closed issues for new questions, otherwise your question will likely get lost.

Dear,

cfg_addr_stream is an interface for a FIFO stream containing the valid signal.

The basic idea of placing the code block in one always_comb is to force the synthesis to build-up a solution, which generates the IDLE state if none of the .valid signals is high. Your code has actually two always blocks, which drive the same signal and the order is not really defined, if I understood it right.

Getting the original code running would be nice, nevertheless there is a work-around by converting the .valid signals of the interface into a logic array and then it should work.