Re: [Iverilog-devel] SystemVerilog functions via vhdlpp

[Martin W. <mai...@ma...>]

Hello Larry,

Extracting the rounded_down_power_of_two function from the output of vhdlpp and converting to a 
simple SystemVerilog test case gives:

   module test();

   function int rounded_down_power_of_two(input int value);
     int n = 32'd0;
     int temp = value;
     while (temp > 32'd1) begin
       temp = temp / 32'd2;
       n = n + 32'd1;
     end
     return n;
   endfunction

   localparam value = rounded_down_power_of_two(34);

   initial begin
     $display("%d", value);
   end

   endmodule

This is illegal, because the standard says:

"Variables declared in a static task, function, or procedural block default to a static lifetime and 
a local scope. However, an explicit static keyword shall be required when an initialization value is 
specified as part of a static variable’s declaration to indicate the user’s intent of executing that 
initialization only once at the beginning of simulation."

(at least the 2012 standard does - I haven't checked earlier versions).

It appears Icarus silently accepts static initialisation, and doesn't currently support 
static/automatic overrides on variable declarations.

Declaring the function as automatic should work round this problem, but it appears this is broken too.

Martin

P.S. I think there is another issue with the converted function - vhdlpp has output unsigned literal 
values, whereas I think they should be signed values.

Larry Doolittle wrote:
> Friends -
>
> See attached test case for VHDL functions, which fails because the
> width of bit_counter comes out as 1, not 4.  I'd like to thank
> Maciej for huge strides towards making this work!  This has turned
> into a SystemVerilog question.  With all the recent changes in
> iverilog git master, I thought I should check if the problem is
> still present.  It is.
>
> It's a pretty small tar file, 3122 bytes spread across 7 files.
> If you bypass the use of next_highest_power_of_two() in use_func.vhd,
> you get
> $ make
> iverilog -Wall -Wno-timescale -g2005-sv -civhdl.cfg  -o use_func_tb use_func_tb.v use_func.vhd
> vvp -n use_func_tb  | awk -f testcode.awk
>           3 PASS
> But with the buggy next_highest_power_of_two() call in place,
> out-of-the-tarball, you get
> $ make
> iverilog -Wall -Wno-timescale -g2005-sv -civhdl.cfg  -o use_func_tb use_func_tb.v use_func.vhd
> vvp -n use_func_tb  | awk -f testcode.awk
>          20 FAIL
> make: *** [use_func_check] Error 1
>
> Back on 28 Jan 2016, when I discussed this with Maciej,
> he discovered that if you change the following line:
>   int \temp =  \value ;
> to
>   int \temp ; \temp = \value ;
> in the generated code, function next_highest_power_of_two(), then it
> works correctly.  The question is if Icarus' SystemVerilog processing
> should be fixed, or ivlpp should stop emitting broken SystemVerilog.
>
>   - Larry
>
>
>
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