RE: [sv-bc] Case Statement Enhancement Proposal Idea

Hi, Brad -

At 10:45 AM 7/5/2007, Brad Pierce wrote:
>To get the effect of a parallel_case that is not a full_case, add a null
>default to the unique-case, such as
>
>always_comb begin
>    y = '0;
>    unique case ({en,a})
>      default;
>      3'b100: y[a]='1;
>      3'b101: y[a]='1;
>      3'b110: y[a]='1;
>      3'b111: y[a]='1;
>    endcase
>end
>
>-- Brad

This is a somewhat well-known trick (kill full_case equivalent by 
adding a case default - also kills the full_case component of unique 
and kills priority altogether).

But there are still a couple of issues with this coding style:

(1) It looks pretty silly to add an empty default to a case 
statement. I like the following code better:

always_comb begin
   unique case ({en,a})
     initial y = '0; // pre-default assignment
     3'b100: y[a]='1; // updates
     3'b101: y[a]='1;
     3'b110: y[a]='1;
     3'b111: y[a]='1;
   endcase
end

Note that begin-end are shown but not needed in this example because 
the initialization assignment is now part of the case statement. Also 
no silly null-default to explain to other engineers or in design 
reviews (because there are many engineers that do not know that 
adding default to a unique-case statement is synthesis-equivalent to 
the old parallel_case).

(2) This is a small example. In recent years, as I have benchmarked 
dc efficiency on larger examples, and I have noted that I frequently 
get better synthesis results by making an initial assignment before 
the case statement and throwing a case-default that sets all of the 
variables to X's inside of the case statement. See the results from 
four years ago in the paper (pages 19-22):
http://www.sunburst-design.com/papers/CummingsSNUG2003SJ_SystemVerilogFSM.pdf

I semi expect to see the same results using dc by making the 
case-initial assignment followed by a case-default of all X's (to be tested)

The pre-default (case-initial) concept is exceptionally useful when 
combined with unique. I am not a big fan of priority.

Regards - Cliff

----------------------------------------------------
Cliff Cummings - Sunburst Design, Inc.
14314 SW Allen Blvd., PMB 501, Beaverton, OR 97005
Phone: 503-641-8446 / FAX: 503-641-8486
cliffc@sunburst-design.com / www.sunburst-design.com
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