Re: [Iverilog-devel] Priority case and default case items.

[Stephen W. <st...@ic...>]

Fair point, although the "default:" also has meaning to the synthesizer and
may render the priority keyword moot in synthesis too. For example:

// Example #1
case (foo)
1: bar = a;
2: bar = b;
default: bar = x;
endcase

and

// Example #2
priority case (foo)
1: bar = a;
2: bar = b;
default: bar = x;
endcase

are indistinguishable to a simulator AND to a synthesizer, but:

// Example #3
case (foo)
1: bar = a;
2: bar = b;
endcase

is very different (it is a latch) from:

// Example #4
priority case (foo)
1: bar = a;
2: bar = b;
endcase

which is a combinational mux. Furthermore, #4 is not necessarily the same
as #1 or #2. If #4 is what is desired, then #2 is probably not a way to
achieve it, although #1 is the best approximation available without the
priority keyword. Thus, I suggest that #2 may deserve a gentle warning at
compile time (but not at run time.)

On Wed, Oct 9, 2019 at 5:54 PM Bryan Murdock <bmu...@gm...> wrote:

> This paper has a lot of explanation about priority (and unique):
>
>
> http://www.sutherland-hdl.com/papers/2005-SNUG-paper_SystemVerilog_unique_and_priority.pdf
>
> My quick summary is that they are directives meant mostly for synthesis
> tools.  If someone provides a default case in their priority case, yes,
> they are making the priority statement redundant in the simulation, but it
> might still be a very important directive for synthesis.  So don't nag them
> about it with a warning 🙂
>
> Bryan
>
>
> On Wed, Oct 9, 2019, 10:39 AM Stephen Williams <st...@ic...> wrote:
>
>> Here's a question. There is this:
>>
>> priority case (foo)
>>  1: stmt1;
>>  2: stmt2;
>>  default: stmt_default;
>> endcase
>>
>> The priority case "shall issue a violation report if no case_item
>> matches."
>>
>> The default: statement makes it so that every possible value of foo
>> has a case item, if the "default" case is considered a case item, and
>> by the BNF, it is indeed a case_item. But the "default:" makes the
>> "priority" moot. So should this print a violation message or not?
>>
>> (I'm thinking, it should not, but a compile time warning may make
>> sense. But I can see logic for the other course too.)
>>
>> --
>> Steve Williams                "The woods are lovely, dark and deep.
>> st...@ic...              But I have promises to keep,
>> http://www.icarus.com         and lines to code before I sleep,
>> http://www.picturel.com       And lines to code before I sleep."
>>
>>
>> _______________________________________________
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>>
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-- 
Steve Williams                "The woods are lovely, dark and deep.
st...@ic... <ste...@gm...>              But I have promises
to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."