Re: [sv-bc] New proposal for Mantis 1360: clarify that separate always_comb's to different variable selects are allowed

Shalom, your suggested change corresponds to my expectations
for always_comb.


Here is an follow-up situation to consider on this.

   module testmod;
      bit [1:0] a;
      function automatic void write_something(input int idx);
         a[idx] = 1;
      endfunction

      always_comb write_something(0);
      always_comb write_something(1);
   endmodule

Such a design will report a conflict in simulation even though I
would expect it to synthesize correctly.  The basic point is the
"longest static prefix" is a locally determined property and
there is no dataflow analysis, inlining, or any similar effect
going on during the analysis.

I would like an example such as the above to explicitly go into
the LRM to make this aspect of the analysis clear.

My basic point here is that while things like "always_comb" bring
synthesis semantics closer to the simulator, simulation is
not going to match synthesis assumptions in all cases.

Gord.




Bresticker, Shalom wrote:
> Please review.
> <<1360_D4.V2.htm>>
> Thanks,
> Shalom
> 
> Shalom Bresticker
> Intel Jerusalem LAD DA
> +972 2 589-6852
> +972 54 721-1033
> 
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
> 
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
> 
> 
> -- 
> This message has been scanned for viruses and
> dangerous content by *MailScanner* <http://www.mailscanner.info/>, and is
> believed to be clean.
> ------------------------------------------------------------------------
> 
> Mantis 1360
> 
> P1800-2008/Draft 4
> 
> Independent elements of a variable can be written by different 
> always_comb processes.
> 
> In Section 9.2.2.2
> 
> CHANGE
> 
> The variables written on the left-hand side of assignments shall not be 
> written to by any other process.
> 
> TO
> 
> The variables written on the left-hand side of assignments shall not be 
> written to by any other process. However, multiple assignments made to 
> independent elements of a variable are allowed as long as their /longest 
> static prefixes/ do not overlap (see _11.5.3_). For example, an unpacked 
> structure or array can have one bit assigned by an always_comb procedure 
> and another bit assigned continuously or by another always_comb 
> procedure, etc. See _6.5_ for more details.
> 
>  
> 

-- 
--------------------------------------------------------------------
Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com


-- 
This message has been scanned for viruses and
dangerous content by MailScanner, and is
believed to be clean.