VHDL Issue Number:     2027
 Language_Version:      VHDL-93
 Classification:        Language Modeling Enhancement or Deficiency
 Summary:               When loop index is static, drivers are created for 
                        each element of array
 Relevant_LRM_Sections: Can't recall, but the issue is on elaboration and 
                        creation of drivers
 Related_Issues:
 Key_Words_and_Phrases: drivers vhdl composite array
 Authors_Name:          ben cohen
 Authors_Phone_Number:  310  721-4830
 Authors_Fax_Number:
 Authors_Email_Address: vhdlcohen@aol.com
 Authors_Affiliation:   consultant / author
 Authors_Address1:      www.vhdlcohen.com
 Authors_Address2:
 Authors_Address3:

 Current Status:        Not an ISAC issue--send to Modeling & Productivity

 Superseded By:

 ------------------------
 Date Submitted:        25 April 2003
 Date Analyzed:         01 July 2004
 Author of Analysis:    Chuck Swart
 Revision Number: $02$
 Date Last Revised: $02July2004$

 Description of Problem ---------------------- 

Users cannot easily comprehend why it is necessary for the elaborator
to create a driver for each element of an array when the signal
assignments are made to a static range.  This deficiency causes the
uses to write each element individually.  Below is a response to an
email I gave to an engineer.  There is no reason as to why the
elaborator cannot determine if an index is static (known range) or
dynamic.  The issue here is MULTIPLE DRIVERS.  Drivers are created at
elaboration time.  Thus, the following loop

    for i in 3 downto 0 loop data(i) <= data(i+1); 
    end loop; 

creates a driver for EACH ELEMENT OF i because the elaborator does not
care and does not compute the value of "i", even though "i" looks
static.

That's vhdl!  

The rationale is that it is easier for the compiler, since loop
indeces can be dynamic (e.g., for i in 0 to Signal_element).  In your
model you have 2 drivers for data(4) - 1 driver in concurrent signal
assignment "data(4) <= jin;" - 1 driver in process (even though
data(4) is not assigned for reason mentioned above).  In your non-loop
example, the drivers are clear to the compiler.

By the way, that stuff is also in my Coding Style book!  Cheers!

     Ben


     Why does my code-checking tool report a combinational feedback
     loop with this:

       ...
     begin
     data(4) <= jin;
     process (data)
     begin
     --   data(3) <= data(4);
     --   data(2) <= data(3);
     --   data(1) <= data(2);
     --   data(0) <= data(1);
       for i in 3 downto 0 loop
        data(i) <= data(i+1);
       end loop;
     end process;
     jout <= data(0);
     end a;

Note submitted by steve bailey:If this is too involved for ISAC, then
I think Jim's modeling & productivity group should take it on as an
enhancement request.  Please let Jim know if ISAC will handle or if MP
should handle it.


Proposed Resolution
 -------------------

Allow elaborator to create drivers for specified range of an array or
composite when the range in the loop index is static.

VASG-ISAC Analysis & Rationale
------------------------------
This is not an ISAC issue.

VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------
Forward to Modeling and Productivity group

VASG-ISAC Recommendation for Future Revisions
---------------------------------------------

TBD

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