VHDL Issue Number:     2022

 Language_Version:      VHDL-2002
 Classification:        Language Modeling Enhancement or Deficiency
 Summary:               Elements of constant composite to be locally static
 Relevant_LRM_Sections: Section on what is locally static
 Related_Issues:
 Key_Words_and_Phrases: locally static array composite
 Authors_Name:          ben cohen
 Authors_Phone_Number:  310  7214830
 Authors_Fax_Number:
 Authors_Email_Address: vhdlcohen@aol.com
 Authors_Affiliation:   Training / author
 Authors_Address1:
 Authors_Address2:
 Authors_Address3:
 
 Current Status:        Forwarded

 Superseded By:

 ------------------------
 Date Submitted:        28 February 2003
 Date Analyzed:         14 December 2004
 Author of Analysis:    Chuck Swart
 Revision Number:       2
 Date Last Revised:     05 January 2005

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

Currently if a constant is a composite, the elements of that constant
are not locally static.  Thus, one cannot write:

        case .. when constant_array(0) => ...

     That does not make sense, since the composite is a constant.
ModelSim used to treat the elements as constant, until I pointed this
LRM non-compliance as a result of someone pointing this out to me from
an example in my book.

     > > > architecture CaseConstant_a of CaseConstant is
     > > >   type AddrState_Typ is array(0 to 3) of Std_Logic_Vector(5 downto);
     > > >   constant Gray :  AddrState_Typ :=  ("000000",
     > > >                                       "000001",
     > > >                                       "000011",
     > > >                                       "000010");
     > > >
     > > >   constant Gray0 : Std_Logic_Vector(5 downto 0) := "000000";
     > > >   signal Wa_s  : Std_Logic_Vector(5 downto 0) := "000000";
     > > >
     > > > begin
     > > >   Test_Lbl: process
     > > >   begin
     > > >     case wa_s is
     > > >       when Gray(0)     => Wa_s <= Gray(1);
     > > >       when Gray0       => Wa_s <= Gray(1);
     >
     > Indexing an array is not locally static (even if everything is locally
     > computable). So Gray(0) cannot be a choice in the case statement.
     > However, a constant is locally static (even if it's elaborated value is
     > not) so Gray0 is a valid choice. I'd say quickhdl is faulty here.



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

 Make static the elements of a composite declared as a constant.

 VASG-ISAC Analysis & Rationale
 ------------------------------
 This issue is resolved in VHDL 200X Fast Track proposal 22, titled
 "Allow aggregates, array operands, and array operators to be locally static."
 This proposal states, in part:

 "A locally static primary is ...  

 A subelement or a slice of a locally static primary, provided that
 any index expressions are locally static expressions and any discrete
 ranges used in slice names are locally static discrete ranges.

 VASG-ISAC Recommendation for IEEE Std 1076-2002
 -----------------------------------------------
 No change.

 VASG-ISAC Recommendation for Future Revisions
 ----------------------------------------
 Implement extensions developed in FT 22.


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