VHDL Issue Number:      1018
Classification:         Language Definition Problem
Summary:                What does "reference" mean?
Related Issues:         0097
Relevant LRM Sections:  4, 10.4, 11.4
Key Words and Phrases:  Order of analysis, use clause, reference
Current Status:         Submitted
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1991/01/29
Author of Submission:   Paul Menchini
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  (919) 361-1913
Author's Fax Number:   
Author's Net Address:   mench@clsi.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.6 $
Date Last Revised:      $Date: 1995/05/15 21:26:37 $


Description of Problem
----------------------
Consider the following VHDL (extracted from test [CH11.S04]04030102.VHD in the
VA Tech test suite).

        package p is
        end p;

        use work.p.all;
        package pp is
        end pp;

        use work.pp.all;
        package ppp is
        end ppp;


        package p is 
        end p;

        use work.pp.all;   -- Failure_here
        package ppp is
        end ppp;   

The assumption is that the reanalysis of package P (by the second to last
design unit in this example) invalidates the package PP (which has the use
clause "use work.p.all;" in its context clause).  Therefore the use clause "use
work.pp.all;" in the context clause of the final design unit is illegal.

This test is intended to check the last sentence from paragraph 4 of Section
11.4 (page 11-4):

        If a library unit is changed (e.g., by reanalysis of the
        corresponding design unit), then all library units that are
        potentially affected by such a change become obsolete and must
        be reanalyzed before they can be used again.

An additional, relevant statement from the LRM is item 1 of paragraph 1 of
Section 11.4 (page 11-3):

    1.  A primary unit whose name is referenced within a given design
        unit must be analyzed prior to the analysis of the given design
        unit.

Central to this issue is whether use clauses reference library units.  The
assumption embodied in the example VHDL is that use clauses reference the
packages named in the selected names of the use clauses.  However, this
assumption is not clearly supported by the LRM.

The term "reference" is not defined by the LRM; the closest attempt comes in
paragraph 2 of the introduction to Chapter 4 (page 4-1):

        For each form of declaration the language rules define a certain
        region of text called the *scope* of the declaration.  Each form
        of declaration associates an identifier with a declared entity.
        Only within its scope, there are places where it is possible to
        use the identifier to refer to the associated declared entity;
        these places are defined by the visibility rules.  At such
        places the identifier is said to be a *name* of the entity; the
        name is said to *denote* the associated entity.  [Emphasis from
        the LRM.]

Moreover, the LRM appears to deliberately avoid the term "reference" in Section
10.4.  For example, paragraph 2 of this section states:

        Each selected name in a use clause *identifies* one or more
        declarations that will potentially become directly visible.  If
        the suffix of the selected name is a simple name or operator
        symbol, then the selected name *identifies* only the
        declaration(s) of that simple name or operator symbol contained
        within the package or library denoted by the prefix of the
        selected name.  If the suffix is the reserved word all, then the
        selected name *identifies* all declarations that are contained
        within the package or library denoted by the prefix of the
        selected name.  [Emphasis mine.]

Since a use clause (from paragraph 1 of Section 10.4) "achives direct
visibility of declarations that are visible by selection", it is my assumption
that use clauses by themselves reference neither library units nor declarations
within packages.


Proposed Resolution
-------------------
Modify the LRM to explicitly state that use clauses, in and of themselves,
reference neither the library units nor the declarations named by the selected
names in the use clauses.

Additionally, define the term "reference" to include names appearing in
expressions, targets, subtype indications, type marks, resolution function
names, the entity name in an architecture declaration or configuration
declaration, the package simple name in a package body, etc.; i.e., everywhere
*except* from within a use clause.  Perhaps the simplest way would be to use a
new syntactic construct in the definition of use clauses instead of the current
"selected name" constuct.

One of the complications is to create a class of "implicit" references.  For
example, references to "Integer" implicitly reference the library unit
"Standard" in library "Std" in the absence of any use clauses or declarations
hiding Std.Standard.Integer.


VASG-ISAC Analysis & Rationale
------------------------------
TBD


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
TBD


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