VHDL Issue Number:      0110
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                May library names be redefined between the analysis
                        of a primary unit and a corresponding secondary unit?
Related Issues:         0111
Relevant LRM Sections:  6.3, 11.2
Key Words and Phrases:  Library names, use clauses, expanded names
Current Status:         Analyzed
1076-1993 Disposition:  Bugs Fixed, Enhancements Outstanding (No ISAC Issues)
Disposition Rationale:  Non-ISAC issues outstanding
Superseded By:          N/A
-----------------------
Date Submitted:         1988/07/07
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:          1991/03/15
Author of Analysis:     Oz Levia
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/08/03 18:31:14 $


Description of Problem
----------------------
Consider the following design units:

        library L;
        entity E is
          ... ;
        end E;

        architecture A of E is
          ... ;
        end A;

The visibility rules of VHDL state that the library logical name L is visible
in the architecture A as well as in the entity E.  If the design units are
analyzed at different times, it is conceivable that the library name L may
denote different libraries during different analyses.  The LRM is silent on the
meaning, if any, implied by this condition, but the Language Definition Problem seems
to require that a library name referred to during the analysis of any secondary
unit denote the same library as was denoted by that name during the analysis of
the primary unit.  Since the LRM does not identify this condition as an error,
we issue a warning message in our Analyzer if this condition is detected.


Proposed Resolution
-------------------
State explicitly that the library denoted by a given library name may not
change during the analysis of a given design unit, nor may it change between
the analysis of a given primary unit and a secondary unit corresponding to that
primary unit.


VASG-ISAC Analysis & Rationale
------------------------------
To require that the logical library name used during analysis of a primary unit
and the analysis of its secondary unit represent "the same" physical location
is to require that implementations detect identity of physical specifications.
Some on the ISAC did not feel this is a reasonable requirement.

The relationship between the logical specification and a physical realization
is one of the hardest relationships to mandate. In general, it is impossible to
mandate that a logical library name represents "the same" physical location
(i.e., directory, file, etc.) at different times.  This is so because of links,
relative path, and symbolic names which different operating systems use to
depict the physical location.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
Do nothing.  The ISAC recommends that implementations do the best they can in
that area (i.e., issue warning or a note if possible.)  No standard requirement
is extended.  Specifically, it is not considered an error if the above
situation arises and as a consequence, it is not a requirement to issue an
error message.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
None.