ISAC: New IR 2129: Bad requirements to check exprs with access type sub exprs

Attached is this new IR. Its also available at the website.

Chuck Swart


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VHDL Issue Number:        2129

Language_Version          Accellera Standard VHDL-2006-D3.0
Classification            Language Definition Problem
Summary                   Bad requirements to check exprs with access type sub exprs
Relevant_LRM_Sections     Section 6.1 Names in which it states:

    If the type of the prefix is an access type, the evaluation of the
    prefix includes the determination of the object designated by the
    corresponding access value. In such a case, it is an error if the
    value of the prefix is a null access value.

Related_Issues            
Key_Words_and_Phrases     
Authors_Name              John Shields
Authors_Phone_Number      503-685-0837
Authors_Fax_Number        
Authors_Email_Address     john_shields@mentor.com
Authors_Affiliation       Mentor Graphics, Inc.
Authors_Address1          8005 SW Boeckman Rd.
Authors_Address2          Wilsonville, OR 97070
Authors_Address3          

Current Status:           Submitted

Superseded By:

------------------------
Date Submitted:           17 January 2008
Date Analyzed:
Author of Analysis:
Revision Number:          0
Date Last Revised:

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

The implication of this appears to be that in the runtime evaluation
of expressions, every subexpression that dereferences an access type
must be checked for null.  In an example like this:
    
    entity top is
    end top;
    
    architecture arch of top is
        type intlistelement;
        type intlistpointer is access intlistelement;
    
        type intlistelement is record
            element : integer;
            nextelem : intlistpointer;
        end record intlistelement;
    
        procedure increment (N: Integer ::   1) is
            variable countptr:intlistpointer;-- := new intlistelement'(0,null);
        begin
            countptr.all.element := countptr.all.element + (n-1);
        end procedure increment;
    
    begin
    
       p: process
       begin
           increment(5);
           wait;
       end process;
    
    end;
    
The rhs of the var assign stmt must check for null.  It is certainly
uncommon in other languages to require this kind of safety and there
is a serious performance penalty.

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

I would like to see this kind of error be erroneous.  It is
incongruous to me that the language requires this, but allows an
access variable to be deallocated and potentially cause erroneous
behavior.

VHDL would better serve users to provide the degree of freedom for
better performance. It is a related consideration to change the memory
allocation model to employ garbage collection, which would go along
way to improve the safety of access types and dynamic data.  These
things will take on increased significance if VHDL make better use of
dynamic data in verification scenarios.

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

VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------
TBD

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