Analysis of IR2085 What happens when a parameter of mode out is not assigned in a procedure?

Note that there is an analysis which proposes two possible solutions, 
but it doesn't make
a recommendation (yet).

Don't forget to prepare for the meeting in one week!


Chuck Swart




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VHDL Issue Number:        2085  

Language_Version          VHDL-2002
Classification            Language Modeling Enhancement or Deficiency
Summary                   What happens when a parameter of mode out is not assigned in a procedure?
Relevant_LRM_Sections     2.1.1.1
Related_Issues            
Key_Words_and_Phrases     procedures, out
Authors_Name              Larry Soule
Authors_Phone_Number      650-584-1919
Authors_Fax_Number        
Authors_Email_Address     larrys@synopsys.com
Authors_Affiliation       Synopsys
Authors_Address1          
Authors_Address2          
Authors_Address3          

Current Status:           Analyzed

Superseded By:

------------------------
Date Submitted:           8 February 2006
Date Analyzed:            09 March 2006
Author of Analysis:       Chuck Swart
Revision Number:          1
Date Last Revised:        09 March 2006

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

Here is an example of a procedure with an out-mode
    parameter that is conditionally assigned.
    
    architecture a of e is
        subtype t is bit_vector(1 to 4);
        constant c : t :=     (others => '1');
    
        procedure p(i : in integer; o : out t) is
        begin
            if i > 0 then
                o := c;
            end if;
        end procedure;
    
    Section 2.1.1.1 says that only for in and inout parameters is the
    value copied in (when using pass by copy).  When exiting the
    procedure, inout and out parameters are copied back in.  If the
    out mode formal is not assigned in the procedure, this has the
    effect of writing t'left back into the actual which seem
    counter-intuitive.
    
    At least 2 VHDL implementations known to me handle this
    differently.

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

Specify that only parameters of mode out that have been assigned
inside the procedure get copied out.
    

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

One way to view this problem is to ask the question: Do out mode
parameters get initialized (presumably to the appropriate 'left
value)? Because of the copy-in semantics for objects of mode in or
mode inout, this question is moot for other modes. Also, The rules for
signal parameters make this question meaningless for signals. That
leaves the question for out mode variables.

The LRM is not completely clear on this issue. Section 12.5 states:

"Execution of a subprogram call involves the elaboration of the
parameter interface list of the corresponding subprogram declaration;
this involves the elaboration of each interface declaration to create
the corresponding formal parameters."

Elaboration of interface declarations is not defined explicitly.
However, section 12.3.1.4 Deals with Object declarations:

"Elaboration of an object declaration that declares an object other
than a file object or an object of a protected type proceeds as
follows:

...

b) If the object declaration includes an explicit initialization
expression, the the initial value of the object is obtained by
evaluating the expression....Otherwise, any implicit initial value for
the object is determined.

c) the object is created.

d) Any initial value is assigned to the object."

Its not clear whether or not this clause refers to interface
declarations.

Clause 4.3.1 Object declarations, states:

    object_declaration ::=
                       constant_declaration
                      |signal_declaration
                      |variable_declaration
                      |file_declaration 

These are syntactic constructs which are not interface declarations.
This seems to imply that object declarations are not intended to
include interface declarations.

Also, in the above clause, the meaning of section b) is unclear.  Out
mode parameters must not contain an explicit initialization expression
(such expressions are interpreted as the value for the parameter if
left unassociated.) So its not clear whether or not an implicit
initial value for the object applies.

There are (at least) two possible solutions to the problem.

1. Extend copy-in semantics to include out mode parameters. This has
the advantage that values which are not updated within the subprogram
are unchanged. This solution also works well with pass by reference
implementations.  (There might be a problem with access types?)

2. Change clause 12.3.1.4 to specifically refer to interface
declarations (or add a new clause). This solution seems to meet the
VHDL principle that all variables are initialized, and the
initialization of interface declaration would be virtually the same as
that for local variables in the subprogram. There is a potential
performance problem, in that arrays which are passed by reference
would have to be initialized each time a subprogram was called.

Its not clear whether either of these solutions would affect backward
compatibility. It does seem that different implementations have
treated this problem in different ways, so some sort of backward
incompatibility may be inevitable.

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


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


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