VHDL Issue Number:              2032
Language_Version:               VHDL-2002
Classification:                 Language Definition Problem
Summary:                        Function "now" is not pure
Relevant_LRM_Sections:          7.4.2 Globally static primaries
                                14.2 Package STANDARD
Related_Issues:
Key_Words_and_Phrases: 
Authors_Name:                   Chuck Swart
Authors_Phone_Number:           503.685.0846
Authors_Fax_Number:             503.685.0921
Authors_Email_Address:          cswart@model.com
Authors_Affiliation:            Model Technology
Authors_Address1:               8005 SW Boeckman Road
Authors_Address2:               Wilsonville, OR 97070
Authors_Address3:      

Current Status:                 VASG-Approved

Superseded By:

------------------------
Date Submitted:                 25 November 2003
Date Analyzed:                  29 July 2004
Author of Analysis:             Ajayharsh Varikat
Revision Number:                0.3
Date Last Revised:              20 July 2005


Description of Problem
----------------------
    Declaring NOW to be pure is a technical mistake.
   
    1. According to the rules for globally static expressions, expressions
    involving NOW can be globally static since "An expression is said to
    be globally static if and only if every operator in the expression
    denotes a pure function and every primary in the expression is a
    globally static primary...and is one of the following:
    i) A function
    call whose function name denotes a pure function and whose actual
    parameters are each globally static expressions"

    Obviously,expressions involving now can compute different values at
        different simulation times.
    So claiming that NOW is pure breaks the notion of staticness.
   
    2. Some optimizations can be performed as early as analysis time if
    functions are known to be pure. If NOW is considered pure, these
    optimizations become much more difficult, since they can only be done
    with knowledge of the bodies of all functions to insure that no calls
    to NOW occur, either directly or indirectly.
   
    3. Historically, the declaration of NOW to be pure caused problems for
    VITAL. However, VITAL 2000 has correctly resolved these issues, and,
    in fact, their glitch handling depends on NOW being impure (their code
    takes different actions for time 0 than for positive time values).
   

Proposed Resolution
-------------------
        Declare NOW to be impure, as it was in VHDL-93


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

In VHDL-93, a pure function returns the same value every time it
is called with a given set of globally static expressions as it's
input parameters. Calling NOW a pure function changes this.

The definition of a globally static expression is affected by this
change in the behavior of pure functions. Defining NOW as a pure function
permits a globally static expression to have different values at different
points of simulation. This conflicts with the normally accepted meaning
of staticness, and makes the new definitions of pure functions and
staticness useless for many kinds of optimizations that simulators
do.

Functions in the VITAL 2000 package that call NOW have been declared
as impure, so this is no longer an issue for VITAL.

On the other hand, changing NOW back to an impure function can impact
user functions that were written based on the VHDL 2002 standard. However,
the ISAC members feel that the penetration of this VHDL 2002 feature
within the user community is very low, and hence there is little
likelyhood of code compatibility issues coming up if we were to revert
to the earlier definition.

VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------
Deprecated in IEEE Std 1076-2002.

VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Revert to the VHDL-93 definition of NOW.