[sv-cc] Decision criteria for meaning of vpiConstantSelect5 (Mantis 1726)

I have collected together some suggestions about how
to decide what vpiConstantSelect means and have attached
it below.  Perhaps this will help in our discussion
of Mantis item 1726.

I will also attach this as a Bugnote.

Regards,
Jim Vellenga

--------------------------------------------------------- 
James H. Vellenga                            978-262-6381 
Architect                              (FAX) 978-262-6636 
Cadence Design Systems, Inc.         vellenga@cadence.com 
270 Billerica Rd
Chelmsford, MA 01824-4179
"We all work with partial information." 
---------------------------------------------------------- 

Some considerations for determining the meaning of vpiConstantSelect
====================================================================

(1) At a minimum, everyone seems to agree that all index expressions 
must be elaboration time constant expressions.  If any are not, then 
vpiConstantSelect returns FALSE.

(2) As Charles has suggested, we can think about what the usage model 
is for the property.  One suggestion was to use vpiConstantSelect to 
determine whether or not we need to check for out-of-range or unknown 
index expressions ("bad indexes").  In this case, an application would 
check vpiConstantSelect, and if it's false, would verify that the 
indexes are in range before trying to get the value of the overall 
expression.

Implications:

-- If any index expression identifies a member of a dynamic array, 
associative array, queue, or string, then vpiConstantSelect would 
return FALSE, since the bounds of the parent array can change 
dynamically during execution.

(3) One could use vpiConstantSelect to determine whether or not one has 
to keep checking for vpiValid before, say, obtaining the expression's 
value.  That is, is this a handle that may lose meaning during 
simulation?

Implications:

-- As with bounds checking, if any index expression identifies a member 
of a dynamic array, associative array, queue, or string, then 
vpiConstantSelect would return FALSE, since the element could go in and 
out of existence during simulation.

-- If any element of the expression is a member of a class variable, 
then vpiConstantSelect would return FALSE, since the element could go 
in and out of existence during simulation.

-- If any element of the expression is a member of an automatic 
variable, then vpiConstantSelect would return FALSE.

-- However, vpiConstantSelect would return TRUE for a statically 
declared vpiClassVar or for a statically declared but dynamically 
allocated array (as opposed to its members), unless the class var or 
array were itself a member of a dynamic object.  That is, in this case, 
the class var or dynamically allocated array does not go in and out of 
existence, although its members or elements do.

(4) We could also decide to try to emulate the meaning and usage of the 
constant_select grammatical element in Appendix A of IEEE Std 
1800=2005.  In this case, a constant_select is something that can be 
used

   o  To extract a subelement of a parameter.

   o  To indicate a subelement of a net as part of an lvalue.

These are values or locations that have to be resolved at elaboration 
time.

Implications:

-- If any of the objects in the hierarchy are dynamic (automatic 
variables, nonstatic data members of a class, a class variable, any of 
the dynamically allocated array types) then vpiConstantSelect should 
return FALSE.

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