RE: [sv-ac] 1704, empty match, and local variable assignments

Hi John,

 

I would like to keep the handling of local variables consistent with the
treatment of all sequence_match_items. So to be consistent with the way
Annex E is already written, I think we should state that it is illegal
to attach any sequence_match_item to an empty match.  I'd like to get
feedback from the group and then I will update the formal writeup.

 

 

REPLACE:

 

17.9 Calling subroutines on match of a sequence

Tasks, task methods, void functions, void function methods, and system
tasks can be called at the end of a successful match of a sequence. 

 

WITH:

 

17.9 Calling subroutines on match of a sequence

Tasks, task methods, void functions, void function methods, and system
tasks can be called at the end of a successful non-empty match of a
sequence. It is illegal to attach a subroutine, or any
sequence_match_item, to an empty match.

 

Note that this section talks about Subroutines on sequence match.  I
extended the second line to clarify that this applies to any
sequence_match_item since there were not other corresponding places to
state it. 

 

Lisa

 

-----Original Message-----
From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf Of John
Havlicek
Sent: Tuesday, January 23, 2007 8:38 PM
To: sv-ac@eda-stds.org
Subject: [sv-ac] 1704, empty match, and local variable assignments

 

All:

 

I thought some this afternoon about 1704 and empty match 

assignments.

 

It seems intuitive to me that if a sequence matches empty, then

no local variable assignment within the sequence should be 

executed during that evaluation thread.  

 

It also seems intuitive that if a sequence matches empty, then 

we do not want a local variable assignment attached to the sequence 

to execute during that evaluation thread.  

 

I think these are consistent with what is written in 1704 now.

 

What we have currently in Annex E is

 

   (R, v=e)  \equiv  (R ##0 (1, v=e))

 

Notice that this says more, namely that if we attach a local

variable assignment to a sequence, then the underlying sequence 

is no longer allowed to match empty.  In other words, Annex E 

today implies that

 

(1)   (R, v=e)  \equiv  (R intersect 1[*1:$], v=e)

 

(This looks clear to me, although I haven't written a proof.)

I don't think there is anything wrong with this, but it might

be criticized as too restrictive.  It is certainly not saying the
following: 

 

(2)  (R, v=e)  \equiv  (R intersect 1[*0]) or (R intersect 1[*1:$], v=e)

 

The righthand side of (2) says that if R matches empty, ignore the 

local variable assignment and go on; otherwise, perform the local
variable 

assignment at the end of a non-empty match of R.

 

Applying the flow rules to the RHS of (2) does no seem to be the right

thing to do if R cannot match empty -- we get an artificial

requirement that v flow in in order for v to flow out of (R, v=e).

On the other hand, if R can match empty, then applying the flow rules

to the RHS of (2) seems to give the desirable condition that v 

flow out of (R, v=e) only if it flows in.

 

We may want to think about changing the semantics of (R, v=e) from

(1) to (2), but there may be some non-trivial work to adapt the flow

rules to work with this.  In any case, such a change should be made

with great care.

 

Best regards,

 

John H.

 

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