| Number | 85
|
| Category | errata
|
| Synopsis | A.6.5: repeat event_control grammar ambiguity
|
| State | lrmdraft
|
| Class | errata-discuss
|
| Arrival-Date | Jul 30 2002
|
| Originator | sharp@cadence.com
|
| Release | 2001b: 9.7 (Syntax 9-8), A.6.5
|
| Environment |
|
| Description |
Blocking and nonblocking assignments allow an intra- assignment timing control, which is represented in the grammar as delay_or_event_control. This can expand to a repeated event control of the form repeat (expression) event_control See syntax 9-1, 9-2 and A.6.5. This has the semantic of evaluating expression and then waiting until event_control has happened that many times before completing the assignment. This is described in section 9.7.7, along with a redundant syntax box 9-12 that includes the same syntaxes as 9-1 and 9-2. Unfortunately, the exact same grammar nonterminal is used in syntax 9-8 for procedural timing controls, which allows the use of a repeat count on a normal event control in a procedural_timing_control_statement. This leads to an ambiguity in the grammar. The source code: repeat (5) @(a) b = c; could be parsed as delay_or_event_control statement_or_null repeat (5) @(a) b = c; or as loop_statement, which expands to repeat (expression) statement repeat (5) @(a) b = c; In other words, is the repeat attached to the event control, or is it around the entire statement which includes the event control? This is not just a benign grammar ambiguity. It makes a semantic difference what the repeat is attached to. If it were attached only to the event control, it would wait for the event to happen the appropriate number of times, then execute the statement once. If it is attached to the statement, then the entire statement will be executed that number of times. In fact, a repeated event control is only possible as an intra-assignment timing control. A repeat elsewhere will be parsed as a loop_statement. |
| Fix |
In A.6.5, after event_expression add the following new nonterminal and definition -- procedural_timing_control ::= delay_control | event_control and change the definition of procedural_timing_control_statement from
procedural_timing_control_statement ::=
delay_or_event_control statement_or_null
to
procedural_timing_control_statement ::=
procedural_timing_control statement_or_null
And in Syntax 9-8 (9.7), DELETE: delay_or_event_control event_expression ADD at the top the new definitions of procedural_timing_control_statement and procedural_timing_control. |
| Audit-Trail |
From: Shalom Bresticker <Shalom.Bresticker@motorola.com> To: sharp@cadence.com Cc: etf-bugs@boyd.com Subject: Re: enhancement/85: repeat event_control grammar ambiguity Date: Thu, 01 Aug 2002 11:15:43 +0300 sharp@cadence.com wrote: > In fact, a repeated event control is only possible as an > intra-assignment timing control. A repeat elsewhere will > be parsed as a loop_statement. It looks like this problem goes back to 1364-1995. The solution I suggest is: procedural_timing_control ::= delay_control | event_control procedural_timing_control_statement ::= procedural_timing_control statement_or_null Note that procedural_timing_control_statement is explicitly mentioned in the text only in 9.7.5, nor does it appear in any of the syntax boxes in the body of the standard. Shalom |
| Unformatted |
|
Hosted by Boyd Technology