Re: [Iverilog-devel] Is there any way to avoid this shift/reduce conflict?

[Evan L. <sa2...@cy...>]

With no knowledge whatever of SV, I imagine the problem is that a 
modport_ports_list might look like this:

in a, b, c, out d, e, f;   // with a ';'?

and it's not immediately obvious to the parser if 'out' is a 
port_direction, or an IDENTIFIER - is that right?

If so, you've got 2 options:

1 - find out whether 'out' is followed by a comma, which is presumably 
why you mention lookahead. At first sight, I'd be inclined to handle 
this by instead re-factoring a port list as something like this:

port_list
   : first_port
   | port_list ',' IDENTIFIER

first_port : port_direction IDENTIFIER ;

2 - add a terminal for PORT_IDENTIFIER, which is identical to 
IDENTIFIER, but doesn't allow the presumably-reserved 'in', 'out', etc. 
This will just work anyway if you've got your ordering correct in the 
lex file - ie. you match on 'in'/'out'/etc before matching identifiers. 
Having said that, I have a very vague recollection that lexing in Icarus 
is table-based in some way, so you may not be able to do this. If so, 
you could have a regex for PORT_IDENTIFIER which excludes the 
directions. Or something like that... :)

BTW, I agree that you should never allow *any* conflicts in your grammars.

-Evan

On 08/01/2015 19:33, Martin Whitaker wrote:
> I'm trying to add support for SystemVerilog modports to the iverilog parser.
> The relevant snippet from parse.y looks like this:
>
> modport_item
>     : IDENTIFIER
>         { pform_start_modport_item(@1, $1); }
>       '(' modport_ports_list ')'
>         { pform_end_modport_item(@1); }
>     ;
>
> modport_ports_list
>     : modport_ports_declaration
>     | modport_ports_list ',' modport_ports_declaration
>     ;
>
> modport_ports_declaration
>     : attribute_list_opt port_direction modport_simple_port_list
>         { pform_add_modport_ports(@2, $1, $2, $3); }
>     | attribute_list_opt import_export modport_tf_port_list
>         { ... }
>     | attribute_list_opt K_clocking IDENTIFIER
>         { ... }
>     ;
>
> modport_simple_port_list
>     : IDENTIFIER
>         { $$ = make_port_list($1, 0); }
>     | '.' IDENTIFIER '(' expression ')'
>         { $$ = make_port_list($2, $4); }
>     | modport_simple_port_list ',' IDENTIFIER
>         { $$ = make_port_list($1, $3, 0); }
>     | modport_simple_port_list ',' '.' IDENTIFIER '(' expression ')'
>         { $$ = make_port_list($1, $4, $6); }
>     ;
>
> which is giving me a shift/reduce conflict. The problem is the parser can't
> distinguish between a ',' that separates items in a modport_simple_port_list
> and one that separates items in a modport_ports_list.
>
> I can't find a way to avoid the conflict with a single level of lookahead. Can
> anyone else?
>
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