Re: [sv-bc] Generate with multiple begins

Nested concurrent begin/end blocks correspond syntactically to vhdl
block statements, which can be nested. Why should vhdl have a feature
which is not permitted in systemverilog? Singling out begin/end blocks
to be unnestable when all other generate constructs are nestable is
non-orthogonal. Furthermore, sequential begin/end blocks are nestable,
so why not concurrent ones?

Finally, a user can get around the nesting restriction by writing dummy
if-generate statements:

    if (1) begin: b1
        if (1) begin: b2
           ...
        end
    end

All these clues suggest that the rule disallowing nested concurrent
begin/end blocks is a mistake.

Paul
----- Original Message -----
From: "Gordon Vreugdenhil" <gordonv@model.com>
To: "Wilson Snyder" <wsnyder@wsnyder.org>
Cc: "SV-BC" <sv-bc@eda.org>
Sent: Monday, January 10, 2011 9:38:17 AM
Subject: Re: [sv-bc] Generate with multiple begins

This dates back to the 2001 LRM. Originally "bare" generate blocks
were legal but were made illegal in the 2005 LRM. Since there
was no technical reason to make them illegal and support for "bare"
generate blocks is strictly a superset of the current language, most
tools likely do accept them (either by default or with a switch) in
order to accept legacy (and ongoing) use of such blocks in current designs.

Gord.

On 1/9/2011 6:24 PM, Wilson Snyder wrote:
> Hello all,
>
> I have a user that sent me the following case which seems to
> work on several simulators, but I don't see that nested
> begin/ends are legal in generates according to the 2009
> grammar. Is this an oversight in the grammar, or intended
> to fail? Can someone clarify please? Thanks!
>
> module t;
> generate
> if (1) begin : g1
> begin : g2
> initial $display("Hello");
> end
> begin : g3
> initial $display("World");
> end
> end
> endgenerate
> endmodule
>
> My reading of the grammar is thus:
>
> loop_generate_construct ::=
> FOR '(' ... ')' generate_block
>
> generate_block ::=
> generate_item
> | [ generate_block_identifier ':' ] BEGIN [ ':' generate_block_identifier ]
> { generate_item }
> yEND [ ':' generate_block_identifier ]
>
> generate_item ::=
> module_or_generate_item
> | interface_or_generate_item
> | checker_or_generate_item
>
> None of these three _item's have a path back to
> generate_block without hitting another generate
> if/case/for.
>
> Looking at it from the bottom up, the only relevant rules which use the
> begin/end keywords are seq_block and generate_block.
>
> generate_block called from
> loop_generate_construct,
> if_generate_construct,
> case_generate_item
>
> seq_block called from
> statement_item
>
> We're not in a statement, so begin must be preceeded by
> loop/if/case; there's no direct path from generate_item
> (etc).
>
> Thanks
>

-- 
--------------------------------------------------------------------
Gordon Vreugdenhil                                503-685-0808
Model Technology (Mentor Graphics)                gordonv@model.com
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