Mantis 1602
P1800-2008/D3a, 13.5.3
Clarify subroutine inout argument behavior with defaults,
Allow output argument defaults.
In
Section 13.5.3
CHANGE
The syntax to declare a
default argument in a subroutine is as follows:
subroutine( [ direction ] [ type ] argument = default_value
);
The optional direction
can be input, inout, or ref (output ports cannot
specify defaults).
The default_value is an expression. The expression is evaluated in the scope containing the
subroutine
declaration each time a call using
the default is made. If the default_value is not used, the
expression is
not evaluated. The use of
default values shall only be allowed with the ANSI style declarations.
When the subroutine is
called, arguments with default values can be omitted from the call, and the
compiler
shall insert their
corresponding values. Unspecified (or empty) arguments can be used as
placeholders for
default arguments, allowing the
use of nonconsecutive default arguments. If an unspecified argument is used
for an argument that does
not have a default value, a compiler error shall be issued.
...
read( ); // error; k has no default value
TO
The syntax to declare a
default argument in a subroutine is as follows:
subroutine( [ direction ] [ type ] argument = default_valueexpression
);
The optional direction can
be input, inout, output, or ref (output ports
cannot specify defaults).
The default_value is an
expression. The expression default_expression is evaluated
in the scope containing the subroutine declaration each time a call using the
default is made. If the default_value is not used,
the expression is not evaluated. If the default is not used, the default expression is not evaluated. The use of
defaults shall only be allowed with ANSI style declarations.
When the subroutine is
called, arguments with default values defaults can be omitted from the call,
and the compiler shall insert their corresponding values. Unspecified (or
empty) arguments can be used as placeholders for default arguments, allowing the use of nonconsecutive default arguments.
If an unspecified argument is used for an argument that does not have a default
value, a compiler error shall be issued.
...
read( ); // error; k
has no default value
read( 1, , 7 ); // error; k has no default
value
The following
example shows an output argument with a default expression.
module m;
logic a, w;
task t1 (output o = a)
; // default binds to m.a
...
endtask :t1
task t2
(output o = b) ; // illegal, b cannot be resolved
...
endtask :t2
task t3 (inout
io = w) ; // default binds to m.w
...
endtask :t1
endmodule :m
module n;
logic a;
initial
begin
m.t1(); // same as
m.t1(m.a), not m.t1(n.a);
// at end of task, value of t1.o is
copied to m.a
m.t3(); // same as
m.t3(m.w)
// value of m.w
is copied to t3.io at start of task and
// value of t3.io is copied to m.w at end of task
end
endmodule :n