Proposed changes for attributes replacing pragmas for DPI import/export declarations

Greetings ITC Techies,

As per my AI, here is a re-write of section 3.2.1 of the
Mentor SCE-MI 2 proposal to replace pragma syntax for
DPI import/export declarations with a string attribute
syntax that can be used both for Verilog 2001 and VHDL.


3.2.1 Attribute syntax adaptation
For SCE-MI 2, we will want to retain the look and feel of the DPI HDL layer
but with a special adaptation for Verilog and VHDL since part of the SCE-MI
requirements is for the API to be multilingual and require no language syntax
extensions. As described in the previous section, the import and export
declarations in SystemVerilog are part of the language syntax. Because these
syntax extensions do not exist for Verilog or VHDL, another mechanism must be
deployed to declare functions as imported or exported.

For both Verilog and VHDL it is proposed to use a attribute syntax for this
purpose. A attribute can be used to alert infrastructure compilers that they
are expected to interpret the value of the attribute and make any special
adjustments necessary to implement bindings to the imported or exported
functions.

Both Verilog 2001 and VHDL allow attachment of attributes to function
declarations. In the proposed scheme, the attribute values will be
string types the follow a syntax similar to the import/export syntax
of SystemVerilog.

Figure 5 shows Verilog and VHDL attribute syntax compared to SystemVerilog
native syntax used when declaring an imported function. For VHDL 1992,
since the FOREIGN attribute is part of package STD, we can easily make
use of it to attribute DPI functions in the manner shown.

-------------------------------------------------------------------------------
Figure 5: Attribute syntax to declare imported DPI function in Verilog and VHDL

SystemVerilog Import Declaration
--------------------------------
[unchanged]

Verilog Import Declaration
--------------------------
(* sce_mi="import DPI-C" *)
function nb_put;
     input ttype;
     input status;
     input [`DATA_WIDTH-1:0] d;
     begin end // Empty function
endfunction

VHDL Import Declaration
-----------------------
function nb_put(
     ttype: std_logic;
     status: std_logic;
     d: std_logic_vector(DATA_WIDTH-1 downto 0))
     return std_logic is
begin end -- Empty function

attribute foreign of nb_put: function is "sce_mi import DPI-C";


In both the Verilog and VHDL cases, an empty “place-holder” function is
used to represent the imported function. It is shown with the string attribute
that alerts the infrastructure compiler that this function represents a foreign
function that is actually defined on the C side.

Figure 6 shows Verilog and VHDL attribute syntax compared to SystemVerilog 
native
syntax used when declaring an exported function.

-------------------------------------------------------------------------------
Figure 6: Attribute syntax to declare exported DPI function in Verilog and VHDL

SystemVerilog Export Declaration
--------------------------------
[unchanged]

Verilog Export Declaration
--------------------------
(* sce_mi="export DPI-C" *)
function configQuery;
     input enableParity;
     begin
         isParityEnabled = enableParity;
         configQuery = currentState;
     end
endfunction

VHDL Export Declaration
-----------------------
function configQuery(
     enableParity: std_logic )
     return std_logic_vector(7 downto 0) is
begin
     isParityEnabled := enableParity;
     return currentState;
end

attribute foreign of configQuery: function is "sce_mi export DPI-C";

For exported functions, in the SystemVerilog, Verilog, and VHDL cases
the function body is declared in a similar way (actually identically
in Verilog since it is a subset of SystemVerilog). But in SystemVerilog
an additional export declaration designates this function as callable
from the C side. Whereas in Verilog and VHDL again, a string attribute
is used.

Note: For VHDL, forward declarations can be used as well such that
the forward declaration can be followed by an attribute declaration
and that can be separate from the exported function definition itself.

-- johnS

______________________________/\/            \     \
John Stickley                   \             \     \
Mgr., Acceleration Methodologies \             \________________
Mentor Graphics - MED             \_