`ifdef inside a macro

Fri Mar 18, 2011 1:36 pm   

Is the macro expansion of the following code defined in SystemVerilog and
if so, what is the expected macro expansion?


`define TEST(x,y) \
`ifdef TEST_STUFF \
x <= y + 42; \
`else \
x <= y * 53; \
`define TEST_STUFF \
`endif

module grmbl(input wire clk,
input wire [31:0] test,test2,
output reg [31:0] o1, o2);
always @(posedge clk) begin
`TEST(o1,test);
`TEST(o2,test2);
end
endmodule


ModelSim expands this as follows, which looks fairly reasonable to me:

module grmbl(input wire clk,
input wire [31:0] test,test2,
output reg [31:0] o1, o2);
always @(posedge clk) begin
o1 <= test * 53; ;
o2 <= test2 + 42; ;
end
endmodule


Precision Synthesis seems to expand it in the same manner as well, but it
gives the following warnings:

Warning: [42044]: Compiler directive `ifdef is present inside macro definition.
Warning: [42044]: Compiler directive `else is present inside macro definition.
Warning: [42044]: Compiler directive `endif is present inside macro definition.


So, I'm beginning to wonder whether the behavior of this situation is actually
defined.

/Andreas

Fri Mar 18, 2011 1:51 pm   

hi Andreas,

On Mar 18, 11:36 am, Andreas Ehliar wrote:

The 1800-2009 LRM clearly says that macros can be used inside
`defines, and that they will be substituted AFTER the `define is
expanded - NOT at the point at which it's defined. However, I
couldn't find anything about the behaviour of directives inside
`define.

It might be safer to turn the whole thing inside-out, with two
different definitions of TEST, even though it's likely to be more
verbose:

`ifdef TEST_STUFF
`define TEST(x,y) x <= y + 42;
`else
`define TEST(x,y) x <= y * 53;
`define TEST_STUFF
`endif

Anyone else know more precisely what's intended by the LRM?
--
Jonathan Bromley