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Implementing $root #2150

Kuree opened this issue Feb 3, 2020 · 1 comment

Implementing $root #2150

Kuree opened this issue Feb 3, 2020 · 1 comment


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@Kuree Kuree commented Feb 3, 2020

I notice that $root was mentioned in some of the earlier release logs such as 3.864. However the master branch no longer works with $root.

Here is a minimal example of code that runs on commercial simulators:

module mod();
integer value = 1;
initial begin
    $display("value is %d", $root.mod.value);

Here is the error I got when running the verilator with lint only option:

$ verilator --lint-only 
%Error: Unsupported: SystemVerilog 2005 reserved word not implemented: '$root'
    $display("value is %d", $root.mod.value);
%Error: syntax error, unexpected '.', expecting TYPE-IDENTIFIER
    $display("value is %d", $root.mod.value);
%Error: Exiting due to 2 error(s)
        ... See the manual and for more assistance.

For a quick reference, $root is defined in 1800-2017 23.3.1, so this should be a IEEE feature.

Although I never touched Verilator internal before, I'm very interested in helping implementing this feature. Please let me know if there is a place I should start looking at.

@Kuree Kuree added the new label Feb 3, 2020

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@wsnyder wsnyder commented Feb 3, 2020

Great, would love to have the assistance to add this. I think the $root reference in the change log was just a way of saying there was two top conflicts not that $root itself worked.

The first step is to make a test_regress format test (see the internals doc), it would be good to make sure there is a e.g, "logic mod;" inside a function so that $root.mod is required to disambiguate which mod is needed.

To support this, add $root to verilog.l replacing the error you see. Add the token to verilog.l, there's comments where it properly goes. Add a new AstRoot in AstNodes.h, and in verilog.y make a AstRoot. Search for where AstDot is used; ultimately the code uses AstDot (the "." in a dotted id) to find the symbols in an AstSym; in the case of $root rather than starting the symbol search in the normal place, it needs to use the root symbol tree, then search down for each dotted component from there as normal.

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