difference between --single_unit and other modes WRT pre-processor.

[Practical-UVM-Step-By-Step]

Mike:
This is a follow up question which is related to, but not the same as the work library question.

When loading the UVM or any other library which has macro definitions with --single-unit, essentially everything works.
you had mentioned that each syntax tree was it's own compilation unit.
I am trying to ascertain if that's true for Macro definitions also and whether they are tied to a compilation unit.

What I am observing is that if I dont use --single_unit, and there are multiple -f files, the uvm_info macro definitions are essentially lost and I need to load the macro definitions again via a include which means I got to edit all my source files.

is there a way to retain all the macro definitions across multiple -f lists without --single-unit or a way to copy the definitions across the compilation units?

Also:
Can you possibly pen a couple of lines on the differnece in behaviour between --single_unit and not using --single_unit? i am sure this will be helpful to a lot of people.

Thanks!

[MikePopoloski]

There is already documentation on --single-unit here and here. Is there something additional I should add?

Yes, macro definitions are part of a compilation unit. You can't parse a compilation unit without knowing the preprocessor definitions to apply to it. There is the --libraries-inherit-macros flag that I added upon request that is basically a hack to allow macros defined in the main compilation unit to be copied over to library units but there's no way to do this 100% reliably in the general case.

[Practical-UVM-Step-By-Step]

Thanks Mike
I think/request you should add the sentence you typed above or some variant of this. IE: Call it out.
many of us wont recognize it because we're thinking about preprocessor and compilation units don't come to mind.

Macro definitions are part of a compilation unit. If your code includes macros, and you do not use --single-unit, please ensure you have the macro definitions available for slang to recognize your code properly.

or any variant of this, if it's ok with you but you get the idea.

BTW: the vendors apparently seem to do something about this under the hood by leaving these pre-processor definitions in a seperate space somehow. The reason I say that is that slang errored out for me, whereas it didnt on a couple of the big vendors. Might be something specific to UVM library, i dont know.

[MikePopoloski]

I assume the tools you're using are just automatically applying the behavior of --single-unit. I don't see how it could work any other way, because imagine a scenario like this:
A.sv:

`ifdef BAR
`define FOO 1
`endif

B.sv:

`define BAR 1

C.sv:

module m;
  `ifdef FOO
    `ifdef BAR
      initial $display("Hello World");
    `endif
  `endif
endmodule

Does this print Hello World if you pass the files in the order A B C ? What about B C A? B A C?

The LRM actually discusses the distinction in 3.12:

The exact mechanism for defining which files constitute a compilation unit is tool-specific. However,
compliant tools shall provide use models that allow both of the following cases:
a) All files on a given compilation command line make a single compilation unit (in which case the
declarations within those files are accessible following normal visibility rules throughout the entire
set of files).
b) Each file is a separate compilation unit (in which case the declarations in each compilation-unit
scope are accessible only within its corresponding file).
...
There are other possible mappings of files to compilation units, and the mechanisms for defining them are
tool-specific and may not be portable.
...
Within a separately compiled unit, compiler directives once seen by a tool apply to all subsequent source
text. However, compiler directives from one separately compiled unit shall not affect other compilation
units. This may result in a difference of behavior between compiling the units separately or as a single
compilation unit containing the entire source.

[Practical-UVM-Step-By-Step]

thanks Mike. your comment about doing this under the hood emulating --single-unit may be on the mark. I didnt think of it that way.
Let me do a few more experiments because the UVM library is universal and there are uvm specific switches to the simulator.

I'm still trying to get to the bottom of that nfs issue I reported earlier midst my other experiements

[MikePopoloski]

I added more docs in 8154635

[udif]

BTW: the vendors apparently seem to do something about this under the hood by leaving these pre-processor definitions in a seperate space somehow. The reason I say that is that slang errored out for me, whereas it didnt on a couple of the big vendors. Might be something specific to UVM library, i dont know.

I think the vendors simply follow the same logic as --single-unit. I've seen package imports in one file (but outside the module scope) affecting the next file.

You might also want to check #927 and #929 which also affects slang behavior compared with other vendors.