Re: Code generation: How to define file-scope static variables?

[David Malcolm <dmalcolm@redhat.com>]

On Mon, 2022-11-28 at 15:28 -0600, Robert Dubner wrote:
> I am part of a team working on a COBOL front end for GCC.
> 
> By reverse engineering other front ends, I learned, some months ago,
> how
> to create a function_decl GENERIC node that is the root of a GENERIC
> tree
> describing an entire function.   
> 
> By calling the routine cgraph_node::finalize_function() with that
> function_decl, the assembly language for that function is created,
> and all
> is well.
> 
> But now I need to be able to create the equivalent of a file-scope
> static
> variable in C.
> 
> This C program file:
> 
> //////////////////
> static int dubner_at_work = 123454321;
> int main(int argc, char **argv)
>   {
>   }
> //////////////////
> 
> produces, in part, this assembly language:
> 
> ###############
>         .file   "ccc.c"
>         .text
>         .data
>         .align 4
>         .type   dubner_at_work, @object
>         .size   dubner_at_work, 4
> dubner_at_work:
>         .long   123454321
>         .text
>         .globl  main
>         .type   main, @function
>         [...]
> ###############
> 
> In my own GENERIC generation code, I believe that I am creating a
> proper
> translation_unit_decl that contains the block and the vars nodes for
> specifying "dubner_at_work".
> 
> But I have been unable, after several days of looking, to figure out
> the
> equivalent of "cgraph_node::finalize_function" for a
> translation_unit_decl.  The resulting assembly language doesn't have
> a
> definition for "dubner_at_work".
> 
> Can anybody describe how I can tell the downstream processing that I
> need
> the translation_unit_decl to actually define storage?

You might find libgccjit's gcc/jit/jit-playback.cc helpful for this, as
it tends to contain minimal code to build trees (generally
simplified/reverse-engineered from the C frontend).

playback::context::global_new_decl makes the VAR_DECL node, and such
trees are added to the jit playback::context's m_globals.  In
playback::context::replay, we have:

      /* Finalize globals. See how FORTRAN 95 does it in gfc_be_parse_file()
         for a simple reference. */
      FOR_EACH_VEC_ELT (m_globals, i, global)
        rest_of_decl_compilation (global, true, true);

      wrapup_global_declarations (m_globals.address(), m_globals.length());

So you'll probably want to do something similar for your globals.

Caveat: this is all reverse-engineered by me/others from the C frontend
(and I haven't touched this code in a while), so I may be missing
things here.

Dave