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

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

[Robert Dubner]

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?

Thanks very much,

Bob Dubner.

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

[David Malcolm]

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

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

[Robert Dubner]

David, thank you very much.  That looks very much like what I was hoping 
for.

I'll dig into it tomorrow.

Heartfelt thanks,

Bob Dubner.

-----Original Message-----
From: David Malcolm <dmalcolm@redhat.com>
Sent: Monday, November 28, 2022 18:01
To: Robert Dubner <rdubner@symas.com>; gcc@gcc.gnu.org
Cc: 'Bob Dubner' <rdubner@dubner.com>
Subject: Re: Code generation: How to define file-scope static variables?

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

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

[Robert Dubner]

David, for completion I am updating this message with what I've learned.  I 
do this in the hope that I might assist the next poor souls that find 
themselves reverse-engineering front ends because they are creating their 
own front end.

I claim only that this worked for me.  I can't you tell what assumptions 
I've been making, because I don't know what I don't know.

That said:  I found that for file-static variables, the trick is to create a 
var_decl for the variable with TREE_STATIC(var_decl)=1, and then to call

	rest_of_decl_compilation (var_decl, true, false); // top_level is true; 
at_end is false

Having done that for an integer_type var_decl with the name 
"dubner_at_work", and with an initial value of 123454321, I now see this at 
the very beginning of the generated .s file:

######################
	.file	"call-scope-1.cbl"
	.text
.Ltext0:
	.file 0 "/home/bob/repos/gcc-cobol/gcc/cobol/failures/call-scope-1" 
"call-scope-1.cbl"
	.data
	.align 4
	.type	dubner_at_work, @object
	.size	dubner_at_work, 4
dubner_at_work:
	.long	123454321
	.section	.rodata	[...]
######################

And that's exactly what I wanted.

Thanks for your help.  It was your mention of "rest_of_compilation" that 
ended this marathon investigation, and I really appreciate it.

Bob Dubner

-----Original Message-----
From: Gcc <gcc-bounces+rdubner=symas.com@gcc.gnu.org> On Behalf Of Robert 
Dubner
Sent: Monday, November 28, 2022 21:55
To: David Malcolm <dmalcolm@redhat.com>; gcc@gcc.gnu.org
Cc: Bob Dubner <rdubner@dubner.com>
Subject: RE: Code generation: How to define file-scope static variables?

David, thank you very much.  That looks very much like what I was hoping 
for.

I'll dig into it tomorrow.

Heartfelt thanks,

Bob Dubner.

-----Original Message-----
From: David Malcolm <dmalcolm@redhat.com>
Sent: Monday, November 28, 2022 18:01
To: Robert Dubner <rdubner@symas.com>; gcc@gcc.gnu.org
Cc: 'Bob Dubner' <rdubner@dubner.com>
Subject: Re: Code generation: How to define file-scope static variables?

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