Symtab cleanup 10/17 remove unnecesary DECL_ARGUMENTS and DECL_RESULT

[Mike Stump]

On Aug 1, 2013, at 8:09 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
> Now when we have abstract origins tracked, this patch makes DECL_ARGUMENTS and
> DECL_RESULT to be removed from FUNCTION_DECLs that are never passed to symbol
> table.  This reduces LTO streaming effort (by about 1/3rd of PARM_DECls)

So, I was tracking down an lto failure in the C++ test suite, g++.dg/ipa/pr46984.C, and it appears to be caused by 4df870fdeec85907db3dcabf1992cf8b63e1d562 aka svn+ssh://gcc.gnu.org/svn/gcc/trunk@201468.  I was trying to find the gcc-patches email for this work, but could not.  :-(  Anyway, the above is the closest work to that, that I can find.

The problem is that DECL_ARGUMENTS of the thunk (aka _ZThn528_N1D3fooEv) is used during thunk code-generation, and thunk code-generation happens during the output of D::foo.

My port is a targetm.asm_out.can_output_mi_thunk false port.

RESULT_DECL is synthesized on the fly like this:

      /* Build the return declaration for the function.  */
      restype = TREE_TYPE (TREE_TYPE (thunk_fndecl));
      if (DECL_RESULT (thunk_fndecl) == NULL_TREE)
        {
          resdecl = build_decl (input_location, RESULT_DECL, 0, restype);
          DECL_ARTIFICIAL (resdecl) = 1;
          DECL_IGNORED_P (resdecl) = 1;
          DECL_RESULT (thunk_fndecl) = resdecl;
        }
      else
        resdecl = DECL_RESULT (thunk_fndecl);

in expand_thunk, but DECL_ARGUMENTS is not.  Either it needs to be, or it has to be saved and restored in some fashion.

input_function is never called for the thunk.  If it had been, then it would have worked I think.  I think this translates into output_function not being called on the thunk.

Anyway, the core dump looks like:

In member function 'non-virtual thunk to D::foo()':
lto1: internal compiler error: Segmentation fault
0x995d3f crash_signal
   ../../gcc/gcc/toplev.c:335
0x6a5828 contains_struct_check
   ../../gcc/gcc/tree.h:3804
0x6a5828 fold_build_pointer_plus_hwi_loc
   ../../gcc/gcc/tree.h:5871
0x6a5828 thunk_adjust
   ../../gcc/gcc/cgraphunit.c:1240
0x6a6ffd expand_thunk(cgraph_node*)
   ../../gcc/gcc/cgraphunit.c:1440
0x6a799d assemble_thunks_and_aliases
   ../../gcc/gcc/cgraphunit.c:1549
0x6a7a32 assemble_thunks_and_aliases
   ../../gcc/gcc/cgraphunit.c:1565
0x6a7bce expand_function
   ../../gcc/gcc/cgraphunit.c:1675
0x6a983c expand_all_functions
   ../../gcc/gcc/cgraphunit.c:1717
0x6a983c compile()
   ../../gcc/gcc/cgraphunit.c:2054
0x62ccd7 lto_main()
   ../../gcc/gcc/lto/lto.c:3843

It appears that 

> 
> Bootstrapped/regtested ppc64-linux, will commit it after further testing on x86_64-linux.
> 
> Honza
> 
> 	* cgraph.h (release_function_body): Declare.
> 	* tree.c (free_lang_data_in_decl): Free, parameters and return values
> 	of unused delcarations.
> Index: cgraph.h
> ===================================================================
> --- cgraph.h	(revision 201408)
> +++ cgraph.h	(working copy)
> @@ -606,6 +606,7 @@ void debug_cgraph_node (struct cgraph_no
> void cgraph_remove_edge (struct cgraph_edge *);
> void cgraph_remove_node (struct cgraph_node *);
> void cgraph_release_function_body (struct cgraph_node *);
> +void release_function_body (tree);
> void cgraph_node_remove_callees (struct cgraph_node *node);
> struct cgraph_edge *cgraph_create_edge (struct cgraph_node *,
> 					struct cgraph_node *,
> Index: tree.c
> ===================================================================
> --- tree.c	(revision 201367)
> +++ tree.c	(working copy)
> @@ -4886,6 +4886,20 @@ free_lang_data_in_decl (tree decl)
> 
>  if (TREE_CODE (decl) == FUNCTION_DECL)
>     {
> +      struct cgraph_node *node;
> +      if (!(node = cgraph_get_node (decl))
> +	  || (!node->symbol.definition && !node->clones))
> +	{
> +	  if (node)
> +	    cgraph_release_function_body (node);
> +	  else
> +	    {
> +	      release_function_body (decl);
> +	      DECL_ARGUMENTS (decl) = NULL;
> +	      DECL_RESULT (decl) = NULL;
> +	      DECL_INITIAL (decl) = error_mark_node;
> +	    }
> +	}
>       if (gimple_has_body_p (decl))
> 	{
> 	  tree t;