Bug 39563

As DW_AT_external-DW_TAG_variable in the inner block is incompatible with current GDB
http://sourceware.org/ml/gdb-patches/2009-03/msg00595.html

the current C++ method of DW_TAG_imported_declaration referencing the file-level DW_AT_external-DW_TAG_variable may be more appropriate.

This seems to be just a C FE problem, C++ FE compiling the exact same testcase gives the expected output (DW_TAG_lexical_block with DW_TAG_variable with DW_AT_external).  So IMHO just the C FE should be fixed to handle it like the C++ FE and if there are gdb bugs, they should be fixed in gdb.

Created attachment 17544 [details]
gcc44-pr39563.patch

Untested fix.

The problem with this patch is that DW_AT_decl_file and DW_AT_decl_line of the
DW_AT_external DW_TAG_variable and DW_TAG_subprogram DIEs is wrong.  Say for:
extern void abort (void);

int var = 1;

int
main (void)
{
  int var = 2;
  if (var != 2)
    abort ();

  {
    extern int var;
    extern void abort (void);
    if (var != 1)
      abort ();
    {
      int var = 3;
      if (var != 3)
abort ();
      {
extern int var;
extern void abort (void);
if (var != 1)
  abort ();
      }
    }
  }

  return 0;
}

for all DW_AT_external DW_TAG_variable DIEs DW_AT_decl_line is 3 and for abort DW_AT_decl_line is the last extern void abort (void) line.

Created attachment 17547 [details]
gcc44-pr39563.patch

Updated patch that preserves the correct location info for the nested extern decls.

Created attachment 17548 [details]
gcc44-pr39563.patch

Patch that actually bootstrapped/regtested successfully.
Jan, could you please test it with gdb/archer?

No regressions for GDB.
GDB requires the extra patch otherwise it still does not work with patched GCC:
http://sourceware.org/ml/gdb-patches/2009-03/msg00595.html

FYI the patch generates one extra file-scope declaration:
int f (void) { extern int var; return var; }

    < c>   DW_AT_producer    : (indirect string, offset: 0x0): GNU C 4.5.0 20090328 (experimental)	
 <1><2d>: Abbrev Number: 2 (DW_TAG_subprogram)
    <2f>   DW_AT_name        : f	
[...]
 <2><50>: Abbrev Number: 3 (DW_TAG_variable)
    <51>   DW_AT_name        : var	
    <55>   DW_AT_decl_file   : 1	
    <56>   DW_AT_decl_line   : 1	
    <57>   DW_AT_type        : <0x5e>	
    <5b>   DW_AT_external    : 1	
    <5c>   DW_AT_declaration : 1	
[...]
 <1><65>: Abbrev Number: 3 (DW_TAG_variable)
    <66>   DW_AT_name        : var	
    <6a>   DW_AT_decl_file   : 1	
    <6b>   DW_AT_decl_line   : 1	
    <6c>   DW_AT_type        : <0x5e>	
    <70>   DW_AT_external    : 1	
    <71>   DW_AT_declaration : 1	

Subject: Bug 39563

Author: jakub
Date: Mon Mar 30 14:35:03 2009
New Revision: 145293

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=145293
Log:
	PR debug/39563
	* c-decl.c (struct c_binding): Add locus field.
	(bind): Add locus argument, set locus field from it.
	(pop_scope): For b->nested VAR_DECL or FUNCTION_DECL,
	add a DECL_EXTERNAL copy of b->decl to current BLOCK_VARS.
	(push_file_scope, pushtag, pushdecl, pushdecl_top_level,
	implicitly_declare, undeclared_variable, lookup_label,
	declare_label, c_make_fname_decl, c_builtin_function,
	c_builtin_function_ext_scope, store_parm_decls_newstyle): Adjust
	bind callers.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-decl.c

Fixed on the trunk.

Fixed in FSF GDB HEAD:
http://sourceware.org/ml/gdb-patches/2009-03/threads.html#00595
http://sourceware.org/ml/gdb-patches/2009-04/threads.html#00040
http://sourceware.org/ml/gdb-cvs/2009-04/msg00021.html
	gdb/
	* dwarf2read.c
	(new_symbol <DW_TAG_variable> <!DW_AT_location> <DW_AT_external>):
	Create the symbol in local scope.
	* symtab.h (cu->list_in_scope <LOC_UNRESOLVED>): New comment part.
	
	gdb/testsuite/
	* gdb.dwarf2/dw2-unresolved-main.c, gdb.dwarf2/dw2-unresolved.S,
	gdb.dwarf2/dw2-unresolved.exp: New.