GCC Bugzilla Bug 23057

In section 4.2 of DWARF STD, it says that:

A Fortran common block may be described by a debugging information entry 
with the tag DW_TAG_common_block. 

But it seems that both gfortran and g77 didn't conform to this.  This makes
trouble for debugging common block.  Here is a testcase to verify this: 

        program info_common
        implicit none
          integer :: a,b
          common /group1/ a,b
          a=1
          b=2
          call ShowCommon()
          stop
        end

        subroutine ShowCommon()
        implicit none
          integer :: num1, num2
          common /group1/ num1, num2
          write(*,*) num1, num2
          return
        end

group1 is a named common block.  After using gfortran to build this case, I use
"readelf -wi" to get the debuginfo of the executable:

 <2><9e>: Abbrev Number: 3 (DW_TAG_variable)
     DW_AT_name        : group1
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 4
     DW_AT_MIPS_linkage_name: group1_
     DW_AT_type        : <e1>        =====>  This point to the type of group1
     DW_AT_external    : 1
     DW_AT_location    : 5 byte block: 3 d0 99 4 8      (DW_OP_addr: 80499d0)
 <1><e1>: Abbrev Number: 5 (DW_TAG_structure_type)
     DW_AT_sibling     : <100>
     DW_AT_byte_size   : 8
 <2><e7>: Abbrev Number: 6 (DW_TAG_member)
     DW_AT_name        : a
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 3
     DW_AT_type        : <100>
     DW_AT_data_member_location: 2 byte block: 23 0     (DW_OP_plus_uconst: 0)
 <2><f3>: Abbrev Number: 6 (DW_TAG_member)
     DW_AT_name        : b
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 3
     DW_AT_type        : <100>
     DW_AT_data_member_location: 2 byte block: 23 4     (DW_OP_plus_uconst: 4)

It tells us that group1 is treated as a two-members structure.

g77 handles this in another way, it treats group1 as an eight-character array.
And eight is exactly the size of common block group1.  Here is the debuginfo for
g77-generated executable:

 <1><12b>: Abbrev Number: 6 (DW_TAG_base_type)
     DW_AT_name        : (indirect string, offset: 0x3b): char
     DW_AT_byte_size   : 1
     DW_AT_encoding    : 8      (unsigned char)
 <1><132>: Abbrev Number: 15 (DW_TAG_array_type)
     DW_AT_sibling     : <143>
     DW_AT_type        : <12b>
 <2><13b>: Abbrev Number: 16 (DW_TAG_subrange_type)
     DW_AT_type        : <70>
     DW_AT_lower_bound : 0
     DW_AT_upper_bound : 7
 <1><143>: Abbrev Number: 17 (DW_TAG_variable)
     DW_AT_name        : (indirect string, offset: 0xbb): group1_
     DW_AT_decl_file   : 1
     DW_AT_decl_line   : 3
     DW_AT_type        : <132>   =====>  This point to the type of group1
     DW_AT_external    : 1
     DW_AT_location    : 5 byte block: 3 a0 99 4 8      (DW_OP_addr: 80499a0)


P.S: I reported this problem some days before.  But there is no response ever
since.  Here is the link:  

  http://gcc.gnu.org/ml/fortran/2005-07/msg00310.html

Thanks.

Confiremd, since the standard says "may", this is an enhancement.

Andrew,
     Do you think you might be interested in tackling the implementation of the
DW_TAG_common_block tag if we could get HJ Lu and/or Jakub Jelinek to provide
technical assistance? Having the ability to debug COMMON blocks in gfortran would
be quite helpful for programs like xplor-nih that heavily use them.
                   Jack

(In reply to comment #2)
Jack,
Actually on ppc-darwin, GCC still uses stabs so I doubt this is the bug you are hitting.

Some more COMMON problems here:
http://gcc.gnu.org/ml/gcc-patches/2007-10/msg01685.html

I submitted a patch that causes debug info for identifiers in COMMON to be
emitted with DW_TAG_common_block.  See

http://gcc.gnu.org/ml/gcc-patches/2007-11/msg00311.html

Fixed in rev 133800, 1 April 2008

Subject: Bug 23057

Author: jakub
Date: Fri Aug 29 18:41:19 2008
New Revision: 139773

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=139773
Log:
        PR fortran/29635
        PR fortran/23057
        * debug.h (struct gcc_debug_hooks): Add NAME and CHILD
        arguments to imported_module_or_decl.
        (debug_nothing_tree_tree): Removed.
        (debug_nothing_tree_tree_tree_bool): New prototype.
        * debug.c (do_nothing_debug_hooks): Adjust.
        (debug_nothing_tree_tree): Removed.
        (debug_nothing_tree_tree_tree_bool): New function.
        * dwarf2out.c (is_symbol_die): Handle DW_TAG_module.
        (gen_variable_die): Put all common vars for the
        same COMMON block under one DW_TAG_common_block.
        (declare_in_namespace): Return new context_die, for Fortran
        return the module DIE instead of adding extra declarations into
        the namespace.
        (gen_type_die_with_usage): Adjust declare_in_namespace caller.
        (gen_namespace_die): If is_fortran (), generate DW_TAG_module
        instead of DW_TAG_namespace.  If DECL_EXTERNAL is set, add
        DW_AT_declaration.
        (dwarf2out_global_decl): Don't skip Fortran global vars.
        (gen_decl_die): Likewise.  Adjust declare_in_namespace callers.
        (dwarf2out_imported_module_or_decl): Add NAME and CHILD arguments.
        If NAME is non-NULL, add DW_AT_name.  If CHILD is non-NULL, put
        DW_TAG_imported_declaration as child of previous
        DW_TAG_imported_module.
        * dbxout.c (dbx_debug_hooks, xcoff_debug_hooks): Adjust.
        * sdbout.c (sdb_debug_hooks): Likewise.
        * vmsdbgout.c (vmsdbg_debug_hooks): Likewise.

        * name-lookup.c (do_using_directive, cp_emit_debug_info_for_using):
        Adjust debug_hooks->imported_module_or_decl callers.

        * f95-lang.c (gfc_init_ts): New function.
        (LANG_HOOKS_INIT_TS): Define.
        * gfortran.h (gfc_use_rename): New type, moved from module.c.
        (gfc_get_use_rename): New macro, moved from module.c.
        (gfc_use_list): New type.
        (gfc_get_use_list): New macro.
        (gfc_namespace): Add use_stmts field.
        (gfc_free_use_stmts): New prototype.
        * Make-lang.in (fortran/trans-decl.o): Depend on debug.h.
        * module.c (gfc_use_rename, gfc_get_use_rename): Moved to
        gfortran.h.
        (gfc_use_module): Chain the USE statement info to
        ns->use_stmts.
        (gfc_free_use_stmts): New function.
        * symbol.c (gfc_free_namespace): Call gfc_free_use_stmts.
        * trans.h (struct module_htab_entry): New type.
        (gfc_find_module, gfc_module_add_decl): New functions.
        * trans.c (gfc_generate_module_code): Create NAMESPACE_DECL for
        the module, adjust DECL_CONTEXTs of module procedures and
        call gfc_module_add_decl for them.
        * trans-common.c (build_common_decl): Set DECL_IGNORED_P
        on the common variable.
        (create_common): Set DECL_IGNORED_P for use associated vars.
        * trans-decl.c: Include debug.h.
        (gfc_get_symbol_decl): Set DECL_IGNORED_P on use_assoc vars from
        modules.
        (build_function_decl): Allow current_function_decl's context
        to be a NAMESPACE_DECL.
        (module_htab, cur_module): New variables.
        (module_htab_do_hash, module_htab_eq, module_htab_decls_hash,
        module_htab_decls_eq, gfc_find_module, gfc_module_add_decl): New
        functions.
        (gfc_create_module_variable): Adjust DECL_CONTEXTs of module
        variables and types and call gfc_module_add_decl for them.
        (gfc_generate_module_vars): Temporarily set cur_module.
        (gfc_trans_use_stmts): New function.
        (gfc_generate_function_code): Call it.
        (gfc_generate_block_data): Set DECL_IGNORED_P on decl.
        * trans-types.c (gfc_get_derived_type): Adjust DECL_CONTEXT
        and TYPE_CONTEXT of module derived types.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/name-lookup.c
    trunk/gcc/dbxout.c
    trunk/gcc/debug.c
    trunk/gcc/debug.h
    trunk/gcc/dwarf2out.c
    trunk/gcc/fortran/ChangeLog
    trunk/gcc/fortran/Make-lang.in
    trunk/gcc/fortran/f95-lang.c
    trunk/gcc/fortran/gfortran.h
    trunk/gcc/fortran/module.c
    trunk/gcc/fortran/symbol.c
    trunk/gcc/fortran/trans-common.c
    trunk/gcc/fortran/trans-decl.c
    trunk/gcc/fortran/trans-types.c
    trunk/gcc/fortran/trans.c
    trunk/gcc/fortran/trans.h
    trunk/gcc/sdbout.c
    trunk/gcc/vmsdbgout.c

Subject: Bug 23057

Author: jakub
Date: Fri Aug 29 18:45:25 2008
New Revision: 139775

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=139775
Log:
        PR fortran/23057
        * dwarf2out.c (gen_variable_die): Represent Fortran COMMON vars
        as DW_TAG_variable children of DW_TAG_common_block rather than
        DW_TAG_member children.  Put DW_AT_external to individual
        DW_TAG_variable DIEs, not to DW_TAG_common_block.

        * gfortran.dg/debug/pr35154-dwarf2.f: Adjust for replacement
        of DW_TAG_member with DW_TAG_variable.

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/dwarf2out.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/gfortran.dg/debug/pr35154-dwarf2.f