[Bug debug/83941] New: Debug info generated with -flto contains useless forwarders

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83941

            Bug ID: 83941
           Summary: Debug info generated with -flto contains useless
                    forwarders
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: lto, missed-optimization
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Currently for

int main()
{
  int i;
  return 0;
}

we emit

 <1><174>: Abbrev Number: 2 (DW_TAG_subprogram)
    <175>   DW_AT_external    : 1
    <175>   DW_AT_name        : (indirect string, offset: 0x152): main
    <179>   DW_AT_decl_file   : 1
    <17a>   DW_AT_decl_line   : 1
    <17b>   DW_AT_decl_column : 5
    <17c>   DW_AT_type        : <0x18f>
    <180>   DW_AT_sibling     : <0x18f>
 <2><184>: Abbrev Number: 3 (DW_TAG_variable)
    <185>   DW_AT_name        : i
    <187>   DW_AT_decl_file   : 1
    <188>   DW_AT_decl_line   : 3
    <189>   DW_AT_decl_column : 7
    <18a>   DW_AT_type        : <0x18f>
 <2><18e>: Abbrev Number: 0
 <1><18f>: Abbrev Number: 4 (DW_TAG_base_type)
    <190>   DW_AT_byte_size   : 4
    <191>   DW_AT_encoding    : 5       (signed)
    <192>   DW_AT_name        : int

in the early unit and

 <1><1c9>: Abbrev Number: 3 (DW_TAG_subprogram)
    <1ca>   DW_AT_abstract_origin: <0x174>
    <1ce>   DW_AT_low_pc      : 0x4004b6
    <1d6>   DW_AT_high_pc     : 0x6
    <1de>   DW_AT_frame_base  : 1 byte block: 9c        (DW_OP_call_frame_cfa)
    <1e0>   DW_AT_GNU_all_call_sites: 1
 <2><1e0>: Abbrev Number: 4 (DW_TAG_variable)
    <1e1>   DW_AT_abstract_origin: <0x184>

in the late one.  Note the "useless" DIE for 'i' which isn't necessary
to make gdb print '<optimized out>' - gdb happily picks up variables
for name lookup without the extra forwarder via the DW_TAG_subprogram
abstract origin children.