[Bug debug/54734] New: Debug info for C++ and LTO misses types

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54734

             Bug #: 54734
           Summary: Debug info for C++ and LTO misses types
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: lto
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: rguenth@gcc.gnu.org


Consider the following reduced testcase (from standard library use):

template <class T> class basic_string;
typedef basic_string<char> sstring;

template <class T>
class basic_string
{
public:
  typedef T type;
};

extern template class basic_string<char>;

int main()
{
  sstring s;
  return sstring::type (0);
}

without LTO we have

 <1><130>: Abbrev Number: 2 (DW_TAG_typedef)
    <131>   DW_AT_name        : (indirect string, offset: 0x98): sstring        
    <135>   DW_AT_decl_file   : 1       
    <136>   DW_AT_decl_line   : 2       
    <137>   DW_AT_type        : <0x13b> 
 <1><13b>: Abbrev Number: 3 (DW_TAG_class_type)
    <13c>   DW_AT_name        : (indirect string, offset: 0xa0):
basic_string<char>     
    <140>   DW_AT_byte_size   : 1       
    <141>   DW_AT_decl_file   : 1       
    <142>   DW_AT_decl_line   : 5       
    <143>   DW_AT_sibling     : <0x15a> 
 <2><147>: Abbrev Number: 2 (DW_TAG_typedef)
    <148>   DW_AT_name        : (indirect string, offset: 0x8e): type   
    <14c>   DW_AT_decl_file   : 1       
    <14d>   DW_AT_decl_line   : 8       
    <14e>   DW_AT_type        : <0x15a> 

while with LTO we get only

 <1><137>: Abbrev Number: 3 (DW_TAG_typedef)
    <138>   DW_AT_name        : (indirect string, offset: 0xf8): sstring        
    <13c>   DW_AT_decl_file   : 1       
    <13d>   DW_AT_decl_line   : 11      
    <13e>   DW_AT_type        : <0x142> 
 <1><142>: Abbrev Number: 4 (DW_TAG_structure_type)
    <143>   DW_AT_name        : (indirect string, offset: 0xeb): basic_string   
    <147>   DW_AT_declaration : 1       

which results from gen_struct_or_union_type_die on the type seeing
a TYPE_STUB_DECL with TYPE_DECL_SUPPRESS_DEBUG set.

Somehow the C++ frontend "manually" outputs the desired info and then for
later processing directs dwarf2out.c to not output anything?

This probably all boils down to LTO "ignoring" debug-hook calls from the
frontend, but is there a way to recover from that?  For example by
clearing TYPE_DECL_SUPPRESS_DEBUG for LTO?  Or is that used in "meaningful"
ways as well?