[Bug debug/51358] New: missing location

[mark at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51358

             Bug #: 51358
           Summary: missing location
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mark@gcc.gnu.org
                CC: dodji@gcc.gnu.org, tromey@redhat.com


This is related to the following systemtap bug report:
http://sourceware.org/bugzilla/show_bug.cgi?id=13420

This is using g++ (GCC) 4.7.0 20111130 (experimental), but can be reproduced
with other versions (g++ (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4) in particular).

Compiling the following with g++ -gdwarf-4 -o length length.cxx

#include <string>

size_t
length(const std::string& str)
{
    int res = str.length();
    return res;
}

int
main()
{
    std::string hello = "Hello World!";
    return 12 != length(hello);
}


Produces:

00000000 00000014 00000000 CIE
  Version:               1
  Augmentation:          "zR"
  Code alignment factor: 1
  Data alignment factor: -8
  Return address column: 16
  Augmentation data:     1b

  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_offset: r16 (rip) at cfa-8
  DW_CFA_nop
  DW_CFA_nop

00000018 0000001c 0000001c FDE cie=00000000 pc=0040088c..004008ae
  DW_CFA_advance_loc: 1 to 0040088d
  DW_CFA_def_cfa_offset: 16
  DW_CFA_offset: r6 (rbp) at cfa-16
  DW_CFA_advance_loc: 3 to 00400890
  DW_CFA_def_cfa_register: r6 (rbp)
  DW_CFA_advance_loc: 29 to 004008ad
  DW_CFA_def_cfa: r7 (rsp) ofs 8
  DW_CFA_nop
  DW_CFA_nop
  DW_CFA_nop

and

 <1><a86>: Abbrev Number: 95 (DW_TAG_subprogram)
    <a87>   DW_AT_external    : 1       
    <a87>   DW_AT_name        : (indirect string, offset: 0x1aec): length       
    <a8b>   DW_AT_decl_file   : 1       
    <a8c>   DW_AT_decl_line   : 4       
    <a8d>   DW_AT_linkage_name: (indirect string, offset: 0x293): _Z6lengthRKSs 
    <a91>   DW_AT_type        : <0x2e2> 
    <a95>   DW_AT_low_pc      : 0x40088c        
    <a9d>   DW_AT_high_pc     : 0x4008ae        
    <aa5>   DW_AT_frame_base  : 1 byte block: 9c        (DW_OP_call_frame_cfa)
    <aa7>   Unknown AT value: 2116: 1   
    <aa7>   DW_AT_sibling     : <0xada> 
 <2><aab>: Abbrev Number: 96 (DW_TAG_formal_parameter)
    <aac>   DW_AT_name        : str     
    <ab0>   DW_AT_decl_file   : 1       
    <ab1>   DW_AT_decl_line   : 4       
    <ab2>   DW_AT_type        : <0xada> 
    <ab6>   DW_AT_location    : 2 byte block: 91 58     (DW_OP_fbreg: -40)

Which seems to suggest that the formal_parameter "str" can always be accessed
through fbreg -40. But this isn't true as you can see by looking at the
generated code:

Dump of assembler code for function length(std::string const&):
   0x000000000040088c <+0>:    push   %rbp
   0x000000000040088d <+1>:    mov    %rsp,%rbp
   0x0000000000400890 <+4>:    sub    $0x20,%rsp
   0x0000000000400894 <+8>:    mov    %rdi,-0x18(%rbp)
   0x0000000000400898 <+12>:    mov    -0x18(%rbp),%rax
   0x000000000040089c <+16>:    mov    %rax,%rdi
   0x000000000040089f <+19>:    callq  0x4006c0 <_ZNKSs6lengthEv@plt>
   0x00000000004008a4 <+24>:    mov    %eax,-0x4(%rbp)
   0x00000000004008a7 <+27>:    mov    -0x4(%rbp),%eax
   0x00000000004008aa <+30>:    cltq   
   0x00000000004008ac <+32>:    leaveq 
   0x00000000004008ad <+33>:    retq   
End of assembler dump.

The argument was actually in %rdi, which isn't pushed on the stack till 400894,
so when we probe at the start of the function (40088c) we will see garbage when
trying to extract the str parameter.

Is there a way a dwarf consumer could have known that?

GCC also doesn't seem to produce line table prologue markers, so it also
doesn't help trying to search for the end of prologue.