Re: c++/1141: cannot print local variables in (some) constructor functions

[Daniel Jacobowitz <drow at mvista dot com>]

The following reply was made to PR c++/1141; it has been noted by GNATS.

From: Daniel Jacobowitz <drow at mvista dot com>
To: germain at cs dot utah dot edu
Cc: gdb-gnats at sources dot redhat dot com, gcc-bugs at gcc dot gnu dot org
Subject: Re: c++/1141: cannot print local variables in (some) constructor functions
Date: Thu, 13 Mar 2003 12:12:16 -0500

 On Thu, Mar 13, 2003 at 04:42:00PM -0000, germain at cs dot utah dot edu wrote:
 > 
 > >Number:         1141
 > >Category:       c++
 > >Synopsis:       cannot print local variables in (some) constructor functions
 > >Confidential:   no
 > >Severity:       serious
 > >Priority:       high
 > >Responsible:    unassigned
 > >State:          open
 > >Class:          change-request
 > >Submitter-Id:   net
 > >Arrival-Date:   Thu Mar 13 16:48:00 UTC 2003
 > >Closed-Date:
 > >Last-Modified:
 > >Originator:     germain at cs dot utah dot edu
 > >Release:        gdb 5.3 (gcc 3.2.(1 or 2))
 > >Organization:
 > >Environment:
 > Linux juggler 2.4.18-17.7.xsmp #1 SMP Tue Oct 8 12:37:04 EDT 2002 i686 unknown
 > >Description:
 > I'm having a problem with the gdb (5.3) debugger being able to
 > tell me the value of local variables in a class function.
 > I'm compiling with g++ 3.2.1.
 > 
 > Here is the code:
 > 
 >  class ABC 
 >  {
 >    public:
 >      ABC(double x);
 >      ABC(int x)
 >        {
 >   int temp = x;
 >   x_ = temp;
 >        }
 >      int x_;
 >  };
 >  
 >  ABC::ABC(double x)
 >    {
 >      double temp = x;
 >      x_ = temp;
 >    }
 >  
 >  int main(int argc, char* argv[])
 >  {
 >    ABC abc(1.0);
 >    ABC def(1);
 >  }
 > 
 > 
 > 
 >         % g++ -g test.cpp -o test
 > 
 >  
 > the problem is that when i step into the constructor taking a double,
 > I cannot print the temp variable ("No symbol "temp" in current
 > context."  When i step into the int (inlined) constructor, I can print
 > the temp variable. 
 > 
 >  
 >  (gdb) break main
 >  (gdb) run
 >  Starting program: /home/germain/PETE/jim 
 >  Breakpoint 1, main (argc=1, argv=0xbffff514) at test.cpp:24
 >  (gdb) s
 >  ABC (this=0xbffff4a4, x=1) at test.cpp:18
 >  (gdb) p temp
 >  No symbol "temp" in current context.
 >  (gdb) fin
 >  Run till exit from #0  ABC (this=0xbffff4a4, x=1) at test.cpp:18
 >  main (argc=1, argv=0xbffff514) at test.cpp:25
 >  (gdb) s
 >  ABC (this=0xbffff4a0, x=1) at test.cpp:10
 >  (gdb) p temp
 >  $1 = 1072693248
 >  (gdb) 
 > 
 > I do not think this is a case of the compiler optimizing the
 > variable away, (no optimization flags are used...) and if it
 > is, then it shouldn't be (IMO).
 > 
 > any help would be appreciated
 
 Thanks for the great bug report.  Unfortunately, this is a GCC bug
 rather than a GDB bug.  Here's the affected debug info.  First, for one
 copy of the constructor:
  <1><f3>: Abbrev Number: 20 (DW_TAG_subprogram)
      DW_AT_sibling     : <129>  
      DW_AT_abstract_origin: <c2>        
      DW_AT_low_pc      : 0x80483d0 134513616    
      DW_AT_high_pc     : 0x8048401 134513665
      DW_AT_frame_base  : 1 byte block: 55       (DW_OP_reg5; )
  <2><106>: Abbrev Number: 21 (DW_TAG_formal_parameter)
      DW_AT_abstract_origin: <cd>
      DW_AT_location    : 2 byte block: 91 8     (DW_OP_fbreg: 8; )
  <2><10e>: Abbrev Number: 21 (DW_TAG_formal_parameter)
      DW_AT_abstract_origin: <d7>
      DW_AT_location    : 2 byte block: 91 78    (DW_OP_fbreg: -8; )
  <2><116>: Abbrev Number: 22 (DW_TAG_lexical_block)
      DW_AT_low_pc      : 0x80483dc 134513628
      DW_AT_high_pc     : 0x8048401 134513665
  <3><11f>: Abbrev Number: 23 (DW_TAG_variable)
      DW_AT_abstract_origin: <e1>
      DW_AT_location    : 2 byte block: 91 70    (DW_OP_fbreg: -16; )
 
 Then for the other, the one you're actually using:
  <1><129>: Abbrev Number: 20 (DW_TAG_subprogram)
      DW_AT_sibling     : <15c>
      DW_AT_abstract_origin: <c2>
      DW_AT_low_pc      : 0x8048402 134513666
      DW_AT_high_pc     : 0x8048433 134513715
      DW_AT_frame_base  : 1 byte block: 55       (DW_OP_reg5; )
  <2><13c>: Abbrev Number: 21 (DW_TAG_formal_parameter)
      DW_AT_abstract_origin: <cd>
      DW_AT_location    : 2 byte block: 91 8     (DW_OP_fbreg: 8; )
  <2><144>: Abbrev Number: 21 (DW_TAG_formal_parameter)
      DW_AT_abstract_origin: <d7>
      DW_AT_location    : 2 byte block: 91 78    (DW_OP_fbreg: -8; )
  <2><14c>: Abbrev Number: 22 (DW_TAG_lexical_block)
      DW_AT_low_pc      : 0x804840e 134513678
      DW_AT_high_pc     : 0x8048433 134513715
  <3><155>: Abbrev Number: 24 (DW_TAG_variable)
      DW_AT_abstract_origin: <e1>
 
 
 Note that the last DW_TAG_variable is missing a DW_AT_location. 
 Without that GDB can't find the variable.  I've reproduced this in both
 GCC 3.2 and GCC 3.4; someone on the GCC side will have to look at it.
 
 -- 
 Daniel Jacobowitz
 MontaVista Software                         Debian GNU/Linux Developer