[Bug gdb/12257] New: gdb ignore line "bar: if(foo)goto bar;"

[Bug gdb/12257] New: gdb ignore line "bar: if(foo)goto bar;"

[safinaskar at mail dot ru]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

           Summary: gdb ignore line "bar: if(foo)goto bar;"
           Product: gdb
           Version: 7.0
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
        AssignedTo: unassigned@sourceware.org
        ReportedBy: safinaskar@mail.ru


foo.c:
-----------------------------------
#include <stdio.h>

int main()
{
        int foo;
        printf("Enter foo: ");
        scanf("%d", &foo);
bar:    if(foo)goto bar;
        return 0;
}
-----------------------------------
0:~# gcc -g foo.c
0:~# gdb a.out
GNU gdb (GDB) 7.0.1-debian
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /root/a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x804842d: file foo.c, line 6.
Starting program: /root/a.out 

Temporary breakpoint 1, main () at foo.c:6
6               printf("Enter foo: ");
(gdb) n
7               scanf("%d", &foo);
(gdb) 
Enter foo: 0
9               return 0;
(gdb) q
A debugging session is active.

        Inferior 1 [process 21382] will be killed.

Quit anyway? (y or n) y
-----------------------------------
But gdb must say:

6               printf("Enter foo: ");
(gdb) n
7               scanf("%d", &foo);
(gdb) 
Enter foo: 0
8       bar:    if(foo)goto bar; 
(gdb) 
9               return 0;
-----------------------------------
0:~# gdb a.out
GNU gdb (GDB) 7.0.1-debian
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /root/a.out...done.
(gdb) start
Temporary breakpoint 1 at 0x804842d: file foo.c, line 6.
Starting program: /root/a.out 

Temporary breakpoint 1, main () at foo.c:6
6               printf("Enter foo: ");
(gdb) n
7               scanf("%d", &foo);
(gdb) 
Enter foo: 1
^C
Program received signal SIGINT, Interrupt.
0x08048458 in main () at foo.c:8
8       bar:    if(foo)goto bar;
(gdb) q
A debugging session is active.

        Inferior 1 [process 21399] will be killed.

Quit anyway? (y or n) y
-----------------------------------
But gdb must say:

6               printf("Enter foo: ");
(gdb) n
7               scanf("%d", &foo);
(gdb) 
Enter foo: 1
8       bar:    if(foo)goto bar; 
(gdb) 
^C
-----------------------------------
0:~# uname -a
Linux debian 2.6.32-5-686 #1 SMP Sat Oct 30 22:47:19 UTC 2010 i686 GNU/Linux
0:~# gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.4 --enable-shared --enable-multiarch
--enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--enable-targets=all --with-arch-32=i586 --with-tune=generic
--enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu
--target=i486-linux-gnu
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-8)

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[safinaskar at mail dot ru]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

Askar Safin <safinaskar at mail dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |safinaskar at mail dot ru

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

Tom Tromey <tromey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |tromey at redhat dot com
         Resolution|                            |INVALID

--- Comment #1 from Tom Tromey <tromey at redhat dot com> 2012-02-07 20:46:45 UTC ---
I think this is a gcc bug, not a gdb bug.

On Fedora 15,with the system gcc, it fails for me if I compile with -g.
But if I use -g -O1, it works

With plain -g, the line number table looks like:

CU: pr.c:
File name                            Line number    Starting address
pr.c                                           4            0x400534
pr.c                                           6            0x40053c
pr.c                                           7            0x40054e
pr.c                                           8            0x400569
pr.c                                           8            0x40056a
pr.c                                           9            0x400571
pr.c                                          10            0x400576

Note the address of the first occurrence of line 8.
Then if I disassemble I see:

        scanf("%d", &foo);
  40054e:       b8 84 06 40 00          mov    $0x400684,%eax
  400553:       48 8d 55 fc             lea    -0x4(%rbp),%rdx
  400557:       48 89 d6                mov    %rdx,%rsi
  40055a:       48 89 c7                mov    %rax,%rdi
  40055d:       b8 00 00 00 00          mov    $0x0,%eax
  400562:       e8 d1 fe ff ff          callq  400438 <__isoc99_scanf@plt>
  400567:       eb 01                   jmp    40056a <main+0x36>
bar:    if(foo)goto bar;
  400569:       90                      nop
  40056a:       8b 45 fc                mov    -0x4(%rbp),%eax
  40056d:       85 c0                   test   %eax,%eax
  40056f:       75 f8                   jne    400569 <main+0x35>
        return 0;

That is, the first "line 8" is that nop, which is skipped after the call.

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[safinaskar at mail dot ru]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

--- Comment #2 from Askar Safin <safinaskar at mail dot ru> 2012-02-07 21:16:57 UTC ---
Thank you, Tom. I posted the bug to
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52160

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[pinskia at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu.org

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-02-07 21:46:08 UTC ---
This is what GCC produces:
Here is what GCC produces:
    .loc 1 7 0
    leaq    -4(%rbp), %rax
    movq    %rax, %rsi
    movl    $.LC1, %edi
    movl    $0, %eax
    call    __isoc99_scanf
    jmp    .L2
.L5:
    .loc 1 8 0
    nop
.L2:
    .loc 1 8 0 is_stmt 0 discriminator 1
    movl    -4(%rbp), %eax
    testl    %eax, %eax
    jne    .L5

This looks fine to me.

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

Tom Tromey <tromey at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |

--- Comment #4 from Tom Tromey <tromey at redhat dot com> 2012-02-08 16:25:45 UTC ---
(In reply to comment #3)

>     .loc 1 8 0 is_stmt 0 discriminator 1

> This looks fine to me.

Yeah, my mistake.

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[Bug gdb/12257] gdb ignore line "bar: if(foo)goto bar;"

[tromey at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

--- Comment #5 from Tom Tromey <tromey at redhat dot com> 2012-02-08 21:38:41 UTC ---
This seems to be a side effect of this code in handle_inferior_event:

  if ((stop_pc == stop_pc_sal.pc)
      && (ecs->event_thread->current_line != stop_pc_sal.line
       || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
    {
      /* We are at the start of a different line.  So stop.  Note that
         we don't stop if we step into the middle of a different line.
         That is said to make things like for (;;) statements work
         better.  */
      if (debug_infrun)
     fprintf_unfiltered (gdb_stdlog,
                 "infrun: stepped to a different line\n");
      ecs->event_thread->control.stop_step = 1;
      print_end_stepping_range_reason ();
      stop_stepping (ecs);
      return;
    }

Here we have:

(top-gdb) p /x stop_pc
$26 = 0x40056a
(top-gdb) p /x stop_pc_sal.pc
$27 = 0x400569

So we skip this stop and instead keep going.

I am not sure whether that comment still makes sense.  It is present
in the initial public checkin of gdb, so I somewhat suspect it is
very out of date now.

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[Bug gdb/12257] gdb ignores line "bar: if(foo)goto bar;"

[safinaskar at mail dot ru]

http://sourceware.org/bugzilla/show_bug.cgi?id=12257

Askar Safin <safinaskar at mail dot ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|gdb ignore line "bar:       |gdb ignores line "bar:
                   |if(foo)goto bar;"           |if(foo)goto bar;"

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[Bug gdb/12257] gdb ignores line "bar: if(foo)goto bar;"

[andrew.burgess at embecosm dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=12257

Andrew Burgess <andrew.burgess at embecosm dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |andrew.burgess at embecosm dot com
             Status|REOPENED                    |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #6 from Andrew Burgess <andrew.burgess at embecosm dot com> ---
I tried this bug using GCC 7.4.0 and 9.2.0 and using GDB 7.9.1, 8.0, and 9.0,
and in all cases we now see the expected behaviour, that is:

 (gdb) start
 Temporary breakpoint 1 at 0x40054f: file test.c, line 6.
 Starting program: /home/andrew/tmp/test 

 Temporary breakpoint 1, main () at test.c:6
 6              printf("Enter foo: ");
 (gdb) n
 7              scanf("%d", &foo);
 (gdb) n
 Enter foo: 0
 8      bar:    if(foo)goto bar;
 (gdb) n
 9              return 0;
 (gdb) q

Given that the last thoughts on this issue were that it was a GDB issue I
intend to close this bug as resolved.  If this is still important to anyone, or
there's a known specific combination of tools that trigger this bug then please
feel free to reopen with the additional tool versions specified.

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