[Bug debug/99457] New: gcc/gdb -gstabs+ is buggy.

[Bug debug/99457] New: gcc/gdb -gstabs+ is buggy.

[jay.krell at cornell dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

            Bug ID: 99457
           Summary: gcc/gdb -gstabs+ is buggy.
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jay.krell at cornell dot edu
  Target Milestone: ---

gcc/gdb -gstabs+ is buggy.
Could be either, I didn't investigate.
The codegen is correct.
That's ok, I didn't really need it, I'll use regular -g.

$ cat 1.c
#include <stdio.h>
int main()
{
        char a[100] = {0};
        printf("%d\n", a[0]);
}

$ gcc --version
gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0

$ gcc -gstabs+ 1.c

 # correct output
$ ./a.out
0

$ gdb --version
GNU gdb (Ubuntu 9.1-0ubuntu1) 9.1

$ gdb ./a.out
(gdb) break main
(gdb) r

Breakpoint 1, main () at 1.c:3
3       {
(gdb) n
4               char a[100] = {0};
(gdb)
5               printf("%d\n", a[0]);
(gdb) p a
$1 =
"@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003\000\000@\003",
'\000' <repeats 11 times>, "\001", '\000' <repeats 49 times>
(gdb) p a[0]
$2 = 64 '@'
(gdb) disas
Dump of assembler code for function main:
   0x0000555555555169 <+0>:     endbr64
   0x000055555555516d <+4>:     push   %rbp
   0x000055555555516e <+5>:     mov    %rsp,%rbp
   0x0000555555555171 <+8>:     sub    $0x70,%rsp
   0x0000555555555175 <+12>:    mov    %fs:0x28,%rax
   0x000055555555517e <+21>:    mov    %rax,-0x8(%rbp)
   0x0000555555555182 <+25>:    xor    %eax,%eax
   0x0000555555555184 <+27>:    movq   $0x0,-0x70(%rbp)
   0x000055555555518c <+35>:    movq   $0x0,-0x68(%rbp)
   0x0000555555555194 <+43>:    movq   $0x0,-0x60(%rbp)
   0x000055555555519c <+51>:    movq   $0x0,-0x58(%rbp)
   0x00005555555551a4 <+59>:    movq   $0x0,-0x50(%rbp)
   0x00005555555551ac <+67>:    movq   $0x0,-0x48(%rbp)
   0x00005555555551b4 <+75>:    movq   $0x0,-0x40(%rbp)
   0x00005555555551bc <+83>:    movq   $0x0,-0x38(%rbp)
   0x00005555555551c4 <+91>:    movq   $0x0,-0x30(%rbp)
   0x00005555555551cc <+99>:    movq   $0x0,-0x28(%rbp)
   0x00005555555551d4 <+107>:   movq   $0x0,-0x20(%rbp)
   0x00005555555551dc <+115>:   movq   $0x0,-0x18(%rbp)
   0x00005555555551e4 <+123>:   movl   $0x0,-0x10(%rbp)
=> 0x00005555555551eb <+130>:   movzbl -0x70(%rbp),%eax
   0x00005555555551ef <+134>:   movsbl %al,%eax
   0x00005555555551f2 <+137>:   mov    %eax,%esi
   0x00005555555551f4 <+139>:   lea    0xe09(%rip),%rdi        # 0x555555556004
   0x00005555555551fb <+146>:   mov    $0x0,%eax
   0x0000555555555200 <+151>:   callq  0x555555555070 <printf@plt>
   0x0000555555555205 <+156>:   mov    $0x0,%eax
   0x000055555555520a <+161>:   mov    -0x8(%rbp),%rdx
(gdb) q

$ gcc -g 1.c
$ gdb ./a.out
(gdb) r

Breakpoint 1, main () at 1.c:3
3       {
(gdb) n
4               char a[100] = {0};
(gdb)
5               printf("%d\n", a[0]);
(gdb) p a
$1 = '\000' <repeats 99 times>

[Bug debug/99457] gcc/gdb -gstabs+ is buggy.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
stabs support really should be removed.

[Bug debug/99457] gcc/gdb -gstabs+ is buggy.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://gcc.gnu.org/pipermail/gcc-patches/2017-July/479552.html

[Bug debug/99457] gcc/gdb -gstabs+ is buggy.

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Can't reproduce (not that anyone would care) with either current 9 branch or
current trunk, there is no reason not to use DWARF.
Sure, on the line that defines the variable you can see garbage in the variable
because it is not initialized yet and STABS has no way to express that (though,
with -O0 gcc doesn't track that anyway), but on the following line which you
get after b main r n it shows the var to be all zeros.

[Bug debug/99457] gcc/gdb -gstabs+ is buggy.

[jay.krell at cornell dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

--- Comment #4 from Jay <jay.krell at cornell dot edu> ---
I thought I stepped far enough, as I showed.
I didn't try current, granted.
And yeah, I don't need -gstabs+ and have now switched to -g.

[Bug debug/99457] gcc/gdb -gstabs+ is buggy.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99457

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Works for me.