[Bug debug/112565] New: Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[Bug debug/112565] New: Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[iamanonymous.cs at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

            Bug ID: 112565
           Summary: Abnormal Jump in Execution using 'stepi' Command in
                    GDB under O2 optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamanonymous.cs at gmail dot com
  Target Milestone: ---

Bug reported initially reported:
https://sourceware.org/bugzilla/show_bug.cgi?id=31070

Tom de Vries suggests that this issue may be attributed to a GCC optimization
bug.


$ cat small.c
int glob_int_arr[100];
int *glob_ptr_int = glob_int_arr;

void simple_global ()
{
  __builtin_prefetch (glob_int_arr, 0, 0);
  __builtin_prefetch (glob_ptr_int, 0, 0);
}

int main()
{
  simple_global ();
  exit (0);
}


$ gcc -w -O2 -g small.c; gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401040: file small.c, line 6.
(gdb) r
Starting program: /root/devil/a.out 

Breakpoint 1, main () at small.c:6
6         __builtin_prefetch (glob_int_arr, 0, 0);
(gdb) si
0x0000000000401044 in simple_global () at small.c:7
7         __builtin_prefetch (glob_ptr_int, 0, 0);
(gdb) si
0x000000000040104b in main () at small.c:13
13        exit (0);
(gdb) si
0x000000000040104d in simple_global () at small.c:6
6         __builtin_prefetch (glob_int_arr, 0, 0);
(gdb)


=================================================
When using the 'stepi' command in GDB, an abnormal jump in the execution occurs
when reaching line 7 of the code. The execution unexpectedly jumps to line 13.


$ gcc --version
gcc (GCC) 14.0.0 20231116 (experimental)
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gdb --version
GNU gdb (GDB) 15.0.50.20231116-git
Copyright (C) 2023 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.

[Bug debug/112565] Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Anonymous from comment #0)
> Tom de Vries suggests that this issue may be attributed to a GCC
> optimization bug.

I do not.

[Bug debug/112565] Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[iamanonymous.cs at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

--- Comment #2 from Anonymous <iamanonymous.cs at gmail dot com> ---
(In reply to Tom de Vries from comment #1)
> (In reply to Anonymous from comment #0)
> > Tom de Vries suggests that this issue may be attributed to a GCC
> > optimization bug.
> 
> I do not.

Thanks for the correction. I apologize for the misunderstanding. I didn't
notice that you mentioned this behavior was expected.

[Bug debug/112565] Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[iamanonymous.cs at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

--- Comment #3 from Anonymous <iamanonymous.cs at gmail dot com> ---
However, I still feel that the current debugging process be somewhat
misleading, and I am uncertain if there are any approaches to enhance the
overall debugging experience.

[Bug debug/112565] Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

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

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

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC can interleave instructions belonging to different source lines so that's
what you see.  Since you advance assembler instructions I don't see a way to
"improve" things here when optimizing.

[Bug debug/112565] Abnormal Jump in Execution using 'stepi' Command in GDB under O2 optimization

[iamanonymous.cs at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112565

--- Comment #5 from Anonymous <iamanonymous.cs at gmail dot com> ---
(In reply to Richard Biener from comment #4)
> GCC can interleave instructions belonging to different source lines so
> that's what you see.  Since you advance assembler instructions I don't see a
> way to
> "improve" things here when optimizing.

Got it. Thanks.