[Bug gdb/30344] New: inconsistent variable values for source-level and instruction-level stepping

[Bug gdb/30344] New: inconsistent variable values for source-level and instruction-level stepping

[yangyibiao at nju dot edu.cn]

https://sourceware.org/bugzilla/show_bug.cgi?id=30344

            Bug ID: 30344
           Summary: inconsistent variable values for source-level and
                    instruction-level stepping
           Product: gdb
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: yangyibiao at nju dot edu.cn
  Target Milestone: ---

Created attachment 14820
  --> https://sourceware.org/bugzilla/attachment.cgi?id=14820&action=edit
triggering source code of small.c

$ gdb --version
GNU gdb (GDB) 13.0.50.20221107-git
Copyright (C) 2022 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.

$ gcc --version
gcc (GCC) 13.0.0 20221107 (experimental)
Copyright (C) 2022 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.



$ cat small.c -n
     1  
     2  int main (int argc, char **argv)
     3  {
     4    char buf[10];
     5  
     6    char c[] = "%i";
     7    unsigned char uc[] = "%i";
     8    const char cc[] = "%i";
     9    const unsigned char cuc[] = "%i";
    10  
    11    __builtin_sprintf(buf, (char *)c, 1);
    12    __builtin_sprintf(buf, (char *)uc, 1);
    13    __builtin_sprintf(buf, (char *)cc, 1);
    14    __builtin_sprintf(buf, (char *)cuc, 1); 
    15    __builtin_sprintf(buf, (const char *)L"foo"); 
    16  
    17    return 0;
    18  }


$ gcc -O2 -g small.c


$ gdb -q a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x401040: file small.c, line 6.
Starting program: /root/devil/batch/a.out 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffe478) at small.c:6
6         char c[] = "%i";
(gdb) s
7         unsigned char uc[] = "%i";
(gdb) s
8         const char cc[] = "%i";
(gdb) info locals
buf = "\000\000\000\000\000\000\000\000\000"
c = "%i"
uc = "%i"
cc = "%i\377"
cuc = "%i\177"
(gdb) 


$ gdb -q a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x401040: file small.c, line 6.
Starting program: /root/devil/batch/a.out 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffe478) at small.c:6
6         char c[] = "%i";
(gdb) si
0x0000000000401044      6         char c[] = "%i";
(gdb) 
0x0000000000401049      7         unsigned char uc[] = "%i";
(gdb) 
0x000000000040104e      9         const unsigned char cuc[] = "%i";
(gdb) 
0x0000000000401053      8         const char cc[] = "%i";
(gdb) info locals
buf = "\000\000\000\000\000\000\000\000\000"
c = "@\000"
uc = "\000\000"
cc = "p\344\377"
cuc = "\377\377\177"
(gdb) 



/**********************************
We can found that, when stepping to source code of line 8, these local
variables has different values. 
**********************************/

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[Bug gdb/30344] inconsistent variable values for source-level and instruction-level stepping

[yangyibiao at nju dot edu.cn]

https://sourceware.org/bugzilla/show_bug.cgi?id=30344

Yibiao Yang <yangyibiao at nju dot edu.cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unknown                     |13.1

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[Bug gdb/30344] inconsistent variable values for source-level and instruction-level stepping

[yangyibiao at nju dot edu.cn]

https://sourceware.org/bugzilla/show_bug.cgi?id=30344

Yibiao Yang <yangyibiao at nju dot edu.cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vries at gcc dot gnu.org,
                   |                            |yangyibiao at nju dot edu.cn

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[Bug gdb/30344] inconsistent variable values for source-level and instruction-level stepping

[yangyibiao at nju dot edu.cn]

https://sourceware.org/bugzilla/show_bug.cgi?id=30344

Yibiao Yang <yangyibiao at nju dot edu.cn> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tromey at sourceware dot org

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[Bug gdb/30344] inconsistent variable values for source-level and instruction-level stepping

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=30344

Tom de Vries <vries at gcc dot gnu.org> changed:

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

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Yibiao Yang from comment #0)
> /**********************************
> We can found that, when stepping to source code of line 8, these local
> variables has different values. 
> **********************************/

But you haven't stepped to the same location (to be precise, the same pc) in
both cases.

In one case, using step, you've stepped to the start of line 8.

In the other case using stepi, you've stepped until hitting the first insn that
is part of line 8.

You should continue stepi-ing until you hit the same location, at which point
you'll see the same values:
...
$ gdb -q a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x400430: file small.c, line 6.
Starting program: /data/vries/gdb/a.out 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffdce8) at small.c:6
6         char c[] = "%i";
(gdb) si
0x0000000000400434      6         char c[] = "%i";
(gdb) 
0x0000000000400439      7         unsigned char uc[] = "%i";
(gdb) 
0x000000000040043e      9         const unsigned char cuc[] = "%i";
(gdb) 
0x0000000000400443      8         const char cc[] = "%i";
(gdb) 
0x0000000000400448      6         char c[] = "%i";
(gdb) 
0x000000000040044d      11        __builtin_sprintf(buf, (char *)c, 1);
(gdb) 
0x0000000000400452      11        __builtin_sprintf(buf, (char *)c, 1);
(gdb) 
0x0000000000400454      7         unsigned char uc[] = "%i";
(gdb) 
0x0000000000400459      11        __builtin_sprintf(buf, (char *)c, 1);
(gdb) 
0x000000000040045e      9         const unsigned char cuc[] = "%i";
(gdb) 
0x0000000000400463      11        __builtin_sprintf(buf, (char *)c, 1);
(gdb) 
0x0000000000400468      8         const char cc[] = "%i";
(gdb) si
0x000000000040046d      6         char c[] = "%i";
(gdb) 
7         unsigned char uc[] = "%i";
(gdb) 
8         const char cc[] = "%i";
(gdb) info locals
buf = "\000\000\000\000\000\000\000\000\000"
c = "%i"
uc = "%i"
cc = "%i\377"
cuc = "%i\177"
(gdb) print /x $pc
$1 = 0x400477
...

Using print /x $pc, we can verify that it is indeed the same location:
...
$ gdb -q a.out
Reading symbols from a.out...
(gdb) start
Temporary breakpoint 1 at 0x400430: file small.c, line 6.
Starting program: /data/vries/gdb/a.out 

Temporary breakpoint 1, main (argc=1, argv=0x7fffffffdce8) at small.c:6
6         char c[] = "%i";
(gdb) s
7         unsigned char uc[] = "%i";
(gdb) 
8         const char cc[] = "%i";
(gdb) p /x $pc
$1 = 0x400477
...

AFAICT, there's no bug in gcc and no bug in gdb.

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