[Bug gdb/31073] New: Inconsistent Variable Values: Discrepancy between Source-Level and Instruction-Level Debugging

["iamanonymous.cs at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31073

            Bug ID: 31073
           Summary: Inconsistent Variable Values: Discrepancy between
                    Source-Level and Instruction-Level Debugging
           Product: gdb
           Version: unknown
            Status: UNCONFIRMED
          Severity: critical
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: iamanonymous.cs at gmail dot com
  Target Milestone: ---

########################
As shows in the following, we can observed that the values of variables x and y
exhibit inconsistencies between source-level debugging and instruction-level
debugging prior to entering the function foo. I'm not sure whether the
inconsistency is a result of incorrect debugging.
########################


###### source-level debugging ######
$ gcc -O2 -g small.c
root@ubuntu:~# gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401060: file small.c, line 30.
(gdb) r
Starting program: /root/a.out 

Breakpoint 1, main () at small.c:30
30        x = -5;
(gdb) s
31        y = -10;
(gdb) s
32        foo (&x, &y);
(gdb) info locals
x = 65531
y = 65526
(gdb) 


###### instruction-level debugging ######
$ gcc -O2 -g small.c; gdb -q a.out
Reading symbols from a.out...
(gdb) b main
Breakpoint 1 at 0x401060: file small.c, line 27.
(gdb) r
Starting program: /root/a.out 

Breakpoint 1, main () at small.c:27
27        x = -5;
(gdb) si
0x0000000000401064      27        x = -5;
(gdb) info locals
x = 0
y = 0
(gdb) si
0x0000000000401069      28        y = -10;
(gdb) info locals
x = 0
y = 0
(gdb) si
0x000000000040106e      29        foo (&x, &y);
(gdb) info locals
x = 0
y = 0
(gdb) 




###### bug-triggering code ######

$ cat small.c
extern void abort (void);
extern void exit (int);

__attribute__ ((noinline)) void
foo(short unsigned int *p1, short unsigned int *p2)
{
  short unsigned int x1, x4;
  int x2, x3, x5, x6;
  unsigned int x7;

  x1 = *p1;
  x2 = (int) x1;
  x3 = x2 * 65536;
  x4 = *p2;
  x5 = (int) x4;
  x6 = x3 + x4;
  x7 = (unsigned int) x6;
  if (x7 <= 268435455U)
    abort ();
  exit (0);
}

int
main()
{
  short unsigned int x, y;
  x = -5;
  y = -10;
  foo (&x, &y);
}



###### gcc and gdb version ######
$ 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.

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