https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112582
Bug ID: 112582
Summary: Inconsistent Variable Values: Discrepancy between
Source-Level and Instruction-Level Debugging
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: ---
########################
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 code optimization.
########################
###### 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.
