Source file (no preprocessing required): (note, I made it C-compatible so I
could see if the bug is present in the C compiler as well, the answer being no,
for reasons that will become obvious immediately)
--------------------------------------------------------------------------------
struct st{
int n;
};
struct st f(struct st x)
{
struct st z;
z=x;
z.n++;
return z;
}
int main()
{
struct st c;
c.n=1234;
f(c);
return 0;
}
--------------------------------------------------------------------------------
Terminal session:
--------------------------------------------------------------------------------
br...@bbi5291-laptop:~/programs/Other$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/i686-pc-linux-gnu/4.5.0/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: ./configure
Thread model: posix
gcc version 4.5.0 (GCC)
br...@bbi5291-laptop:~/programs/Other$ g++ -g bug.c
br...@bbi5291-laptop:~/programs/Other$ gdb a.out
GNU gdb 6.8-debian
Copyright (C) 2008 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. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...
(gdb) break 8
Breakpoint 1 at 0x80483d2: file bug.c, line 8.
(gdb) run
Starting program: /home/brian/programs/Other/a.out
Breakpoint 1, f (x={n = 1234}) at bug.c:8
8 z.n++;
Current language: auto; currently c++
(gdb) print z.n
$1 = 2424820
(gdb) q
The program is running. Exit anyway? (y or n) y
br...@bbi5291-laptop:~/programs/Other$ g++ -g -fno-elide-constructors bug.c
br...@bbi5291-laptop:~/programs/Other$ gdb a.out
GNU gdb 6.8-debian
Copyright (C) 2008 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. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...
(gdb) break 8
Breakpoint 1 at 0x80483e0: file bug.c, line 8.
(gdb) run
Starting program: /home/brian/programs/Other/a.out
Breakpoint 1, f (x={n = 1234}) at bug.c:8
8 z.n++;
Current language: auto; currently c++
(gdb) print z.n
$1 = 1234
(gdb) q
The program is running. Exit anyway? (y or n) y
br...@bbi5291-laptop:~/programs/Other$
--------------------------------------------------------------------------------
It is evident to me from these test runs that a bug in the return value
optimization is causing the wrong address for the variable z (since this is
presumably elided altogether when the optimization is used) to be reported in
the debug information. I first noticed this bug in g++ 4.4.1, and installed g++
4.5.0 and found that it was there too; it does not occur in g++ 4.3.4 or
earlier.
--
Summary: Return value optimization produces inaccurate debug info
Product: gcc
Version: 4.5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: bbi5291 at gmail dot com
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44731
