confused interaction between gcc and gdb

confused interaction between gcc and gdb

[Brian Banister]

I have encountered a bug which I'm not sure if it should be attributed to gcc or gdb.
Example code is below.  Inside Foo::operator+, gdb will on some systems have the
wrong location for the variable "res" such that looking at res.top in gdb shows a bogus number.  The actual code execution is fine.  I then have 2 "workarounds" to make gdb see the correct location for res.  The first workaround is to make the return variable of operator+ not compile-time deterministic (add -DGDB_HAPPY to compile line).  The second workaround is to remove my empty destructor (-DGDB_HAPPY_ALT).  I can make guesses on why the first case matters.  I have no clue on the second case.

Without the workarounds, I've found the following systems to work or not:
Ubuntu 9.04, gcc 4.3.3, gdb 6.8 - Buggy
Ubuntu 9.04, gcc 4.2.4, gdb 6.8 - Works
Ubuntu 9.04, gcc 4.1.3, gdb 6.8 - Works
Mac Snow Leopard, gcc 4.2.1, gdb 6.3.50-20050815 (Apple version gdb-1346) - Works
Centos 5.2, gcc 4.1.2, gdb 6.5-37.el5_r.2rh - Buggy (tried both 32-bit and 64-bit)
Gentoo, gcc 4.3.4, gdb 6.6, - Buggy

I've found an old (but still open) gdb bug which may be related: 8761 "if gdb cannot find function return value location, random unitialized [sic] value is used"
Given different results on Ubuntu using the same gdb, I'm not convinced its gdb.

Is this the known gdb bug, and how should I be proceeding?

---- code ----
// Compiled with "g++ -o cpptest cpptest.cpp -g" fails on some systems
// Works with "g++ -o cpptest cpptest.cpp -g -DGDB_HAPPY"
// or with "g++ -o cpptest cpptest.cpp -g -DGDB_HAPPY_ALT"

class Foo {
public:
        Foo(int x=0) { this->top = (x)?1:0; }
#ifndef GDB_HAPPY_ALT
    ~Foo() {};
#endif
        Foo operator+(const Foo& rhs) const;
protected:
        int top;
};

Foo Foo::operator+(const Foo& rhs) const {
        Foo dummy1, res, dummy2;
    int i = 0;

    i++;
    /* Look at res HERE in gdb. res.top should be 0 */

#ifdef GDB_HAPPY
    if (rhs.top) {
      return res;
    } else {
      return dummy1;
    }
#else
    return res;
#endif
}

int main(int argc, char* argv[])
{
        Foo A(1), B(2), C;
        C = A + B;
        return 0;
}