Bug#256312: How to jump in inline assembler with -fPIC?

["Richard B. Kreckel" <kreckel@ginac.de>]

Hi,

Bug #256312 is a problem that can be reproduced with 20 lines of C++ code:

---<boingboing.cc>-------------------------------------------------------------
extern "C" int putchar(int);

extern "C" void catchpoint(void);
struct thrower_t {
    inline thrower_t() {
        __asm__("jmp %*%0" : : "rm" ((void*)(catchpoint)));
    }
};
#ifdef BOINGBOING
static thrower_t thrower;
#endif


class herring_t {
public:
    herring_t() {
        putchar('A');
    }
};
static herring_t red_herring;


struct catcher_t {
    inline catcher_t() {
        __asm__("\ncatchpoint:");
    }
};
static catcher_t catcher;


int main()
{
    putchar('\n');
}
--------------------------------------------------------------------------------

The purpose of the code is to either jump over the construction of the
red_herring object or not, depending on whether -DBOINGBOING was def'ed.
All this is part of a scheme for ordering static C++ objects, in case you
are wondering why one writes such code.

When I compile that program on ia32 I get the expected result: it either
prints 'A' followed by a newline or just a newline:

gromit:~$ uname -a
Linux gromit 2.6.7 #1 Thu Jun 17 22:32:35 CEST 2004 i686 GNU/Linux
gromit:~$ g++ -O boingboing.cc  &&  ./a.out
A
gromit:~$ g++ -DBOINGBOING -O boingboing.cc  &&  ./a.out

gromit:~$ g++ -fPIC -DBOINGBOING -O boingboing.cc  &&  ./a.out

Now, on amd64, the program segfaults if -fPIC has been set and
-DBOINGBOING is def'ed.  Apparently the jump to the catchpoint label goes
havoc:

kreckel@ravel:~$ uname -a
Linux ravel 2.6.5 #3 SMP Tue Apr 13 13:41:54 UTC 2004 x86_64 GNU/Linux
kreckel@ravel:~$ g++ -O boingboing.cc  &&  ./a.out
A
kreckel@ravel:~$ g++ -DBOINGBOING -O boingboing.cc  &&  ./a.out

kreckel@ravel:~$ g++ -fPIC -DBOINGBOING -O boingboing.cc  &&  ./a.out
Segmentation fault

I see the differences in GCC's config/i386/i386.md but I am not an expert
in GCC machine description files and the ABI.  Does anybody have a
suggestion how to write the __asm__() statement in the thrower_t
constructor?

TIA
   -richy.
-- 
  .''`.  Richard B. Kreckel
 : :' :  <kreckel@debian.org>
 `. `'   <kreckel@ginac.de>
   `-    <http://www.ginac.de/~kreckel/>