Optimized code fails to assign struct member

[Jan Wielemaker]

Hi,

I've upgraded to Ubuntu 12.10 on Intel i7 2600 (64-bit version) and -as a developer- compiled SWI-Prolog using
the standard gcc 4.7 compiler. All seems to work, except for the `semweb' package. It works with compiled
without optimization, but fails at -O2.

After a bit searching with gdb, it appears that a structure assignment is simply omitted. I've forced it to work
by adding a dummy function and call this using the structure field as argument.

Not sure how to report this as building the whole system is a bit work. So, I'll first provide a pointer to the
work-around commit. This can be found at the address below. The first change is irrelevant, but indicates
the location of the missing assignment.

http://www.swi-prolog.org/git/packages/semweb.git/blobdiff/192d052db9b7c44519723fa3186a5786a1562127..af7fb1bb50ec30295bc2e1a7473bbe36ee0c03bf:/rdf_db.c

The dummy function itself is simply

#if __GNUC__ == 4 && __GNUC_MINOR__ == 7
void
gcc_47_dummy(void *ptr)
{ static void *p;

  if ( p != ptr )
    p = ptr;
}
#endif

Of course, I know that adding dummy calls can make the difference without the compiler being at fault, but

  - I can see in gdb that the assignment is not done without the hack. Of course, I also know that gdb
    sometimes does weird things with optimized code. Adding a print statement fixes the problem ...
    The failed assignment does however explain subsequent (wrong) behaviour.

I'll leave it here, providing further details as deemed necessary on request.

      Thanks --- Jan

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.