gcc-4.6.0-10.fc15.x86_64 breaks grub-0.97-71.fc15 and later

[Jakub Jelinek]

On Wed, Jul 06, 2011 at 07:19:10AM +0200, Joshua C. wrote:
> >> So why does the gcc-4.6.0-10.fc15.x86_64 breaks my package???
> >
> > Note that this problem has been reported as bug 718722.
> >
> 
> I think this is a separate issue from rbz#718722. My Problem is that
> the current compiler produces different results than the one used to
> build the packages in koji. This shouldn't be the case. I think this
> is a gcc issues not a grub problem.

It is much more probable it is an application bug (relying on undefined
behavior etc.) than a gcc bug, though of course gcc bug can't be ruled out.

If you suspect a miscompilation, let somebody familiar with the source code
first try to see if some gcc flags make it working again (e.g. if compiling
with -O0 makes it work, or -O2 -fno-strict-aliasing, etc.).  If yes, try to do
a binary search between objects compiled with the working options and
non-working to narrow the problem to one source file.  If not, do a similar
binary search between older compiler compiled objects and new compiler
compiled objects.

Using a debugger or debugging printouts might be alternative
to the binary search to narrow the problem down, binary search is kind of
brute force.

Once you know which source file is problematic, first compile it with -W -Wall,
look at all the warnings, see if some of them might not show up problem in the
code, if not, try to narrow down the problem to a particular function (and
see if it can be reproduced even with -fno-inline, that helps to narrow it down
to a function), noinline and optimize attributes might help with that too,
then try to create a self-contained testcase calling that
function with the right arguments and abort or somehow else signal if that
function misbehaves.

	Jakub