Re: [Qemu-devel] [PATCH 23/25] fdc: Move floppy geometry guessing back from block.c

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/09/2012 11:46 AM, Eric Blake wrote:
> On 07/09/2012 10:07 AM, Markus Armbruster wrote:
>
>>>> This is an unconditional use of fd_type[0].  If floppy == NULL, this is
>>>> dereferencing an uninitialized value.
>>>>
>>>> I'm not sure why the explicit initialization was removed...
>>
>> Brain fart on my part, sorry.  The old loop assigns only if the drive
>> exists.  The new loop assigns unconditionally.  Except the whole loop is
>> still conditional.
>>
>> Testing can't flag this, because floppy is never null.
>>
>>> Looks broken indeed. I just wonder why my gcc (or the buildbots) didn't
>>> complain.
>>
>> Me too.  Looks like I should upgrade to a more recent gcc.
>
> It's probably not the version of the gcc you used, but whether or not
> your CFLAGS include -O2.  Gcc has the (IMO very annoying) limitation
> that uninitialized-use analysis can only be performed if you are also
> doing optimization.  You have to use a tool like clang or Coverity if
> you want more reliable uninitialized-use analysis even while building
> -O0 debug images.
>

Specifically, without -O, GCC doesn't do data flow analysis so any warning that 
requires DFA won't get triggered.

So in general, if you are normally building with -O0, make sure to also build 
with -O in order to get full warnings.

Regards,

Anthony Liguori