Re: [Qemu-devel] Build error with git commit 8eb29f1bf5a974dc4c11d2d1f5e7c7f7a62be116 on x86_64

[Thomas Huth <thuth@redhat.com>]

On 26/02/2019 09.58, Andrew Randrianasulu wrote:
> В сообщении от Tuesday 26 February 2019 11:54:12 вы написали:
>> On 25/02/2019 18.29, Andrew Randrianasulu wrote:
>>> В сообщении от Monday 25 February 2019 19:19:01 Philippe Mathieu-Daudc3a9
>>>
>>> написал(а):
>>>> Hi Andrew,
>>>>
>>>> On 2/23/19 1:35 AM, Andrew Randrianasulu wrote:
>>>>> Hello!
>>>>>
>>>>> I just pulled latest git
>>>>
>>>> [...]
>>>>
>>>>> and default build with simple ./configure on slackware 14.2 x86-64 box
>>>>> failed like this:
>>>>>
>>>>> root@slax:~/src/qemu# LANG=C make -j 5
>>>>>         CHK version_gen.h
>>>>>   CC      qobject/block-qdict.o
>>>>>   CC      util/thread-pool.o
>>>>>   CC      util/main-loop.o
>>>>>   CC      util/qemu-timer.o
>>>>>   CC      util/iohandler.o
>>>>>   CC      util/aio-posix.o
>>>>> qobject/block-qdict.c: In function 'qdict_array_split':
>>>>> qobject/block-qdict.c:259:9: error: 'subqdict' may be used
>>>>> uninitialized in this function [-Werror=maybe-uninitialized]
>>>>>          qlist_append_obj(*dst, subqobj ?: QOBJECT(subqdict));
>>>>>          ^
>>>>
>>>> That's odd, I can not reproduce with a simple ./configure:
>>>>
>>>> $ cat /etc/slackware-version
>>>> Slackware 14.2
>>>>
>>>> $ gcc --version
>>>> gcc (GCC) 5.5.0
>>>
>>> Well, then may be this is false positive, right now another qemu instance
>>> is busy inside same chroot, will try patch posted in earlier mail (after
>>> removing CFLAG I added for compiling qemu at all after this error).
>>>
>>> Thanks for trying to reproduce.
>>
>> Just to be sure: You don't compile with -O3 or -O1 or -O0 in the CFLAGS
>> here, do you?
> 
> This time  it was -O3, but I got some corruption in ppc64le guest's X, so I plan 
> to revert this -O3 back to default ....

Ok, then that's the problem here: GCC often produces some additional
"may be unused" warnings with -O3, and we normally only guarantee that
QEMU compiles without warnings when using the standard -O2 optimization
level.
So if you want to compile with -O3, you also have to specify
--disable-werror (or add -Wno-error=maybe-unitialized to the CFLAGS).
But unless you have really an urgent need for O3, I'd rather recommend
to compile with the well-tested O2 optimization level instead.

 Thomas