[PATCH 0/2] Work around broken clang. Again.

[Michal Prívozník]

On 3/16/23 15:51, Andrea Bolognani wrote:
> On Wed, Mar 15, 2023 at 08:10:26PM +0100, Michal Privoznik wrote:
>> We ran into so many clang bugs recently, that I'm strongly in favor of
>> making it optional and aim on gcc only. Debugging these issues burns our
>> time needlessly.
>>
> [...]
>>
>> After these patches, there is still one qemuxml2argvtest case failing,
>> and it's simply because no matter how hard I try, I can't stop clang
>> from optimizing the function.
>>
> [...]
>>
>> At this point, I think we can call clang broken and focus our time on
>> developing libvirt. Not coming up with hacks around broken compilers.
> 
> clang is the default compiler for FreeBSD and macOS, both of which
> are officially supported platforms. Unless we are willing to change
> that, ignoring clang is simply out of the question.

I'm fine with doing that change. FreeBSD offers option to install gcc.
And for macOS - the fact we compile there says absolutely nothing.
Anybody willing to make libvirt work on macOS is more than welcome to
post patches.

> 
> 
> I've only looked at your patches very briefly and I'm trying to wrap
> something else up before the end of the week, so unfortunately I'm
> unlikely to be able to do a proper review within a reasonable
> timeframe. Plus IIUC even with these patches applied we'd still have
> at least one failing test case, so they're not a complete solution.

Yep.

> 
> So, in the interest of returning the CI to green as soon as possible,
> I would recommend reverting 95ae91fdd4da quickly. We can then worry
> about improving the situation compared to the (admittedly poor)
> status quo as a follow-up, once that urgency is gone.

That won't do any good as it's not the root cause of this problem. The
problem is (was until Dan fixed it) that even though I've specifically
marked some functions to be not optimized, clang ignored that and
optimized them anyway. And in this particular case it was
virNumaCPUSetToNodeset() and virNumaGetNodeOfCPU() which are new. They
were not implemented in the commit you're referencing.

> 
> A thought about VIR_OPTNONE. It seems to me that we'd want to apply
> this to all the functions that currently are marked with G_NO_INLINE
> for mocking purposes. So, wouldn't it make sense to instead have a
> single VIR_MOCKABLE annotation that combines the two, and use that
> everywhere?
> 

The problem is, __attribute__((optnone)) works only when passed at
function definition, while noinline can be passed at function
definition.

At any rate, this is not solved (for new enough clang, i.e. 11.0+) and
for even newer clang, where completely unrelated issue is happening,
I've posted another fix [1]. Which brings me back to the question from
the cover letter - how many clang related problems are acceptable for
us?

1: https://listman.redhat.com/archives/libvir-list/2023-March/238923.html

Michal