Re: [Ksummit-discuss] [CORE TOPIC] [TECH TOPIC] Support (or move towards to) LLVM

[Shuah Khan <shuahkhan@gmail.com>]

On Thu, Jul 21, 2016 at 3:54 AM, David Woodhouse <dwmw2@infradead.org> wrote:
> On Wed, 2016-07-20 at 19:04 +0200, Jiri Kosina wrote:
>> On Wed, 20 Jul 2016, Mark Brown wrote:
>>
>> > > > There's a push from certain quarters to move away from GCC to LLVM.
>> >
>> > > This might actually be an interesting topic per se.
>> >
>> > Yes, indeed.
>>
>> Let's make this a real proposal then ... (subject changed). I am again a
>> bit unsure about the core / tech division here.
>>
>> People who should be invited: proponents of the push from the certain
>> quarters mentioned by Mark above, and ideally some LLVM folks as well.
>>
>> I've never actually used llvm to compile the kernel (which makes me rather
>> poor contributor should any such discussion happen), but I've been on the
>> "receiving side", debugging a crash that turned out to be llvm messing up
>> with IF in a way that interfers with local_irq_save(), and also suffered
>> from the followup frustration when I found out that this has been reported
>> to llvm folks ages ago, and they haven't bothered to fix it (it's now at
>> least worked around, in a very sub-optimal way (lahf/sahf)).
>
> I got involved in building the kernel with LLVM a little while ago,
> after accidentally implementing .code16 support in LLVM — for other
> reasons, but it allowed the arch/x86/boot/ bits to be built with LLVM.
>
> Apart from resolutely not wanting to implement variable length arrays
> on the stack, the LLVM folks actually seem quite keen to make things
> work. I'm interested in the problem you report above.. and note the
> absence of a bug number. Can you provide it?
>
> You're right that it does take a while to get some things fixed, but
> people *are* doing a fairly good job of identifying them, filing bugs,
> and implementing workarounds until the bugs can be fixed.
>
> Building with LLVM has also helped to find some real kernel bugs. I'd
> be keen to get this working more widely.
>

Would you be willing to share your experiences and the nature of bugs
you were able to find using LLVM. Maybe that could be folded into this
discussion as a real life experience.

thanks,
-- Shuah