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

[David Woodhouse <dwmw2@infradead.org>]

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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.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation

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