Linux and LLVM

Linux and LLVM

[Grozdan]

Hi,

First, I'd like to say that I'm not interested in any holy wars,
trolling, ego-boosters, or whatever, and basically want to ask a
simple question and get the views of the Linux developers on it, at
least from those who are interested in answering it or have time to do
so as I know that most kernel devs are very busy :)

Recently, FreeBSD has reported that it's slowly switching from GCC to
LLVM/CLANG for compilers. What are the thoughts of the Linux kernel
devs about this and do they things that it's a good idea to go in the
same direction as FreeBSD (switching over to LLVM)?

Please CC me as I'm not subscribed to this specific mailing list

Thanks :)

Re: Linux and LLVM

[Jeff Garzik]

Grozdan wrote:
> Hi,
> 
> First, I'd like to say that I'm not interested in any holy wars,
> trolling, ego-boosters, or whatever, and basically want to ask a
> simple question and get the views of the Linux developers on it, at
> least from those who are interested in answering it or have time to do
> so as I know that most kernel devs are very busy :)
> 
> Recently, FreeBSD has reported that it's slowly switching from GCC to
> LLVM/CLANG for compilers. What are the thoughts of the Linux kernel
> devs about this and do they things that it's a good idea to go in the
> same direction as FreeBSD (switching over to LLVM)?

It is highly unlikely we will switch compilers away from gcc -- we have 
enough problems just supporting multiple gcc versions.

That said, you could join the effort to get LLVM to compile under the 
kernel.  Here is a meta-bug for that: 
http://llvm.org/bugs/show_bug.cgi?id=4068

It is a moot question until LLVM can actually build a bootable kernel :)

Regards,

	Jeff

Re: Linux and LLVM

[Grozdan]

2009/5/8 Jeff Garzik <jeff@garzik.org>:
> Grozdan wrote:
>>
>> Hi,
>>
>> First, I'd like to say that I'm not interested in any holy wars,
>> trolling, ego-boosters, or whatever, and basically want to ask a
>> simple question and get the views of the Linux developers on it, at
>> least from those who are interested in answering it or have time to do
>> so as I know that most kernel devs are very busy :)
>>
>> Recently, FreeBSD has reported that it's slowly switching from GCC to
>> LLVM/CLANG for compilers. What are the thoughts of the Linux kernel
>> devs about this and do they things that it's a good idea to go in the
>> same direction as FreeBSD (switching over to LLVM)?
>
> It is highly unlikely we will switch compilers away from gcc -- we have
> enough problems just supporting multiple gcc versions.
>
> That said, you could join the effort to get LLVM to compile under the
> kernel.  Here is a meta-bug for that:
> http://llvm.org/bugs/show_bug.cgi?id=4068
>
> It is a moot question until LLVM can actually build a bootable kernel :)

Thanks for the quick answer! I was not aware that currently there's
lots of work until CLANG can compile a bootable Linux kernel. Thanks
for the link too. I'll dig through it

>
> Regards,
>
>        Jeff
>
>
>
>

Re: Linux and LLVM

[Theodore Tso]

On Fri, May 08, 2009 at 09:50:04PM +0200, Grozdan wrote:
> 2009/5/8 Jeff Garzik <jeff@garzik.org>:
> > Grozdan wrote:
> >>
> > It is highly unlikely we will switch compilers away from gcc -- we have
> > enough problems just supporting multiple gcc versions.
> >
> > That said, you could join the effort to get LLVM to compile under the
> > kernel.  Here is a meta-bug for that:
> > http://llvm.org/bugs/show_bug.cgi?id=4068
> >
> > It is a moot question until LLVM can actually build a bootable kernel :)
> 
> Thanks for the quick answer! I was not aware that currently there's
> lots of work until CLANG can compile a bootable Linux kernel. Thanks
> for the link too. I'll dig through it

Note also that the list of architectures currently supported by Linux:

http://en.wikipedia.org/wiki/Linux_kernel_portability_and_supported_architectures

Seems to be a superset of the architectures supported by LLVM, and the
CLANG web page states that it is production quality only on the x86
and x86_64 architectures.  So "switch away from" seems to be a rather
ambitious goal.  

As Jeff has suggested, getting CLANG to at least be able to compile a
single Linux kernel, on one architecture is clearly the first step.

The next step will be to work on fixing regressions on a wide variety
of different kernel configurations such that in people's mind, that
when a kernel bug is submitted, the probability that it is a compiler
bug must be roughly the same as in GCC --- in other words, that for
any give bug report submitted, P(LLVM/CLANG compiler bug) must be less
than P(GCC compiler bug).  At that point, assuming there is a large
enough community of users who prefer to use LLVM/CLANG (presumably
because its code generator is proving itself superior to gcc), it
would be possible to call LLVM/CLANG "supported" under Linux.

I suspect we have quite a ways to go before we get to that point, and
even then there is the issue of the architectures which aren't yet
supported by LLVM.

Regards,

					- Ted