Clang (LLVM C compiler) builds a working Linux kernel

http://article.gmane.org/gmane.comp.compilers.clang.devel/11432

by zdw • 16 years ago

188 59 16 years ago

article.gmane.org

tzury• 16 years ago

The LLVM project is one of those open-source project which originally aimed to do some great things and eventually found out to do even greater.

fleitz2• 16 years ago

It's even self hosting. This is huge.

Rusky3• 16 years ago

The Linux kernel is explicitly built using GNU extensions, which means that in addition to implementing the C standard, they have to deal with kernel-specific issues as well as whatever ideas GCC has. Clang as a work-alike for GCC will be/is incredibly useful and an incredible accomplishment.

While being able to compile Linux is definitely helpful and a good sign of compiler maturity, the kernel developers target GCC and I'm not sure that will ever change. This (and similar projects) will probably somewhat limit Clang adoption as people will always need to keep GCC around rather than relying on the portability of using standards. :(

Hopefully the competition Clang brings to the table will push at least some open source projects to be a little less reliant on GNU.

chc2• 16 years ago

On the other hand, Apple is pushing LLVM very hard — Xcode is growing more and more integrated with it in each release — and I would not be surprised to see Clang declared the one true Mac and iOS compiler while GCC is shown the way out. So I think we might have some healthy competition on our hands.

rwl2• 16 years ago

What's wrong with relying on GNU? It's Free software that will love on as long as the concept does. You can be assured that GNU will never have the rug pulled out from under it by a corporate affiliate, anyway.

mfukar• 16 years ago

I don't think he meant there's anything wrong with it, only that clang/llvm might not feel like implementing certain GNU C extensions.

Besides, pluralism is always good, right?

wash8• 16 years ago

I'm a hacker. This means I tend to have a lot of strong opinions about a lot of things. There are things I don't like about GNU, there are things I do like about GNU. The same can be said for Linux, Clang, LLVM, Apple, and a long list of other things. I mostly try to ignore my personal opinions, because I'd rather code than argue about epolitics.

Rusky3• 16 years ago

Nothing's wrong with GNU; the problem is when it becomes a de-facto standard. Having more than one compiler available will improve both as they compete, and it keeps code more time-proof and gives developers and users more options.

sedachv• 16 years ago

I always get a chuckle out of the knuckleheads claiming "C is portable." Even ignoring all the libc and type size assumptions, there are so many "portable" programs out there that won't compile on anything but GCC because the extensions are on by default and people sometimes don't even know they're using them. This gets real amusing when a new version of GCC decides to deprecate them (one example of this I ran into recently was lvalue casts).

j_baker2• 16 years ago

Why would one want to do this (aside from it being a cool project)? What advantages does clang have over gcc for kernel hacking?

hendler• 16 years ago

(disclaimer: I can't answer this with authority)

The reasons I get excited:

* speed: faster compilation

* research: interesting code introspection you can't get from GCC.

* proof of concept: if you can build the linux kernel, you can probably build almost any C project

* edit: as mentioned in other comments, competition is good.

Legion• 16 years ago

This isn't a "hey, go do this" post. This is a milestone in the development of Clang.

j_baker2• 16 years ago

Of course. I was just curious if this had any other value.

fleitz2• 16 years ago

It may make certain bugs more apparent.

If you have development going on with a different compiler and it's actively maintained to work on both compilers then overall you'd get similar advantages in your code base to those that write cross-platform software. Incidentally, you get the same effect with cross-platform as you're usually targeting different compilers.

Also, with less compiler specific code it will be easier to get the kernel working with other compilers, perhaps ICC.

mzl4• 16 years ago

Unfortunately, Clang is still a bit shaky at times. In our (C++) project, we get a miscompilation in Clang that we don't get with gcc, the visual studio compiler, nor the intel compiler. It will be great when it is more stable: clang cuts the compile time by half in comparison with gcc (from 18 minutes to 9 minutes on my computer).

aidenn0• 16 years ago

In general having 2 compatible implementations with little or no shared-code base allows for more easily finding bugs in your code. A lot of times bugs won't show up in testing because one compiler just happens to mask them. This isn't always a problem, since the bugs may stay masked. However, in a changing codebase they are likely to pop up in the future.

btilly• 16 years ago

I think that http://blog.llvm.org/2010/04/amazing-feats-of-clang-error-re... gives a number of nice features that clang brings to the table which could be very useful for kernel hackers.

wash8• 16 years ago

Clang's diagnostics and static analysis? Take, for example, https://patchwork.kernel.org/patch/36060/, a serious problem in the kernel that was recently discovered. Given the size of Linux, it would be highly inefficient to manually find and fix every instances of this issue.

Currently, Clang doesn't support the work-around option that GCC provides to prevent the aforementioned issue. I could just implement the GNU work-around, but with Clang, it's far easier to write a scanner which will identify every place in the Linux source code where those dangerous semantics appear.

Clang might not be mature enough to compile Linux for distribution. The difference between GCC and Clang is that Clang is not a compiler. Clang is a modular API that provides the tools to build C language front-ends to the LLVM compiler infrastructure. The Clang compiler driver is just one implementation of an application built using the Clang libraries.

Why would one want to do this? Well...

   http://sourceforge.net/mailarchive/message.php?msg_name=2010http://sourceforge.net/mailarchive/message.php?msg_name=20101007001758.6bcd3d3b%40Pegasus1007001758.6bcd3d3b%40Pegasus

   http://clang-analyzer.llvm.org/

   http://clang.llvm.org/docs/libIndex.html

   http://llvm.org/ProjectsWithLLVM/#LENS

   http://llvm.org/ProjectsWithLLVM/#spedi

Think outside of the box :)

caf• 16 years ago

I'm pretty sure that bug was the canonical example of the usefulness of Coccinelle in kernel development. This semantic patch was used to find and fix all the instances of it:

    // Copyright: (C) 2009 Gilles Muller, Julia Lawall, INRIA, DIKU.  GPLv2.
    
    @@
    type T;
    expression E;
    identifier i,fld;
    statement S;
    @@
    
    - T i = E->fld;
    + T i;
      ... when != E
          when != i
      if (E == NULL) S
    + i = E->fld;

See http://coccinelle.lip6.fr/impact_linux.php for more.

jacquesm• 16 years ago

That's quite the milestone to achieve. I'm actually really happy to see this happen in light of the negative rap C seems to be getting just about everywhere.

C is far from dead, it may have its warts but it is still very much in use in all kinds of places and a solid knowledge of C has never hurt anybody (as far as I know).

Being able to compile the linux kernel and to get it to boot is a non-trivial exercise in GCC compatibility, I'm very impressed with this.

I wonder if there ever will be an official 'blessing' by the kernel devs of the LLVM C compiler.

zdw• 16 years ago

It's unlikely that it'll get that 'blessing' - a while back certain versions of GCC were recommended against by Linus.

Also, on some esoteric platforms, GCC dropped compatibility. For example, OpenBSD still supports an older version of GCC for VAX and m88k platforms IIRC.

igravious• 16 years ago

I don't see why not in the long run if the devs bend over backward to maintain GCC compatibility on x86. It is never a great idea to have a single point of failure even if that point is the wonderful and awe-inspiring GCC. It is amazing to think that nearly 20 years after conception Linux maybe be built with an alternate tool chain. This is an incredible feat that has far-reaching consequences for the health of the ecosystem. Maybe Linus wasn't keen on the idea but if some people step up and bake in the support it'll be de facto blessed.

Slightly off-topic, does anyone know if the Gold linker works with Clang?

wash8• 16 years ago

Gold should work.

ICC has been able to compile Linux for awhile, I think.

alanh2• 16 years ago

With all the flak Apple has taken over dropping its commitment to one "-VM," the JVM, I'd just like to point out Apple is the primary corporate sponsor of LLVM (I understand). How many plates must they keep in the air? This one is flying pretty high.

wash8• 16 years ago

I'm a 19 year old college student, actually. I have no affiliation with Apple, nor a formal relation with Clang or LLVM.

alanh2• 16 years ago

Sorry, but your HN bio is empty and you didn't submit the article. Did you spearhead the effort? And is my comment regarding Apple and Clang incorrect? Thanks!

codedivine• 16 years ago

Sure Apple is one of the bigger sponsors of LLVM but LLVM started out as an academic project and even now lots of academics contribute to it. Apple hired the guy who originally wrote LLVM and now uses it a lot. But many other companies also utilize LLVM and contribute back to it (though not as heavily). For example, some AMD compiler folks I know said they were going to commit some changes to LLVM (not sure if they did). Adobe also uses it AFAIK for some specific tools.

However, LLVM and JVM dont have much in common. JVM is popular more for its baked in libraries than its instruction set or VM efficiency (though HotSpot is a darned good VM too).

bigmac• 16 years ago

It looks like this was targeting an x86_64 version of the kernel. For a good reference on LLVM-supported backends, see this table: http://llvm.org/docs/CodeGenerator.html#targetfeatures

Clearly, x86 gets the most love. x86, ARM, PPC, and SPARC are listed as "Generally Reliable." Quite an accomplishment, given the difficulty of building up a new compiler from scratch. For now, MIPS is notably missing from the party.

wash8• 16 years ago

Unfortunately, I'm a student with limited resources. I have three boxes, a macbook and two Atom desktops, all x86_64.

newman314• 16 years ago

If you promise to get things working on ARM, I might be able to get you something.

IdeaHamster• 16 years ago

…but it still can't compile Ruby 1.9 :-P

program• 16 years ago

I am very happy. I was very pleased when back in may 20 2010 they announced that clang passed its first fully-successful Boost regression test run. But this is another big step ahead.

The clang static analyzer is now part of my everyday life as a programmer and everytime I read "did you mean..." on the console I am amazed as the first time. It saves me a lot of time (even compile time.)

rbanffy4• 16 years ago

My only worry, in the light of Oracle's spat with Google over Dalvik and its lack of teeth over the OpenJDK, is what kind of patent protection the BSD-like license of LLVM offers against Apple in case it decides every Linux kernel on every non-iPhone smartphone (something that will probably happen in a couple years) infringes some patent they have that touches LLVM in some way.

Any lawyer wants to chime in?

bramcohen• 16 years ago

The GPL doesn't offer any special protection against third party patents, and there's no way it could, given the way patents work.

rbanffy4• 16 years ago

I am not considering 3rd party patents, but Apple's. If Apple includes some patented tech into a contribution to LLVM or Clang, what guarantees I have they will not sue non-Apple downstream users?

coliveira• 16 years ago

Well, the technology used (the C language) is 30 years old. What is new is LLVM, but that was not created by Apple either.

mhd2• 16 years ago

Wasn't tcc able to do the same[1] quite a while ago?

[1] http://bellard.org/tcc/

praptak• 16 years ago

Even better, it could compile and boot Linux directly from its source code: http://bellard.org/tcc/tccboot.html

mahmud• 16 years ago

I don't think tinycc and llvm are even in the same league in terms of optimizations.

Tinycc is, well, tiny. You can get a working compiler just using syntax-directed translation schemes and it would compile any valid program. The output would even boot, but it would make baby-Muchnick cry.

mhd2• 16 years ago

True, no one is comparing their performance characteristics, it's just that getting the Linux kernel to compile isn't a unique feat where you need to be on the same level as gcc and llvm. You "just" need ANSI C with GNU extensions. And not break on a couple dozen corner cases.

ddp• 16 years ago

This is good news. FYI, Clang/LLVM's been building FreeBSD for a while now: http://wiki.freebsd.org/BuildingFreeBSDWithClang

brianm• 16 years ago

FWIW, I use clang over gcc (unless there is a major overriding reason to use gcc) just for the fabulous error messages.

http://clang.llvm.org/diagnostics.html gives some good examples, and my experience has been that the examples given on that page are pretty spot on.

Clang (LLVM C compiler) builds a working Linux kernel

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