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 :)

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.

Oh. Well, I guess they beat me to the punch.

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.

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).

I do a lot of C++ template metaprogramming, so I end up running into 3-5 Clang internal compiler errors (In my circles, "ICE"s) a week, if not more. With GCC I maybe run into 3-5 a month.

The difference is that I can pinpoint the error. Clang dies gracefully, with a stack trace, source file location, and, for parse errors, the tokens that are currently being parsed. The problem might not always be easy to fix, but I never have to waste time trying to find it.

If you want a more stable Clang, then build one. ;)

File a bug report. They are very receptive of them. Especially if you get a miscompilation.

I hope to do so, I really like the project and want it to succeed. Unfortunately, I'm not yet sure how to reduce the 100000 lines to something manageable with the problem clear :)

Do you suspect the bug to be in clang (the front-end) itself, or in LLVM's optimizer or back-end? For the latter cases, bugpoint is included in the LLVM source tree and does a good job of narrowing things down, so long as you provide a program whose output should be deterministic (this may involve some work on your part if your program usually depends on data from the outside world). It is described here http://llvm.org/docs/Bugpoint.html . It may be useful for Clang bugs too, I'm not sure.

People may help you by suggesting strategies for finding a small test case that tickles the same problem if you post to the mailing list here http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev . I've posted a couple times and found people surprisingly responsive and helpful.

That looks like an interesting tool, I'll have to try it out. The program is deterministic given a fixed seed.

The bug depends on whether or not optimization is on, so it might be in the backend. What I've heard is that the C++ FE in particular still does have some troubles with miscompilations, so that is why I suspect it.

Clang is really good at C and Objective-C. C++ is kind of its weak point, most likely because C++ is a couple of orders of magnitude more complex than either of those languages.

I was actually surprised that it could compile our stuff at all, and that it runs mostly all right. We do use a lot of the language, and we need at least gcc version 4.2 for example.

(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.

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

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

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.

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.