[LLVMdev] compile linux kernel

[Dale Johannesen]

On Sep 27, 2008, at 4:34 PM, Ashish Bijlani wrote:

> Thanks for the help. I've a couple of questions though:
>
> How does LLVM deal with inline assembly?

It's been implemented piece by piece on an as-needed basis.  At this  
point most of the things people actually use should work.   llvm-gcc  
has seen the Linux kernel, so most usages in there ought to work.

The symptoms here look more like a preprocessor problem than an asm  
problem.  Your asm snippet works fine with llvm's top of tree, and I  
suspect the real problem is something else.  If you compile with -save- 
temps what does the .i file look like?

> I'm trying to compile kernel and I get this error probably because
> LLVM is not able to handle inline assembly. I'm using LLVM-2.3
>
> code snippet from "arch/x86_64/kernel/asm-offsets.c"
> ....
>
> #define DEFINE(sym, val) \
>        asm volatile("\n->" #sym " %0 " #val : : "i" (val))
> ....
>
>    DEFINE(__NR_syscall_max, sizeof(syscalls) - 1);
>    return 0;
>
> # make CROSS_COMPILE=llvm- V=1
>
>  llvm-gcc -Wp,-MD,arch/x86_64/kernel/.syscall.o.d  -nostdinc -isystem
> /home/ashish/llvm/llvm-gcc4.2-2.3.source/obj/../install/lib/gcc/ 
> x86_64-unknown-linux-gnu/4.2.1/include
> -D__KERNEL__ -Iinclude  -include include/linux/autoconf.h -emit-llvm
> -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing
> -fno-common -Werror-implicit-function-declaration -O2  -mtune=generic
> -m64 -mno-red-zone -mcmodel=kernel -pipe -Wno-sign-compare
> -fno-asynchronous-unwind-tables -funit-at-a-time -mno-sse -mno-mmx
> -mno-sse2 -mno-3dnow -maccumulate-outgoing-args
> -fomit-frame-pointer -g  -fno-stack-protector
> -Wdeclaration-after-statement -Wno-pointer-sign
> -D"KBUILD_STR(s)=#s" -D"KBUILD_BASENAME=KBUILD_STR(syscall)"
> -D"KBUILD_MODNAME=KBUILD_STR(syscall)" -c -o
> arch/x86_64/kernel/.tmp_syscall.o arch/x86_64/kernel/syscall.c
> arch/x86_64/kernel/syscall.c:22: error: '__NR_syscall_max' undeclared
> here (not in a function)
> arch/x86_64/kernel/syscall.c:24: error: array index in initializer not
> of integer type
> arch/x86_64/kernel/syscall.c:24: error: (near initialization for
> 'sys_call_table')
> In file included from arch/x86_64/kernel/syscall.c:25:
> include/asm-x86_64/unistd.h:16: error: array index in non-array  
> initializer
> include/asm-x86_64/unistd.h:16: error: (near initialization for
> 'sys_call_table')
> include/asm-x86_64/unistd.h:18: error: array index in non-array  
> initializer
>
> Note that I use "-emit-llvm" option with llvm-gcc. Is this correct of
> am I missing something?
> How can I fix this error?
>
> Thanks,
> Ashish
>
> On Sat, Sep 27, 2008 at 7:03 PM, Andrew Lenharth  
> <andrewl at lenharth.org> wrote:
>> On Fri, Sep 26, 2008 at 9:19 PM, Ashish Bijlani
>> <ashish.bijlani at gmail.com> wrote:
>>> Hi,
>>>
>>> I'm trying to compile linux-2.6.23.16 with llvm-2.3. Is it at all
>>> possible?
>>
>> Yes, but it requires significant hacking, and the result for 2.6 is a
>> mostly bitcode kernel with a few userspace shared libraries linked in
>> as objcode (yes, the kernel builds .so files and includes them in the
>> binary).  There are a few changes here and there you can make to the
>> kernel code to produce significantly nicer bitcodes too (e.g. rather
>> than having module level asm  that makes symbols weak, you can just
>> have gcc do it).
>>
>> The modification to the make system is not difficult, but it requires
>> some work and an understanding of the kernel build system.
>>
>> Andrew
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