[LLVMdev] Compile Linux Kernel module into LLVM bitcode

[chenping]

Greatings,       I am encountering the same issue with Kevin. Does anyone know how to compile the linux kernel to .bc file?     Is there any oneline guide for this? Thanks!~
Bests,Ping     

> Date: Sat, 1 Jun 2013 20:15:56 +0200
> From: baldrick at free.fr
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Compile Linux Kernel module into LLVM bitcode
> 
> Hi Kevin,
> 
> On 01/06/13 09:59, Kevin Boos wrote:
> > Greetings,
> >
> > I am trying to compile a linux kernel module (currently a small part of the gpu driver) into the bitcode ".bc" format so that I can run a pass on it using the "opt" command. This pass will count the number of times copy_to/from_user() is invoked.
> >
> > Compiling the gnu driver kernel modules works with clang as a front end. However, I am unable to get the "-emit-llvm" flag to work.
> >
> > I am using the typical Makefile system (kbuild) included with the kernel, as such:
> > $ make V=1 CC=clang CFLAGS="-emit-llvm -c"  drivers/gpu/drm/drm_drv.o
> >
> > I have tried setting KBUILD_CFLAGS or KCFLAGS instead of CFLAGS, but nothing I do gets it to work. Trying these few different variations gets me to the point where it doesn't actually use the  "-emit-llvm" flag in the clang (previously gcc) commands that are shown with the verbose V=1 option.  If the "-emit-llvm" flag *does* make it into the clang command, it doesn't create the .bc file
> 
> -emit-llvm doesn't change the name of the file output, it changes what is put
> in the output file (bitcode rather than object code).  So it is normal not to
> see a file called xyz.bc, it will still be called xyz.o.  If you look inside
> it you should see that it starts with BC (followed by nasty stuff).  You can
> use llvm-dis to turn this into a file containing human readable IR.  The
> following error output shows that indeed bitcode is being output:
> 
>   and sometimes errors out like this:
> >
> > ====================
> > objdump: scripts/mod/.tmp_empty.o: File format not recognized
> >    if [ "-pg" = "-pg" ]; then if [ scripts/mod/empty.o != "scripts/mod/empty.o" ]; then /home/kevin/split_io_Linux/scripts/recordmcount  "scripts/mod/empty.o"; fi; fi;
> >    gcc -Wp,-MD,scripts/mod/.mk_elfconfig.d -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer     -o scripts/mod/mk_elfconfig scripts/mod/mk_elfconfig.c
> >    scripts/mod/mk_elfconfig < scripts/mod/empty.o > scripts/mod/elfconfig.h
> > Error: not ELF
> 
> Yup, it's not ELF, it's bitcode!  This is going to be a problem for something
> like the linux kernel that does a lot of tricky stuff with generated object
> files.  I suggest you only try compiling your particular module as bitcode,
> and compiling everything else normally.  For example, compile everything using
> clang but without -emit-llvm, and tell make to produce verbose output.  This
> should show you the clang command line it uses to compile your module.  Run
> that command line by hand, but adding -emit-llvm.  This should produce bitcode
> for your module.
> 
> > make[2]: *** [scripts/mod/elfconfig.h] Error 1
> > make[1]: *** [scripts/mod] Error 2
> > make: *** [scripts] Error 2
> > ====================
> >
> >
> > Essentially, I need to know how to use LLVM/clang with a linux kernel makefile to output the .bc bitcode file. I thought clang was a good drop-in replacement for gcc, but it hasn't worked out for me yet.
> 
> You didn't just drop it in, you made a major change, telling it to output
> bitcode rather than ELF.
> 
> Ciao, Duncan.
> 
>   How can I change the "make" command so that it produces .bc?
> >
> > Thanks in advance,
> > Kevin
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> >
> 
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