[LLVMdev] regarding LLVM Pass

[Eli Friedman]

On Wed, Mar 23, 2011 at 9:15 PM, Gondi, Kalpana <kgondi2 at uic.edu> wrote:
> Hi All,
>  I am a newbie to LLVM and I would like to write an LLVM pass where I can
> transform C code. Say, I would like to introduce a print statement after
> every loop. Could you please provide me any hints as how I should proceed
> to write such transformation using LLVM?
> Also, I would like to analyze C Code and transform. Say, I would like to
> use Alias analysis and decide to introduce print statements for some
> pointer variables inside the code itself. Any suggestion as where I should
> start?
>
> I've been going through documentation like, writing an LLVM Pass , but I
> guess, I need much more exposure than just going through such
> documentation.

If you haven't looked at http://llvm.org/docs/tutorial/ , I would
suggest taking a look; it's only partially relevant to what you're
asking, but should give you a better feel for how stuff works.

> Also, do I need to look at CLANG? How is it different from writing the
> LLVM pass to perform the above mentioned tasks?

It's different: clang has a rewriter you can use, for example, to
insert a call after every loop.  You end up with a different
definition of "loop", though: at the clang level, you'll see the AST
nodes for "for" and "while" loops; at the IR level, you'll see
constructs which are structurally loops.

> Finally, did anyone compile Linux kernel using LLVM and booted the same? I
> am facing the error like "unsupported inline asm:...".

IIRC, llvm-gcc is affected by some bugs related to inline asm which
will likely never be fixed that affect the Linux kernel (llvm-gcc is
considered deprecated).  See
http://llvm.org/bugs/attachment.cgi?id=3486 for building it with
clang; the kernel tends to use all sorts of obscure/nasty gcc flags
and constructs, which makes things tricky.

-Eli