[LLVMdev] regarding LLVM Pass

[John Criswell]

On 3/23/11 11:15 PM, Gondi, Kalpana 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?

You essentially want to create a call instruction to printf.  Look for 
the doxygen documentation on the llvm.org web site and look for the 
llvm::CallInst class.  The Create() method of CallInst is what you want 
to use.

> 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?

Try to find examples that use the AliasAnalysis interface.  The 
interface itself should be defined in a header file; it should be pretty 
easy to use, although the underlying implementation is still pretty 
simple, as far as I know.

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

Clang is useful for working with source-level ASTs.

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

Yes and no, depending on one's perspective.  I ported Linux 2.4 to a 
virtual architecture, meaning that I ripped out all the inline asm code 
and replaced it with calls to my VM which implemented it own assembly 
code.  The C code parts of the kernel were compiled with LLVM.

I think other people have compiled Linux 2.6 out-of-the-box (or pretty 
close to it) with newer versions of LLVM.  I'll let them comment.

-- John T.

> Please help me with all these. And I appreciate your support and patience.
>
> Thanks,
> GK
>
>
>
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