llvm dev - Nov 2008 - [LLVMdev] How do I get the result of an instruction?

Thanks Eli! 
But I still have a question. What you have written is in IR. When I am writing
the pass, I donot have access to the result of the instruction   
(%resulttoprint in your example). All I have is a Instruction* right? So, how do
I get the result of the instruction? Am I missing something here?

Thanks,
Bhavani 



--- On Sun, 11/16/08, Eli Friedman <eli.friedman at gmail.com> wrote:
> From: Eli Friedman <eli.friedman at gmail.com>
> Subject: Re: [LLVMdev] How do I get the result of an instruction?
> To: bhavi63 at yahoo.com
> Cc: "LLVM Developers Mailing List" <llvmdev at
cs.uiuc.edu>, "John Criswell" <criswell at uiuc.edu>
> Date: Sunday, November 16, 2008, 9:22 PM
> On Sun, Nov 16, 2008 at 7:54 AM, bhavani krishnan
> <bhavi63 at yahoo.com> wrote:
> > Hi,
> >
> > I am writing an optimization pass where I need to
> instrument the code such that I need to store the results of
> some instructions in file. Using llc -march=cpp option I
> figured out how to add a function(say writeToFile) which
> takes char* parameter and writes to file. Now, I need put in
> a CallInst which calls writeToFile passing the Instruction
> result as parameter. How do I do this?
> >
> > So, in my optimization pass...
> > Func *myprint = makewriteToFile() //creates a function
> which writes to file
> > for (Function::iterator i = func->begin(), e > func->end(); i
!= e; ++i)
> > {
> >        blk=i;
> >        for (BasicBlock::iterator j = blk->begin(),
> k = blk->end(); j != k; ++j){
> >            Instruction *inst = j;
> >            //if inst satisfies my condition, write
> reults to file
> >            CallInst *CallPrint > CallInst::Create(myprint, ???,
"", j);
> >            CallPrint->setTailCall(true);
> >       }
> > }
> > What do I put in the ???. How do I cast Instruction
> *inst into char * which can be passed into the function?
> 
> Well, you can do something like the following:
> define i32 @f() nounwind {
> entry:
> 	%x = alloca i32
> 	%resulttoprint = call i32 (...)* @a() nounwind
> ;start instrumentation
> 	store i32 %resulttoprint, i32* %x, align 4
> 	%x1 = bitcast i32* %x to i8*
> 	call void @print(i8* %x1) nounwind
> ;end instrumentation
> 	ret i32 %resulttoprint
> }
> 
> That said, you might need to do something that's aware
> of the type of
> the result; printing a value in human-readable form
> requires calling
> something like printf.
> 
> -Eli