[LLVMdev] Printing Function Arguments

Nick Lewycky nicholas at mxc.ca
Sun Sep 27 20:53:22 PDT 2009

ivtm wrote:
> Hey Oscar,
> I want to extract information from the instruction.
> Think writing a simple interpreter.
> I already have the CallInst instance (described above in the message).
> Via ci->getOperand(1) say I can get the 'i32 8' parameter and I can get the
> 'i32' and '8' separately as Nick described.
> But I need to extract the %0 from the CallInst instance somehow. I am not
> sure what the exact method to call is.
> I need that btw, for every instruction that writes, e.g. loads, adds, muls,
> etc...There has got to be a generic method for that. 

This is a common misunderstanding. When you look at something like:

   %a = add i32 %x, %y

it's common to think "Well, I've got the Instruction* which is on the 
right hand side, how do I get the %a on the left hand side"? The answer 
is that the whole thing is actually:

   %I->getName() = I->getOpcode() I->op_begin()..I->op_end()

There is only the Instruction *I. It _is_ the value that it produces. 
There is no register it's being stored into, that Instruction* is itself 
the definition of the register!

Note that registers are immutable (being a static single assignment 
form) and that we have an infinite register set.


> I am looking at the existing LLVM passes to figure out how they extract
> stuff, but since many are only computing the small relevant info that they
> need, they do not always need to extract all the components of an
> instruction. (I've looked at the interpreter too, but it is doing other
> stuff).
> Anyway..concretely, I need to extract the return register for an
> instruction.
> Óscar Fuentes wrote:
>> ivtm <martinaide1 at yahoo.com> writes:
>>> Another question, I need to get the "%0" below in the:
>>> %0 = tail call i32 (...)* @__FFF (i32 8) nounwind; <i32> [#uses=1]
>>> that is, the return register. 
>> What information do you want, exactly?
>> In your example, %0 is the CallInst. So if you have
>> CallInst *ci = CallInst::Create(...
>> then use `ci' whenever you want to use %0.
>>> I am wondering in general, where should I look in the llvm codebase for
>>> parsing instructions ?
>>> I am looking at existing passes and also the header files like
>>> Function.h,
>>> etc to see what methods they have, but it takes a while to figure out the
>>> basic methods...
>> This is a bit better than looking at header files:
>> http://llvm.org/doxygen/classes.html
>> -- 
>> Óscar
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