[LLVMdev] Operand, instruction

help__me_please krishnadhan at cse.iitb.ac.in
Wed Apr 14 00:25:10 PDT 2010

Thanks for the reply.

Nick Lewycky wrote:
> There's an entire tutorial at http://llvm.org/docs/tutorial which 
> includes covers the IRBuilder to generate all sorts of IR, including the 
> add instruction. You can skip ahead to chapter 3 if you just want to see 
> LLVM API, but I suggest you read the whole tutorial if you have the time.
> Nick
> help__me_please wrote:
>> Thanks for reply.
>> I have used AllocaInst, it's working but i think it's only for allocating
>> some memory for new variable. And CallInst creates a call instruction. I
>> am
>> looking for creating a add or sub instruction.
>> I used function instruction(), which gives me error "error: cannot
>> allocate
>> an object of abstract type ‘llvm::Instruction’" and also
>> "Instruction.h:28:
>> note:   because the following virtual functions are pure within
>> ‘llvm::Instruction’:", "Instruction.h:50: note: 	virtual
>> llvm::Instruction*
>> llvm::Instruction::clone(llvm::LLVMContext&) const"
>> Also i have used BinaryOperator::create(), but it gives error that there
>> is
>> no function called create().
>> Thanks again for your reply.
>> John Criswell-2 wrote:
>>> help__me_please wrote:
>>>> Can you please give an example of creating an instruction (for example
>>>> add
>>>> instructions with two operand a and b)? I am trying instruction() for a
>>>> while, but no success yet.
>>> You need to look for the appropriate subclass of llvm::Instruction and
>>> find the method for creating a new instruction.  The method is usually a
>>> static method and takes arguments pointing to the values to use as
>>> operands.  Doxygen is your best resource for finding these methods.
>>> For example, if you look at
>>> http://llvm.org/doxygen/classllvm_1_1AllocaInst.html, you can see that
>>> the AllocaInst class (which represents an alloca instruction) has a
>>> standard constructor method that takes the type of object to allocate,
>>> the name of the new alloca instruction, an instruction before which to
>>> insert the alloca instruction, etc.
>>> As another example, the CallInst class
>>> (http://llvm.org/doxygen/classllvm_1_1CallInst.html) represents a call
>>> instruction and has a static Create() method that you can use to create
>>> a new call instruction.
>>> -- John T.
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