[LLVMdev] Operand, instruction

John Criswell criswell at uiuc.edu
Tue Apr 13 23:38:19 PDT 2010

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"

An add or sub should be a BinaryOperator, if memory serves me 
correctly.  Looking at mainline doxygen, there's a whole bunch of 
CreateNSWAdd, CreateNSWMul, etc. methods.  It's one of those that you want.

-- John T.

> 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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