Question

In llvm, can one trace back to the instruction that defines the value for a particular register? For example, if I have an instruction as:

  %add14 = add i32 %add7, %add5

Is here a way for me to trace back to the instruction where add5 is defined?

Answer 1

First of all, there are no registers in LLVM IR: all those things with % in their names are just names of values. You don’t store information inside those things, they are not variables or memory locations, they are just names. I recommend reading about SSA form, which helps explains this further.

In any case, what you need to do is invoke the getOperand(n) method on the instruction to get its nth operand – for example, getOperand(0) in your example will return the value named %add7. You can then check whether that value is indeed an instruction (as opposed to, say, a function argument) by checking its type (isa<Instruction>).

To emphasize – calling the getOperand method will give you the actual place in which the operand is defined, nothing else is required.