Add an llvm instruction - c++11

Question

I'm new to LLVM and I was wondering if you could help me building a pass to duplicate instructions in LLVM IR, the problem I'm facing is that the cloned instructions couldn't be returned using (user class), is this the correct way to do it ? are there any other ways (excluding this http://llvm.org/docs/ExtendingLLVM.html)
My pass:
BasicBlock *B = I->getParent();
if (auto *op = dyn_cast<BinaryOperator>(&*I))
{
auto temp = op->clone();
B->getInstList().insert(op, temp);
temp->setName(op->getName());
if (temp->getOpcode() == Instruction::Add)
{
IRBuilder<> builder(temp); //building the cloned instruction
Value *lhs = temp->getOperand(0);
Value *rhs = temp->getOperand(1);
Value *add1 = builder.CreateAdd(lhs, rhs);
for (auto &v : temp->uses()) {
User *user = v.getUser(); // A User is anything with operands.
user->setOperand(v.getOperandNo(), add1);
}
}
}

Answer 1

BasicBlock *B = I->getParent();
if (auto *op = dyn_cast<BinaryOperator>(&*I))
{
auto temp = op->clone();
B->getInstList().insert(op, temp);
temp->setName(op->getName());
At this point you have successfully cloned your instruction and inserted it in the BasicBlock where the original instruction lives.
if (temp->getOpcode() == Instruction::Add)
{
IRBuilder<> builder(temp); //building the cloned instruction
Value *lhs = temp->getOperand(0);
Value *rhs = temp->getOperand(1);
Value *add1 = builder.CreateAdd(lhs, rhs);
Now you are building an IRBuilder. An IRBuilder as a helper class that allows you easy insertion of instructions in your code. But it does not build your temp instruction. The temp instruction is already there from calling clone an inserting it in the BasicBlock.
You create another copy of your original instruction (add1).
for (auto &v : temp->uses()) {
User *user = v.getUser(); // A User is anything with operands.
user->setOperand(v.getOperandNo(), add1);
}
Here you are updating all users of temp. But at this point temp has no users. temp is just a clone of your original instruction. You have created two copies of your original instruction that are not used and will be removed by dead code elimination.
What you want to do is to replace all uses of op by one of your copies.
An easier way to achieve this is to use RAUW ReplaceAllUsesWith.
BasicBlock *B = I->getParent();
if (auto *op = dyn_cast<BinaryOperator>(&*I))
{
auto temp = op->clone();
B->getInstList().insert(op, temp);
temp->setName(op->getName());
op->replaceAllUsesWith(temp);
}
With RAUW now op is dead (i.e., has no users) and your cloned instruction is alive.