[LLVMdev] Is it safe to insert instructions in batches into BBs?

[Marcello Maggioni]

Hi,

I'm getting a very strange behaviour while adding already created 
instructions in batches into basicblocks instead of creating and 
inserting them immediately.

Because I need to insert instructions in a certain specific order inside 
multiple different BBs I found it easy to use the IRBuilder to create 
instructions without inserting them into a BB, storing them somewhere 
(vector, map ... etc) and later on inserting all of them in one go into 
their positions in the BBs.

What I do is similar to this:

IRBuilder<> IB(Context);

Vector.push_back(IB.CreateXXX());
Vector.push_back(IB.CreateXXX());
Vector.push_back(IB.CreateXXX());
...
// Other stuff

for (SmallVectorImpl<Instruction*>::iterator I = Vector.begin(), E = 
Vector.end(); I != E; ++I) {

   SomeBB->getInstList().insert(SomeInstruction, *I);

}

If I add the instructions like this I get strange results. It's like if 
llvm gets corrupted or something similar, because in the backend strange 
things happen, like MCRegisterInfo returning wrong register aliasing 
information in a random fashion (sometimes happens and sometimes doesn't) .

If I create and add the instructions on the fly like this:

IB.SetInsertionPoint(SomeInstruction);

for () {
   IB.CreateXXX();
}

Everything is fine.

So , in the end, is it safe to add instructions like in the first method 
I mentioned or actually the only safe way is to use the second way? In 
case it should be safe, what could be the cause of the random breakages?

Cheers,
Marcello

PS = Some of the instructions I'm adding are using one the output value 
of the other.