[LLVMdev] Question adding dummy basic blocks

Zhang Qiuyu qiuyu at ee.ucla.edu
Tue Dec 7 00:42:10 PST 2004


I got a problem when I am trying to add a dummy basic block.

Let say there are two blocks, A and B.


I am trying to generate new BB called C which is located between A and B, but not break the edge of AB. The graph is like the following

 \       /
   \   /
There is new BB 'C' with edges AC and CB.

It is kind of like what breakcriticaledge pass does. But we just pick up basic block(TIBB) whose the num of successor is 1 (like unconditional branch). And we try to replace the Terminator Inst of this basic block by contrusting a conditional branch(A to B and A to C). Meanwhile we generate a new basic block and unconditional branch which is from C to B.

Finally, We got some problems due to PHINODE. I knew the reason, but I failed to  fix it.

The code is like the following

if( TI->getNumSuccessors() ==1){
   BasicBlock *TIBB=TI->getParent();
   BasicBlock *DestBB=TI->getSuccessor(0);

   BasicBlock *NewBB = new BasicBlock(TIBB->getName()+DestBB->getName()+"dummy_bb",&F);
   new BranchInst(DestBB, NewBB);
   BranchInst *NewBr = new BranchInst(DestBB, NewBB, V_dummy);
   ReplaceInstWithInst(TI, NewBr);

// the following code, we are trying to change value of PHINODE, and trying to find some examples to solve, but we failed 
// Since the DestBB is added to one more predecessor, and we need addIncoming value. 
// TIBB got one more successor, do we need change something?
// For NewBB, we need add the incoming value from TIBB?
// For the code listed in breakcriticaledge, it makes sure. But for our case, I don't know to do that.

Probably, the reasons are like above, but I really don't know how sovle them. 
I would appreciate if you can give me some hint. 


> > 2) For splitting BB, the way what I did is to pick up instruction
> > randomly and do splitting before this instruction. But sometime I got
> > error when I run the Pass. It seems some of instruction set cannot be
> > splitted, right? the instruction like 'phi..'. If so, how could we check
> > this instruction to see if it can be splitted or not.
> PHI nodes are special and can only be at certain places in the CFG (for
> example, the number of incoming values to a PHI must match the number of
> predecessors in the CFG.  All PHI nodes are guaranteed to start at the
> head of a block, so just split after them.

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