[LLVMdev] Inserting MachineBasicBlock(s) before a MachineBasicBlock

[Fernando Magno Quintao Pereira]

>
> I want to be able to do two things with LLVM (both just before code
> emission):
>
> 1. Insert a MachineBasicBlock just before a MachineBasicBlock.
> There is a function called AddPredecessor(). However, the comment says that
> it does not update the actual CFG. I want to redirect all CFG edges that are
> incoming to this MachineBasicBlock to the new one I create, and add just one
> outgoing edge (no branch) to the newly formed MachineBasicBlock to jump to
> the original MachineBasicBlock. How can I do this?

Hi,

    I had an old pass to break critical edges, and I am sending you the 
code below. I Last tested it with LLVM 2.1. It inserts a basic block 
between src and dst.

Fernando

------------------------------------------------------------------

MachineBasicBlock* CriticalEdgeRemoval_Fer::SplitCriticalMachineEdge(
   MachineBasicBlock* src,
   MachineBasicBlock* dst
) {
   const BasicBlock* srcBB = src->getBasicBlock();

   MachineBasicBlock* crit_mbb = new MachineBasicBlock(srcBB);

   // modify the llvm control flow graph
   src->removeSuccessor(dst);
   src->addSuccessor(crit_mbb);
   crit_mbb->addSuccessor(dst);

   // insert the new block into the machine function.
   src->getParent()->getBasicBlockList().insert(src->getParent()->end(),
                                                crit_mbb);

   // insert a unconditional branch linking the new block to dst
   const TargetMachine& TM = src->getParent()->getTarget();
   const TargetInstrInfo* TII = TM.getInstrInfo();
   std::vector<MachineOperand> emptyConditions;
   TII->InsertBranch(*crit_mbb, dst, (MachineBasicBlock*)0, 
emptyConditions);

   // modify every branch in src that points to dst to point to the new
   // machine basic block instead:
   MachineBasicBlock::iterator mii = src->end();
   bool found_branch = false;
   while (mii != src->begin()) {
     mii--;
     // if there are no more branches, finish the loop
     if (!TII->isTerminatorInstr(mii->getOpcode())) {
       break;
     }

     // Scan the operands of this branch, replacing any uses of dst with
     // crit_mbb.
     for (unsigned i = 0, e = mii->getNumOperands(); i != e; ++i) {
       MachineOperand & mo = mii->getOperand(i);
       if (mo.isMachineBasicBlock() &&
           mo.getMachineBasicBlock() == dst) {
         found_branch = true;
         mo.setMachineBasicBlock(crit_mbb);
       }
     }
   }

   // TODO: This is tentative. It may be necessary to fix this code. Maybe
   // I am inserting too many gotos, but I am trusting that the asm printer
   // will optimize the unnecessary gotos.
   if(!found_branch) {
     TII->InsertBranch(*src, crit_mbb, (MachineBasicBlock*)0, 
emptyConditions);
   }

   /// Change all the phi functions in dst, so that the incoming block be
   /// crit_mbb, instead of src
   for(mii = dst->begin(); mii != dst->end(); mii++) {
     /// the first instructions are always phi functions.
     if(mii->getOpcode() != TargetInstrInfo::PHI)
       break;

     for (unsigned u = 0; u != mii->getNumOperands(); ++u)
       if (mii->getOperand(u).isMachineBasicBlock() &&
           mii->getOperand(u).getMachineBasicBlock() == src)
         mii->getOperand(u).setMachineBasicBlock(crit_mbb);
   }

   return crit_mbb;
}