C++ DominatorTree::splitBlock方法代码示例

/// UpdateAnalysisInformation - Update DominatorTree, LoopInfo, and LCCSA
/// analysis information.
static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
                                      ArrayRef<BasicBlock *> Preds,
                                      Pass *P, bool &HasLoopExit) {
  if (!P) return;

  LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
  Loop *L = LI ? LI->getLoopFor(OldBB) : 0;

  // If we need to preserve loop analyses, collect some information about how
  // this split will affect loops.
  bool IsLoopEntry = !!L;
  bool SplitMakesNewLoopHeader = false;
  if (LI) {
    bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID);
    for (ArrayRef<BasicBlock*>::iterator
           i = Preds.begin(), e = Preds.end(); i != e; ++i) {
      BasicBlock *Pred = *i;

      // If we need to preserve LCSSA, determine if any of the preds is a loop
      // exit.
      if (PreserveLCSSA)
        if (Loop *PL = LI->getLoopFor(Pred))
          if (!PL->contains(OldBB))
            HasLoopExit = true;

      // If we need to preserve LoopInfo, note whether any of the preds crosses
      // an interesting loop boundary.
      if (!L) continue;
      if (L->contains(Pred))
        IsLoopEntry = false;
      else
        SplitMakesNewLoopHeader = true;
    }
  }

  // Update dominator tree if available.
  DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
  if (DT)
    DT->splitBlock(NewBB);

  if (!L) return;

  if (IsLoopEntry) {
    // Add the new block to the nearest enclosing loop (and not an adjacent
    // loop). To find this, examine each of the predecessors and determine which
    // loops enclose them, and select the most-nested loop which contains the
    // loop containing the block being split.
    Loop *InnermostPredLoop = 0;
    for (ArrayRef<BasicBlock*>::iterator
           i = Preds.begin(), e = Preds.end(); i != e; ++i) {
      BasicBlock *Pred = *i;
      if (Loop *PredLoop = LI->getLoopFor(Pred)) {
        // Seek a loop which actually contains the block being split (to avoid
        // adjacent loops).
        while (PredLoop && !PredLoop->contains(OldBB))
          PredLoop = PredLoop->getParentLoop();

        // Select the most-nested of these loops which contains the block.
        if (PredLoop && PredLoop->contains(OldBB) &&
            (!InnermostPredLoop ||
             InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
          InnermostPredLoop = PredLoop;
      }
    }

    if (InnermostPredLoop)
      InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase());
  } else {
    L->addBasicBlockToLoop(NewBB, LI->getBase());
    if (SplitMakesNewLoopHeader)
      L->moveToHeader(NewBB);
  }
}

/// SplitBlockPredecessors - This method transforms BB by introducing a new
/// basic block into the function, and moving some of the predecessors of BB to
/// be predecessors of the new block.  The new predecessors are indicated by the
/// Preds array, which has NumPreds elements in it.  The new block is given a
/// suffix of 'Suffix'.
///
/// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and
/// DominanceFrontier, but no other analyses.
BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB, 
                                         BasicBlock *const *Preds,
                                         unsigned NumPreds, const char *Suffix,
                                         Pass *P) {
  // Create new basic block, insert right before the original block.
  BasicBlock *NewBB =
    BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB);
  
  // The new block unconditionally branches to the old block.
  BranchInst *BI = BranchInst::Create(BB, NewBB);
  
  // Move the edges from Preds to point to NewBB instead of BB.
  for (unsigned i = 0; i != NumPreds; ++i)
    Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
  
  // Update dominator tree and dominator frontier if available.
  DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0;
  if (DT)
    DT->splitBlock(NewBB);
  if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0)
    DF->splitBlock(NewBB);
  AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0;
  
  
  // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
  // node becomes an incoming value for BB's phi node.  However, if the Preds
  // list is empty, we need to insert dummy entries into the PHI nodes in BB to
  // account for the newly created predecessor.
  if (NumPreds == 0) {
    // Insert dummy values as the incoming value.
    for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
      cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
    return NewBB;
  }
  
  // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
  for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
    PHINode *PN = cast<PHINode>(I++);
    
    // Check to see if all of the values coming in are the same.  If so, we
    // don't need to create a new PHI node.
    Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
    for (unsigned i = 1; i != NumPreds; ++i)
      if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
        InVal = 0;
        break;
      }
    
    if (InVal) {
      // If all incoming values for the new PHI would be the same, just don't
      // make a new PHI.  Instead, just remove the incoming values from the old
      // PHI.
      for (unsigned i = 0; i != NumPreds; ++i)
        PN->removeIncomingValue(Preds[i], false);
    } else {
      // If the values coming into the block are not the same, we need a PHI.
      // Create the new PHI node, insert it into NewBB at the end of the block
      PHINode *NewPHI =
        PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
      if (AA) AA->copyValue(PN, NewPHI);
      
      // Move all of the PHI values for 'Preds' to the new PHI.
      for (unsigned i = 0; i != NumPreds; ++i) {
        Value *V = PN->removeIncomingValue(Preds[i], false);
        NewPHI->addIncoming(V, Preds[i]);
      }
      InVal = NewPHI;
    }
    
    // Add an incoming value to the PHI node in the loop for the preheader
    // edge.
    PN->addIncoming(InVal, NewBB);
    
    // Check to see if we can eliminate this phi node.
    if (Value *V = PN->hasConstantValue(DT != 0)) {
      Instruction *I = dyn_cast<Instruction>(V);
      if (!I || DT == 0 || DT->dominates(I, PN)) {
        PN->replaceAllUsesWith(V);
        if (AA) AA->deleteValue(PN);
        PN->eraseFromParent();
      }
    }
  }
  
  return NewBB;
}