BreakCriticalEdges.cpp

//===- BreakCriticalEdges.cpp - Critical Edge Elimination Pass ------------===//
//
// BreakCriticalEdges pass - Break all of the critical edges in the CFG by
// inserting a dummy basic block.  This pass may be "required" by passes that
// cannot deal with critical edges.  For this usage, the structure type is
// forward declared.  This pass obviously invalidates the CFG, but can update
// forward dominator (set, immediate dominators, and tree) information.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Function.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Support/CFG.h"
#include "Support/Statistic.h"

namespace {
  Statistic<> NumBroken("break-crit-edges", "Number of blocks inserted");

  struct BreakCriticalEdges : public FunctionPass {
    virtual bool runOnFunction(Function &F);
    
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreserved<DominatorSet>();
      AU.addPreserved<ImmediateDominators>();
      AU.addPreserved<DominatorTree>();
      AU.addPreservedID(LoopPreheadersID);   // No preheaders deleted.
    }
  };

  RegisterOpt<BreakCriticalEdges> X("break-crit-edges",
                                    "Break critical edges in CFG");
}

// Publically exposed interface to pass...
const PassInfo *BreakCriticalEdgesID = X.getPassInfo();
Pass *createBreakCriticalEdgesPass() { return new BreakCriticalEdges(); }


// isCriticalEdge - Return true if the specified edge is a critical edge.
// Critical edges are edges from a block with multiple successors to a block
// with multiple predecessors.
//
bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum) {
  assert(SuccNum < TI->getNumSuccessors() && "Illegal edge specification!");
  if (TI->getNumSuccessors() == 1) return false;

  const BasicBlock *Dest = TI->getSuccessor(SuccNum);
  pred_const_iterator I = pred_begin(Dest), E = pred_end(Dest);

  // If there is more than one predecessor, this is a critical edge...
  assert(I != E && "No preds, but we have an edge to the block?");
  ++I;        // Skip one edge due to the incoming arc from TI.
  return I != E;
}

// SplitCriticalEdge - Insert a new node node to split the critical edge.  This
// will update DominatorSet, ImmediateDominator and DominatorTree information if
// it is available, thus calling this pass will not invalidate either of them.
//
void SplitCriticalEdge(TerminatorInst *TI, unsigned SuccNum, Pass *P) {
  assert(isCriticalEdge(TI, SuccNum) &&
         "Cannot break a critical edge, if it isn't a critical edge");
  BasicBlock *TIBB = TI->getParent();

  // Create a new basic block, linking it into the CFG.
  BasicBlock *NewBB = new BasicBlock(TIBB->getName()+"_crit_edge");
  BasicBlock *DestBB = TI->getSuccessor(SuccNum);
  // Create our unconditional branch...
  BranchInst *BI = new BranchInst(DestBB);
  NewBB->getInstList().push_back(BI);
  
  // Branch to the new block, breaking the edge...
  TI->setSuccessor(SuccNum, NewBB);

  // Insert the block into the function... right after the block TI lives in.
  Function &F = *TIBB->getParent();
  F.getBasicBlockList().insert(TIBB->getNext(), NewBB);

  // If there are any PHI nodes in DestBB, we need to update them so that they
  // merge incoming values from NewBB instead of from TIBB.
  //
  for (BasicBlock::iterator I = DestBB->begin();
       PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
    // We no longer enter through TIBB, now we come in through NewBB.
    PN->replaceUsesOfWith(TIBB, NewBB);
  }

  // If we don't have a pass object, we can't update anything...
  if (P == 0) return;

  // Now update analysis information.  These are the analyses that we are
  // currently capable of updating...
  //

  // Should we update DominatorSet information?
  if (DominatorSet *DS = P->getAnalysisToUpdate<DominatorSet>()) {
    // The blocks that dominate the new one are the blocks that dominate TIBB
    // plus the new block itself.
    DominatorSet::DomSetType DomSet = DS->getDominators(TIBB);
    DomSet.insert(NewBB);  // A block always dominates itself.
    DS->addBasicBlock(NewBB, DomSet);
  }

  // Should we update ImmdediateDominator information?
  if (ImmediateDominators *ID = P->getAnalysisToUpdate<ImmediateDominators>()) {
    // TIBB is the new immediate dominator for NewBB.  NewBB doesn't dominate
    // anything.
    ID->addNewBlock(NewBB, TIBB);
  }
  
  // Should we update DominatorTree information?
  if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>()) {
    DominatorTree::Node *TINode = DT->getNode(TIBB);
    
    // The new block is not the immediate dominator for any other nodes, but
    // TINode is the immediate dominator for the new node.
    //
    DT->createNewNode(NewBB, TINode);
  }
}

// runOnFunction - Loop over all of the edges in the CFG, breaking critical
// edges as they are found.
//
bool BreakCriticalEdges::runOnFunction(Function &F) {
  bool Changed = false;
  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    TerminatorInst *TI = I->getTerminator();
    if (TI->getNumSuccessors() > 1)
      for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
        if (isCriticalEdge(TI, i)) {
          SplitCriticalEdge(TI, i, this);
          ++NumBroken;
          Changed = true;
        }
  }

  return Changed;
}