path: root/lib/Transforms/Utils/LoopSimplify.cpp

//===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
//
// Insert Loop pre-headers and exit blocks into the CFG for each function in the
// module.  This pass updates loop information and dominator information.
//
// Loop pre-header insertion guarantees that there is a single, non-critical
// entry edge from outside of the loop to the loop header.  This simplifies a
// number of analyses and transformations, such as LICM.
//
// Loop exit-block insertion guarantees that all exit blocks from the loop
// (blocks which are outside of the loop that have predecessors inside of the
// loop) are dominated by the loop header.  This simplifies transformations such
// as store-sinking that are built into LICM.
//
// Note that the simplifycfg pass will clean up blocks which are split out but
// end up being unnecessary, so usage of this pass does not neccesarily
// pessimize generated code.
//
//===----------------------------------------------------------------------===//

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

namespace {
  Statistic<> NumInserted("preheaders", "Number of pre-header nodes inserted");

  struct Preheaders : public FunctionPass {
    virtual bool runOnFunction(Function &F);
    
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      // We need loop information to identify the loops...
      AU.addRequired<LoopInfo>();
      AU.addRequired<DominatorSet>();

      AU.addPreserved<LoopInfo>();
      AU.addPreserved<DominatorSet>();
      AU.addPreserved<ImmediateDominators>();
      AU.addPreserved<DominatorTree>();
      AU.addPreserved<DominanceFrontier>();
      AU.addPreservedID(BreakCriticalEdgesID);  // No crit edges added....
    }
  private:
    bool ProcessLoop(Loop *L);
    BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
                                       const std::vector<BasicBlock*> &Preds);
    void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
    void InsertPreheaderForLoop(Loop *L);
  };

  RegisterOpt<Preheaders> X("preheaders", "Natural loop pre-header insertion");
}

// Publically exposed interface to pass...
const PassInfo *LoopPreheadersID = X.getPassInfo();
Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }


/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
/// it in any convenient order) inserting preheaders...
///
bool Preheaders::runOnFunction(Function &F) {
  bool Changed = false;
  LoopInfo &LI = getAnalysis<LoopInfo>();

  for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
    Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);

  return Changed;
}


/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
/// all loops have preheaders.
///
bool Preheaders::ProcessLoop(Loop *L) {
  bool Changed = false;

  // Does the loop already have a preheader?  If so, don't modify the loop...
  if (L->getLoopPreheader() == 0) {
    InsertPreheaderForLoop(L);
    NumInserted++;
    Changed = true;
  }

  // Regardless of whether or not we added a preheader to the loop we must
  // guarantee that the preheader dominates all exit nodes.  If there are any
  // exit nodes not dominated, split them now.
  DominatorSet &DS = getAnalysis<DominatorSet>();
  BasicBlock *Header = L->getHeader();
  for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
    if (!DS.dominates(Header, L->getExitBlocks()[i])) {
      RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
      assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
             "RewriteLoopExitBlock failed?");
      NumInserted++;
      Changed = true;
    }

  const std::vector<Loop*> &SubLoops = L->getSubLoops();
  for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
    Changed |= ProcessLoop(SubLoops[i]);
  return Changed;
}

/// SplitBlockPredecessors - Split the specified block into two blocks.  We want
/// to move the predecessors specified in the Preds list to point to the new
/// block, leaving the remaining predecessors pointing to BB.  This method
/// updates the SSA PHINode's, but no other analyses.
///
BasicBlock *Preheaders::SplitBlockPredecessors(BasicBlock *BB,
                                               const char *Suffix,
                                       const std::vector<BasicBlock*> &Preds) {
  
  // Create new basic block, insert right before the original block...
  BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);

  // The preheader first gets an unconditional branch to the loop header...
  BranchInst *BI = new BranchInst(BB);
  NewBB->getInstList().push_back(BI);
  
  // For every PHI node in the block, insert a PHI node into NewBB where the
  // incoming values from the out of loop edges are moved to NewBB.  We have two
  // possible cases here.  If the loop is dead, we just insert dummy entries
  // into the PHI nodes for the new edge.  If the loop is not dead, we move the
  // incoming edges in BB into new PHI nodes in NewBB.
  //
  if (!Preds.empty()) {  // Is the loop not obviously dead?
    for (BasicBlock::iterator I = BB->begin();
         PHINode *PN = dyn_cast<PHINode>(I); ++I) {
      
      // Create the new PHI node, insert it into NewBB at the end of the block
      PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
        
      // Move all of the edges from blocks outside the loop to the new PHI
      for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
        Value *V = PN->removeIncomingValue(Preds[i]);
        NewPHI->addIncoming(V, Preds[i]);
      }
      
      // Add an incoming value to the PHI node in the loop for the preheader
      // edge
      PN->addIncoming(NewPHI, NewBB);
    }
    
    // Now that the PHI nodes are updated, actually move the edges from
    // Preds to point to NewBB instead of BB.
    //
    for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
      TerminatorInst *TI = Preds[i]->getTerminator();
      for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
        if (TI->getSuccessor(s) == BB)
          TI->setSuccessor(s, NewBB);
    }
    
  } else {                       // Otherwise the loop is dead...
    for (BasicBlock::iterator I = BB->begin();
         PHINode *PN = dyn_cast<PHINode>(I); ++I)
      // Insert dummy values as the incoming value...
      PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
  }  
  return NewBB;
}

// ChangeExitBlock - This recursive function is used to change any exit blocks
// that use OldExit to use NewExit instead.  This is recursive because children
// may need to be processed as well.
//
static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
  if (L->hasExitBlock(OldExit)) {
    L->changeExitBlock(OldExit, NewExit);
    const std::vector<Loop*> &SubLoops = L->getSubLoops();
    for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
      ChangeExitBlock(SubLoops[i], OldExit, NewExit);
  }
}


/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
/// preheader, this method is called to insert one.  This method has two phases:
/// preheader insertion and analysis updating.
///
void Preheaders::InsertPreheaderForLoop(Loop *L) {
  BasicBlock *Header = L->getHeader();

  // Compute the set of predecessors of the loop that are not in the loop.
  std::vector<BasicBlock*> OutsideBlocks;
  for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
       PI != PE; ++PI)
      if (!L->contains(*PI))           // Coming in from outside the loop?
        OutsideBlocks.push_back(*PI);  // Keep track of it...
  
  // Split out the loop pre-header
  BasicBlock *NewBB =
    SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
  
  //===--------------------------------------------------------------------===//
  //  Update analysis results now that we have preformed the transformation
  //
  
  // We know that we have loop information to update... update it now.
  if (Loop *Parent = L->getParentLoop())
    Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());

  // If the header for the loop used to be an exit node for another loop, then
  // we need to update this to know that the loop-preheader is now the exit
  // node.  Note that the only loop that could have our header as an exit node
  // is a sibling loop, ie, one with the same parent loop, or one if it's
  // children.
  //
  const std::vector<Loop*> *ParentSubLoops;
  if (Loop *Parent = L->getParentLoop())
    ParentSubLoops = &Parent->getSubLoops();
  else       // Must check top-level loops...
    ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();

  // Loop over all sibling loops, performing the substitution (recursively to
  // include child loops)...
  for (unsigned i =