LoopRotation.cpp

//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Devang Patel and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Loop Rotation Pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "loop-rotation"

#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/SmallVector.h"
#include <map>

using namespace llvm;

#define MAX_HEADER_SIZE 16

STATISTIC(NumRotated, "Number of loops rotated");
namespace {

  cl::opt<unsigned>
  RotateThreshold("rotate-threshold", cl::init(200), cl::Hidden,
                  cl::desc("The cut-off point for loop rotating"));

  class VISIBILITY_HIDDEN InsnReplacementData {
  public:
    InsnReplacementData(Instruction *O, Instruction *P, Instruction *H) 
      : Original(O), PreHeader(P), Header(H) {}
  public:
    Instruction *Original; // Original instruction
    Instruction *PreHeader; // New pre-header replacement
    Instruction *Header; // New header replacement
  };

  class VISIBILITY_HIDDEN LoopRotate : public LoopPass {

  public:
    bool runOnLoop(Loop *L, LPPassManager &LPM);
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequiredID(LCSSAID);
      AU.addPreservedID(LCSSAID);
      //AU.addRequired<LoopInfo>();
      //AU.addPreserved<LoopInfo>();
    }

    // Helper functions

    /// Do actual work
    bool rotateLoop(Loop *L, LPPassManager &LPM);
    
    /// Initialize local data
    void initialize();

    /// Make sure all Exit block PHINodes have required incoming values.
    /// If incoming value is constant or defined outside the loop then
    /// PHINode may not have an entry for new pre-header. 
    void  updateExitBlock();

    /// Return true if this instruction is used outside original header.
    bool usedOutsideOriginalHeader(Instruction *In);

    /// Find Replacement information for instruction. Return NULL if it is
    /// not available.
    InsnReplacementData *findReplacementData(Instruction *I);

  private:

    Loop *L;
    BasicBlock *OrigHeader;
    BasicBlock *OrigPreHeader;
    BasicBlock *OrigLatch;
    BasicBlock *NewHeader;
    BasicBlock *NewPreHeader;
    BasicBlock *Exit;

    SmallVector<InsnReplacementData, MAX_HEADER_SIZE> RD;
  };
  
  RegisterPass<LoopRotate> X ("loop-rotate", "Rotate Loops");
}

LoopPass *llvm::createLoopRotatePass() { return new LoopRotate(); }

bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
  
  bool RotatedOneLoop = false;
  initialize();

  // One loop can be rotated multiple times.
  while (rotateLoop(Lp,LPM)) {
    RotatedOneLoop = true;
    initialize();
  }

  return RotatedOneLoop;
}

bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {

  L = Lp;
  if ( NumRotated >= RotateThreshold) 
    return false;

  OrigHeader =  L->getHeader();
  OrigPreHeader = L->getLoopPreheader();
  OrigLatch = L->getLoopLatch();

  // If loop has only one block then there is not much to rotate.
  if (L->getBlocks().size() <= 1)
    return false;

  if (!OrigHeader || !OrigLatch || !OrigPreHeader)
    return false;

  // If loop header is not one of the loop exit block then
  // either this loop is already rotated or it is not 
  // suitable for loop rotation transformations.
  if (!L->isLoopExit(OrigHeader))
    return false;

  BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
  if (!BI)
    return false;

  std::vector<BasicBlock *> ExitBlocks;
  L->getExitBlocks(ExitBlocks);
  if (ExitBlocks.size() > 1)
    return false;

  // Find new Loop header. NewHeader is a Header's one and only successor
  // that is inside loop.  Header's all other successors are out side the
  // loop. Otherwise loop is not suitable for rotation.
  for (unsigned index = 0; index < BI->getNumSuccessors(); ++index) {
    BasicBlock *S = BI->getSuccessor(index);
    if (L->contains(S)) {
      if (!NewHeader) 
        NewHeader = S;
      else
        // Loop Header has two successors inside loop. This loop is
        // not suitable for rotation.
        return false;
    } else {
      if (!Exit)
        Exit = S;
      else
        // Loop has multiple exits.
        return false;
    }
  }
  assert (NewHeader && "Unable to determine new loop header");

  // Check size of original header and reject
  // loop if it is very big.
  if (OrigHeader->getInstList().size() > MAX_HEADER_SIZE)
    return false;

  // Now, this loop is suitable for rotation.

  // Copy Prepare PHI nodes and other instructions from original header
  // into new pre-header. Unlike original header, new pre-header is
  // not a member of loop. New pre-header has only one predecessor,
  // that is original loop pre-header.
  //
  // New loop header is one and only successor of original header that 
  // is inside the loop. All other original header successors are outside 
  // the loop. Copy PHI Nodes from original header into new loop header. 
  // Add second incoming value, from new loop pre-header into these phi 
  // nodes. If a value defined in original header is used outside original 
  // header then new loop header will need new phi nodes with two incoming 
  // values, one definition from original header and second definition is 
  // from new loop pre-header (which is a clone of original header definition).

  NewPreHeader = new BasicBlock("bb.nph", OrigHeader->getParent(), OrigHeader);
  for (BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
       I != E; ++I) {
    Instruction *In = I;

    if (PHINode *PN = dyn_cast<PHINode>(I)) {

      // Create new PHI node with one value incoming from OrigPreHeader.
      // NewPreHeader has only one predecessor, OrigPreHeader.
      PHINode *NPH = new PHINode(In->getType(), In->getName());
      NPH->addIncoming(PN->getIncomingValueForBlock(OrigPreHeader), 
		       OrigPreHeader);
      NewPreHeader->getInstList().push_back(NPH);
      
      // Create new PHI node with two incoming values for NewHeader.
      // One incoming value is from OrigLatch (through OrigHeader) and 
      // second incoming value is from NewPreHeader.
      PHINode *NH = new PHINode(In->getType(), In->getName());
      NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
      NH->addIncoming(NPH, NewPreHeader);
      NewHeader->getInstList().push_front(NH);

      RD.push_back(InsnReplacementData(In, NPH, NH));
    } else {
      // This is not a PHI instruction. Insert its clone into NewPreHeader.
      // If this instruction is using a value from same basic block then
      // update it to use value from cloned instruction.
      Instruction *C = In->clone();
      C->setName(In->getName());
      NewPreHeader->getInstList().push_back(C);

      // If this instruction is used outside this basic block then
      // create new PHINode for this instruction.
      Instruction *NewHeaderReplacement = NULL;
      if (usedOutsideOriginalHeader(In)) {
        PHINode *PN = new PHINode(In->getType(), In->getName());
        PN->addIncoming(In, OrigHeader);
        PN->addIncoming(C, NewPreHeader);
        NewHeader->getInstList().push_front(PN);
        NewHeaderReplacement = PN;
      } 
      RD.push_back(InsnReplacementData(In, C, NewHeaderReplacement));
    }
  }

  // Update new pre-header.
  // Rename values that are defined in original header to reflects values
  // defined in new pre-header.
  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
         I = RD.begin(), E = RD.end(); I != E; ++I) {
    
    InsnReplacementData IRD = (*I);
    Instruction *In = IRD.Original;
    Instruction *C = IRD.PreHeader;
    
    if (C->getParent() != NewPreHeader)
      continue;

    // PHINodes uses value from pre-header predecessors.
    if (isa<PHINode>(In))
      continue;

    for (unsigned opi = 0; opi < In->getNumOperands(); ++opi) {
      if (Instruction *OpPhi = dyn_cast<PHINode>(In->getOperand(opi))) {
        if (InsnReplacementData *D = findReplacementData(OpPhi))
          C->setOperand(opi, D->PreHeader);
      }
      else if (Instruction *OpInsn = 
               dyn_cast<Instruction>(In->getOperand(opi))) {
        if (InsnReplacementData *D = findReplacementData(OpInsn))
          C->setOperand(opi, D->PreHeader);
      }
    }
  }

  // Rename uses of original header instructions to reflect their new
  // definitions (either from new pre-header node or from newly created
  // new header PHINodes.
  //
  // Original header instructions are used in
  // 1) Original header:
  //
  //    If instruction is used in non-phi instructions then it is using
  //    defintion from original heder iteself. Do not replace this use
  //    with definition from new header or new pre-header.
  //
  //    If instruction is used in phi node then it is an incoming 
  //    value. Rename its use to reflect new definition from new-preheader
  //    or new header.
  //
  // 2) Inside loop but not in original header
  //
  //    Replace this use to reflect definition from new header.
  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
         I = RD.begin(), E = RD.end(); I != E; ++I) {

    InsnReplacementData IRD = (*I);
    if (!IRD.Header)
      continue;

    Instruction *OldPhi = IRD.Original;
    Instruction *NewPhi = IRD.Header;

    // Before replacing uses, collect them first, so that iterator is
    // not invalidated.
    SmallVector<Instruction *, 16> AllUses;
    for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
         UI != UE; ++UI ) {
      Instruction *U = cast<Instruction>(UI);
      AllUses.push_back(U);
    }

    for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(), 
           UE = AllUses.end(); UI != UE; ++UI) {
      Instruction *U = *UI;
      BasicBlock *Parent = U->getParent();

      // Used inside original header
      if (Parent == OrigHeader) {
        // Do not rename uses inside original header non-phi instructions.
        if (!isa<PHINode>(U))
          continue;
        PHINode *PU = dyn_cast<PHINode>(U);
        // Do not rename uses inside original header phi nodes, if the
        // incoming value is for new header.
        if (PU->getBasicBlockIndex(NewHeader) != -1
            && PU->getIncomingValueForBlock(NewHeader) == U)
          continue;

       U->replaceUsesOfWith(OldPhi, NewPhi);
       continue;
      }

      // Used inside loop, but not in original header.
      if (L->contains(U->getParent())) {
        if (U != NewPhi )
          U->replaceUsesOfWith(OldPhi, NewPhi);
        continue;
      }

      // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
      assert ( U->getParent() == Exit && "Need to propagate new PHI into Exit blocks");
      assert (isa<PHINode>(U) && "Use in Exit Block that is not PHINode");        

      PHINode *UPhi = cast<PHINode>(U);

      // UPhi already has one incoming argument from original header. 
      // Add second incoming argument from new Pre header.
      
      UPhi->addIncoming(IRD.PreHeader, NewPreHeader);
    }
  }
  
  /// Make sure all Exit block PHINodes have required incoming values.
  updateExitBlock();

  // Update CFG

  // Removing incoming branch from loop preheader to original header.
  // Now original header is inside the loop.
  OrigHeader->removePredecessor(OrigPreHeader);

  // Establish NewPreHeader as loop preheader. Add unconditional branch
  // from original loop pre-header to new loop pre-header. Add NewPreHEader
  // in loop nest.
  BranchInst *PH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
  PH_BI->setSuccessor(0, NewPreHeader);
  LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
  if (Loop *PL = LI.getLoopFor(OrigPreHeader))
    PL->addBasicBlockToLoop(NewPreHeader, LI);

  // Make NewHeader as the new header for the loop.
  L->moveToHeader(NewHeader);

  NumRotated++;
  return true;
}


/// Make sure all Exit block PHINodes have required incoming values.
/// If incoming value is constant or defined outside the loop then
/// PHINode may not have an entry for new pre-header. 
void LoopRotate::updateExitBlock() {

  for (BasicBlock::iterator I = Exit->begin(), E = Exit->end();
       I != E; ++I) {

    if (!isa<PHINode>(I))
      break;

    PHINode *PN = dyn_cast<PHINode>(I);

    if (PN->getBasicBlockIndex(NewPreHeader) == -1) {
      Value *V = PN->getIncomingValueForBlock(OrigHeader);
      if (isa<Constant>(V))
        PN->addIncoming(V, NewPreHeader);
      else {
        InsnReplacementData *IRD = findReplacementData(cast<Instruction>(V));
        assert (IRD && IRD->PreHeader && "Missing New Preheader Instruction");
        PN->addIncoming(IRD->PreHeader, NewPreHeader);
      }
    }
  }
}


/// Initialize local data
void LoopRotate::initialize() {
  L = NULL;
  OrigHeader = NULL;
  OrigPreHeader = NULL;
  NewHeader = NULL;
  NewPreHeader = NULL;
  Exit = NULL;

  RD.clear();
}

/// Return true if this instruction is used outside original header.
bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {

  for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
       UI != UE; ++UI) {
    Instruction *U = cast<Instruction>(UI);
    if (U->getParent() != OrigHeader) {
      if (L->contains(U->getParent()))
        return true;
    }
  }

  return false;
}

/// Find Replacement information for instruction. Return NULL if it is
/// not available.
InsnReplacementData *LoopRotate::findReplacementData(Instruction *In) {

  // Since RD is small, linear walk is OK.
  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
         I = RD.begin(), E = RD.end(); I != E; ++I) 
    if ((*I).Original == In)
      return &(*I);

  return NULL;
}