llvm/lib/Transforms/Utils/UnrollLoop.cpp

//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements some loop unrolling utilities. It does not define any
// actual pass or policy, but provides a single function to perform loop
// unrolling.
//
// It works best when loops have been canonicalized by the -indvars pass,
// allowing it to determine the trip counts of loops easily.
//
// The process of unrolling can produce extraneous basic blocks linked with
// unconditional branches.  This will be corrected in the future.
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "loop-unroll"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include "llvm/BasicBlock.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"

using namespace llvm;

/* TODO: Should these be here or in LoopUnroll? */
STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
STATISTIC(NumUnrolled,    "Number of loops unrolled (completely or otherwise)");

/// RemapInstruction - Convert the instruction operands from referencing the
/// current values into those specified by ValueMap.
static inline void RemapInstruction(Instruction *I,
                                    DenseMap<const Value *, Value*> &ValueMap) {
  for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
    Value *Op = I->getOperand(op);
    DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
    if (It != ValueMap.end()) Op = It->second;
    I->setOperand(op, Op);
  }
}

/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
/// only has one predecessor, and that predecessor only has one successor.
/// The LoopInfo Analysis that is passed will be kept consistent.
/// Returns the new combined block.
static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) {
  // Merge basic blocks into their predecessor if there is only one distinct
  // pred, and if there is only one distinct successor of the predecessor, and
  // if there are no PHI nodes.
  BasicBlock *OnlyPred = BB->getSinglePredecessor();
  if (!OnlyPred) return 0;

  if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
    return 0;

  DOUT << "Merging: " << *BB << "into: " << *OnlyPred;

  // Resolve any PHI nodes at the start of the block.  They are all
  // guaranteed to have exactly one entry if they exist, unless there are
  // multiple duplicate (but guaranteed to be equal) entries for the
  // incoming edges.  This occurs when there are multiple edges from
  // OnlyPred to OnlySucc.
  //
  while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
    PN->replaceAllUsesWith(PN->getIncomingValue(0));
    BB->getInstList().pop_front();  // Delete the phi node...
  }

  // Delete the unconditional branch from the predecessor...
  OnlyPred->getInstList().pop_back();

  // Move all definitions in the successor to the predecessor...
  OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());

  // Make all PHI nodes that referred to BB now refer to Pred as their
  // source...
  BB->replaceAllUsesWith(OnlyPred);

  std::string OldName = BB->getName();

  // Erase basic block from the function...
  LI->removeBlock(BB);
  BB->eraseFromParent();

  // Inherit predecessor's name if it exists...
  if (!OldName.empty() && !OnlyPred->hasName())
    OnlyPred->setName(OldName);

  return OnlyPred;
}

/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
/// if unrolling was succesful, or false if the loop was unmodified. Unrolling
/// can only fail when the loop's latch block is not terminated by a conditional
/// branch instruction. However, if the trip count (and multiple) are not known,
/// loop unrolling will mostly produce more code that is no faster.
///
/// The LoopInfo Analysis that is passed will be kept consistent.
///
/// If a LoopPassManager is passed in, and the loop is fully removed, it will be
/// removed from the LoopPassManager as well. LPM can also be NULL.
bool llvm::UnrollLoop(Loop *L, unsigned Count, LoopInfo* LI,
                      LPPassManager* LPM) {
  assert(L->isLCSSAForm());

  BasicBlock *Header = L->getHeader();
  BasicBlock *LatchBlock = L->getLoopLatch();
  BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());

  Function *Func = Header->getParent();
  Function::iterator BBInsertPt = next(Function::iterator(LatchBlock));

  if (!BI || BI->isUnconditional()) {
    // The loop-rotate pass can be helpful to avoid this in many cases.
    DOUT << "  Can't unroll; loop not terminated by a conditional branch.\n";
    return false;
  }

  // Find trip count
  unsigned TripCount = L->getSmallConstantTripCount();
  // Find trip multiple if count is not available
  unsigned TripMultiple = 1;
  if (TripCount == 0)
    TripMultiple = L->getSmallConstantTripMultiple();

  if (TripCount != 0)
    DOUT << "  Trip Count = " << TripCount << "\n";
  if (TripMultiple != 1)
    DOUT << "  Trip Multiple = " << TripMultiple << "\n";

  // Effectively "DCE" unrolled iterations that are beyond the tripcount
  // and will never be executed.
  if (TripCount != 0 && Count > TripCount)
    Count = TripCount;

  assert(Count > 0);
  assert(TripMultiple > 0);
  assert(TripCount == 0 || TripCount % TripMultiple == 0);

  // Are we eliminating the loop control altogether?
  bool CompletelyUnroll = Count == TripCount;

  // If we know the trip count, we know the multiple...
  unsigned BreakoutTrip = 0;
  if (TripCount != 0) {
    BreakoutTrip = TripCount % Count;
    TripMultiple = 0;
  } else {
    // Figure out what multiple to use.
    BreakoutTrip = TripMultiple =
      (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
  }

  if (CompletelyUnroll) {
    DOUT << "COMPLETELY UNROLLING loop %" << Header->getName()
         << " with trip count " << TripCount << "!\n";
  } else {
    DOUT << "UNROLLING loop %" << Header->getName()
         << " by " << Count;
    if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
      DOUT << " with a breakout at trip " << BreakoutTrip;
    } else if (TripMultiple != 1) {
      DOUT << " with " << TripMultiple << " trips per branch";
    }
    DOUT << "!\n";
  }

  // Make a copy of the original LoopBlocks list so we can keep referring
  // to it while hacking on the loop.
  std::vector<BasicBlock*> LoopBlocks = L->getBlocks();

  bool ContinueOnTrue = BI->getSuccessor(0) == Header;
  BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);

  // For the first iteration of the loop, we should use the precloned values for
  // PHI nodes.  Insert associations now.
  typedef DenseMap<const Value*, Value*> ValueMapTy;
  ValueMapTy LastValueMap;
  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
    PHINode *PN = cast<PHINode>(I);
    if (Instruction *I = 
                dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock)))
      if (L->contains(I->getParent()))
        LastValueMap[I] = I;
  }

  // Keep track of all the headers and latches that we create. These are
  // needed by the logic that inserts the branches to connect all the
  // new blocks.
  std::vector<BasicBlock*> Headers;
  std::vector<BasicBlock*> Latches;
  Headers.reserve(Count);
  Latches.reserve(Count);
  Headers.push_back(Header);
  Latches.push_back(LatchBlock);

  // Iterate through all but the first iterations, cloning blocks from
  // the first iteration to populate the subsequent iterations.
  for (unsigned It = 1; It != Count; ++It) {
    char SuffixBuffer[100];
    sprintf(SuffixBuffer, ".%d", It);
    
    std::vector<BasicBlock*> NewBlocks;
    NewBlocks.reserve(LoopBlocks.size());
    
    // Iterate through all the blocks in the original loop.
    for (std::vector<BasicBlock*>::const_iterator BBI = LoopBlocks.begin(),
         E = LoopBlocks.end(); BBI != E; ++BBI) {
      bool SuppressExitEdges = false;
      BasicBlock *BB = *BBI;
      ValueMapTy ValueMap;
      BasicBlock *New = CloneBasicBlock(BB, ValueMap, SuffixBuffer);
      NewBlocks.push_back(New);
      Func->getBasicBlockList().insert(BBInsertPt, New);
      L->addBasicBlockToLoop(New, LI->getBase());

      // Special handling for the loop header block.
      if (BB == Header) {
        // Keep track of new headers as we create them, so that we can insert
        // the proper branches later.
        Headers[It] = New;

        // Loop over all of the PHI nodes in the block, changing them to use
        // the incoming values from the previous block.
        for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
          PHINode *NewPHI = cast<PHINode>(ValueMap[I]);
          Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
          if (Instruction *InValI = dyn_cast<Instruction>(InVal))
            if (It > 1 && L->contains(InValI->getParent()))
              InVal = LastValueMap[InValI];
          ValueMap[I] = InVal;
          New->getInstList().erase(NewPHI);
        }
      }

      // Special handling for the loop latch block.
      if (BB == LatchBlock) {
        // Keep track of new latches as we create them, so that we can insert
        // the proper branches later.
        Latches[It] = New;

        // If knowledge of the trip count and/or multiple will allow us
        // to emit unconditional branches in some of the new latch blocks,
        // those blocks shouldn't be referenced by PHIs that reference
        // the original latch.
        unsigned NextIt = (It + 1) % Count;
        SuppressExitEdges =
          NextIt != BreakoutTrip &&
          (TripMultiple == 0 || NextIt % TripMultiple != 0);
      }

      // Update our running map of newest clones
      LastValueMap[BB] = New;
      for (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end();
           VI != VE; ++VI)
        LastValueMap[VI->first] = VI->second;

      // Add incoming values to phi nodes that reference this block. The last
      // latch block may need to be referenced by the first header, and any
      // block with an exit edge may be referenced from outside the loop.
      for (Value::use_iterator UI = BB->use_begin(), UE = BB->use_end();
           UI != UE; ) {
        PHINode *PN = dyn_cast<PHINode>(*UI++);
        if (PN &&
            ((BB == LatchBlock && It == Count - 1 && !CompletelyUnroll) ||
             (!SuppressExitEdges && !L->contains(PN->getParent())))) {
          Value *InVal = PN->getIncomingValueForBlock(BB);
          // If this value was defined in the loop, take the value defined
          // by the last iteration of the loop.
          ValueMapTy::iterator VI = LastValueMap.find(InVal);
          if (VI != LastValueMap.end())
            InVal = VI->second;
          PN->addIncoming(InVal, New);
        }
      }
    }
    
    // Remap all instructions in the most recent iteration
    for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
      for (BasicBlock::iterator I = NewBlocks[i]->begin(),
           E = NewBlocks[i]->end(); I != E; ++I)
        RemapInstruction(I, LastValueMap);
  }

  // Now that all the basic blocks for the unrolled iterations are in place,
  // set up the branches to connect them.
  for (unsigned It = 0; It != Count; ++It) {
    // The original branch was replicated in each unrolled iteration.
    BranchInst *Term = cast<BranchInst>(Latches[It]->getTerminator());

    // The branch destination.
    unsigned NextIt = (It + 1) % Count;
    BasicBlock *Dest = Headers[NextIt];
    bool NeedConditional = true;
    bool HasExit = true;

    // For a complete unroll, make the last iteration end with an
    // unconditional branch to the exit block.
    if (CompletelyUnroll && NextIt == 0) {
      Dest = LoopExit;
      NeedConditional = false;
    }

    // If we know the trip count or a multiple of it, we can safely use an
    // unconditional branch for some iterations.
    if (NextIt != BreakoutTrip &&
        (TripMultiple == 0 || NextIt % TripMultiple != 0)) {
      NeedConditional = false;
      HasExit = false;
    }

    if (NeedConditional) {
      // Update the conditional branch's successor for the following
      // iteration.
      Term->setSuccessor(!ContinueOnTrue, Dest);
    } else {
      Term->setUnconditionalDest(Dest);
      // Merge adjacent basic blocks, if possible.
      if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI)) {
        std::replace(Latches.begin(), Latches.end(), Dest, Fold);
        std::replace(Headers.begin(), Headers.end(), Dest, Fold);
      }
    }

    // Special handling for the first iteration. If the first latch is
    // now unconditionally branching to the second header, then it is
    // no longer an exit node. Delete PHI references to it both from
    // the first header and from outsie the loop.
    if (It == 0)
      for (Value::use_iterator UI = LatchBlock->use_begin(),
           UE = LatchBlock->use_end(); UI != UE; ) {
        PHINode *PN = dyn_cast<PHINode>(*UI++);
        if (PN && (PN->getParent() == Header ? Count > 1 : !HasExit))
          PN->removeIncomingValue(LatchBlock);
      }
  }
  
  // At this point, unrolling is complete and the code is well formed. 
  // Now, do some simplifications.

  // If we're doing complete unrolling, loop over the PHI nodes in the
  // original block, setting them to their incoming values.
  if (CompletelyUnroll) {
    BasicBlock *Preheader = L->getLoopPreheader();
    for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ) {
      PHINode *PN = cast<PHINode>(I++);
      PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
      Header->getInstList().erase(PN);
    }
  }

  // We now do a quick sweep over the inserted code, doing constant
  // propagation and dead code elimination as we go.
  for (Loop::block_iterator BI = L->block_begin(), BBE = L->block_end();
       BI != BBE; ++BI) {
    BasicBlock *BB = *BI;
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
      Instruction *Inst = I++;

      if (isInstructionTriviallyDead(Inst))
        BB->getInstList().erase(Inst);
      else if (Constant *C = ConstantFoldInstruction(Inst)) {
        Inst->replaceAllUsesWith(C);
        BB->getInstList().erase(Inst);
      }
    }
  }

  NumCompletelyUnrolled += CompletelyUnroll;
  ++NumUnrolled;
  // Remove the loop from the LoopPassManager if it's completely removed.
  if (CompletelyUnroll && LPM != NULL)
    LPM->deleteLoopFromQueue(L);

  // If we didn't completely unroll the loop, it should still be in LCSSA form.
  if (!CompletelyUnroll)
    assert(L->isLCSSAForm());

  return true;
}