lib/Transforms/Scalar/LoopRotation.cpp

//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file 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-rotate"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Function.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

#define MAX_HEADER_SIZE 16

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

  class LoopRotate : public LoopPass {
  public:
    static char ID; // Pass ID, replacement for typeid
    LoopRotate() : LoopPass(ID) {
      initializeLoopRotatePass(*PassRegistry::getPassRegistry());
    }

    // Rotate Loop L as many times as possible. Return true if
    // loop is rotated at least once.
    bool runOnLoop(Loop *L, LPPassManager &LPM);

    // LCSSA form makes instruction renaming easier.
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreserved<DominatorTree>();
      AU.addRequired<LoopInfo>();
      AU.addPreserved<LoopInfo>();
      AU.addRequiredID(LoopSimplifyID);
      AU.addPreservedID(LoopSimplifyID);
      AU.addRequiredID(LCSSAID);
      AU.addPreservedID(LCSSAID);
      AU.addPreserved<ScalarEvolution>();
    }

    // Helper functions

    /// Do actual work
    bool rotateLoop(Loop *L);
    
    /// After loop rotation, loop pre-header has multiple sucessors.
    /// Insert one forwarding basic block to ensure that loop pre-header
    /// has only one successor.
    void preserveCanonicalLoopForm(Loop *L, BasicBlock *OrigHeader,
                                   BasicBlock *OrigPreHeader,
                                   BasicBlock *OrigLatch, BasicBlock *NewHeader,
                                   BasicBlock *Exit);

  private:
    LoopInfo *LI;
  };
}
  
char LoopRotate::ID = 0;
INITIALIZE_PASS_BEGIN(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_END(LoopRotate, "loop-rotate", "Rotate Loops", false, false)

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

/// Rotate Loop L as many times as possible. Return true if
/// the loop is rotated at least once.
bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
  LI = &getAnalysis<LoopInfo>();

  // One loop can be rotated multiple times.
  bool MadeChange = false;
  while (rotateLoop(L))
    MadeChange = true;

  return MadeChange;
}

/// Rotate loop LP. Return true if the loop is rotated.
bool LoopRotate::rotateLoop(Loop *L) {
  // If the loop has only one block then there is not much to rotate.
  if (L->getBlocks().size() == 1)
    return false;
  
  BasicBlock *OrigHeader = L->getHeader();
  
  BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
  if (BI == 0 || BI->isUnconditional())
    return false;
  
  // If the loop header is not one of the loop exiting blocks then
  // either this loop is already rotated or it is not
  // suitable for loop rotation transformations.
  if (!L->isLoopExiting(OrigHeader))
    return false;

  // Updating PHInodes in loops with multiple exits adds complexity. 
  // Keep it simple, and restrict loop rotation to loops with one exit only.
  // In future, lift this restriction and support for multiple exits if
  // required.
  SmallVector<BasicBlock*, 8> ExitBlocks;
  L->getExitBlocks(ExitBlocks);
  if (ExitBlocks.size() > 1)
    return false;

  // Check size of original header and reject loop if it is very big.
  {
    CodeMetrics Metrics;
    Metrics.analyzeBasicBlock(OrigHeader);
    if (Metrics.NumInsts > MAX_HEADER_SIZE)
      return false;
  }

  // Now, this loop is suitable for rotation.
  BasicBlock *OrigPreHeader = L->getLoopPreheader();
  BasicBlock *OrigLatch = L->getLoopLatch();
  assert(OrigPreHeader && OrigLatch && "Loop not in canonical form?");

  // Anything ScalarEvolution may know about this loop or the PHI nodes
  // in its header will soon be invalidated.
  if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
    SE->forgetLoop(L);

  // Find new Loop header. NewHeader is a Header's one and only successor
  // that is inside loop.  Header's other successor is outside the
  // loop.  Otherwise loop is not suitable for rotation.
  BasicBlock *Exit = BI->getSuccessor(0);
  BasicBlock *NewHeader = BI->getSuccessor(1);
  if (L->contains(Exit))
    std::swap(Exit, NewHeader);
  assert(NewHeader && "Unable to determine new loop header");
  assert(L->contains(NewHeader) && !L->contains(Exit) && 
         "Unable to determine loop header and exit blocks");
  
  // This code assumes that the new header has exactly one predecessor.
  // Remove any single-entry PHI nodes in it.
  assert(NewHeader->getSinglePredecessor() &&
         "New header doesn't have one pred!");
  FoldSingleEntryPHINodes(NewHeader);

  // Begin by walking OrigHeader and populating ValueMap with an entry for
  // each Instruction.
  BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
  ValueToValueMapTy ValueMap;

  // For PHI nodes, the value available in OldPreHeader is just the
  // incoming value from OldPreHeader.
  for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
    ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));

  // For the rest of the instructions, either hoist to the OrigPreheader if
  // possible or create a clone in the OldPreHeader if not.
  TerminatorInst *LoopEntryBranch = OrigPreHeader->getTerminator();
  while (I != E) {
    Instruction *Inst = I++;
    
    // If the instruction's operands are invariant and it doesn't read or write
    // memory, then it is safe to hoist.  Doing this doesn't change the order of
    // execution in the preheader, but does prevent the instruction from
    // executing in each iteration of the loop.  This means it is safe to hoist
    // something that might trap, but isn't safe to hoist something that reads
    // memory (without proving that the loop doesn't write).
    if (L->hasLoopInvariantOperands(Inst) &&
        !Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
        !isa<TerminatorInst>(Inst)) {
      Inst->moveBefore(LoopEntryBranch);
      continue;
    }
    
    // Otherwise, create a duplicate of the instruction.
    Instruction *C = Inst->clone();
    
    // Eagerly remap the operands of the instruction.
    RemapInstruction(C, ValueMap,
                     RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
    
    // With the operands remapped, see if the instruction constant folds or is
    // otherwise simplifyable.  This commonly occurs because the entry from PHI
    // nodes allows icmps and other instructions to fold.
    Value *V = SimplifyInstruction(C);
    if (V && LI->replacementPreservesLCSSAForm(C, V)) {
      // If so, then delete the temporary instruction and stick the folded value
      // in the map.
      delete C;
      ValueMap[Inst] = V;
    } else {
      // Otherwise, stick the new instruction into the new block!
      C->setName(Inst->getName());
      C->insertBefore(LoopEntryBranch);
      ValueMap[Inst] = C;
    }
  }

  // Along with all the other instructions, we just cloned OrigHeader's
  // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
  // successors by duplicating their incoming values for OrigHeader.
  TerminatorInst *TI = OrigHeader->getTerminator();
  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
    for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin();
         PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
      PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreHeader);

  // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
  // OrigPreHeader's old terminator (the original branch into the loop), and
  // remove the corresponding incoming values from the PHI nodes in OrigHeader.
  LoopEntryBranch->eraseFromParent();
  for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
    PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));

  // Now fix up users of the instructions in OrigHeader, inserting PHI nodes
  // as necessary.
  SSAUpdater SSA;
  for (I = OrigHeader->begin(); I != E; ++I) {
    Value *OrigHeaderVal = I;
    Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];

    // If there are no uses of the value (e.g. because it returns void), there
    // is nothing to rewrite.
    if (OrigHeaderVal->use_empty() && OrigPreHeaderVal->use_empty())
      continue;
    
    // The value now exits in two versions: the initial value in the preheader
    // and the loop "next" value in the original header.
    SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
    SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
    SSA.AddAvailableValue(OrigPreHeader, OrigPreHeaderVal);

    // Visit each use of the OrigHeader instruction.
    for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
         UE = OrigHeaderVal->use_end(); UI != UE; ) {
      // Grab the use before incrementing the iterator.
      Use &U = UI.getUse();

      // Increment the iterator before removing the use from the list.
      ++UI;

      // SSAUpdater can't handle a non-PHI use in the same block as an
      // earlier def. We can easily handle those cases manually.
      Instruction *UserInst = cast<Instruction>(U.getUser());
      if (!isa<PHINode>(UserInst)) {
        BasicBlock *UserBB = UserInst->getParent();

        // The original users in the OrigHeader are already using the
        // original definitions.
        if (UserBB == OrigHeader)
          continue;

        // Users in the OrigPreHeader need to use the value to which the
        // original definitions are mapped.
        if (UserBB == OrigPreHeader) {
          U = OrigPreHeaderVal;
          continue;
        }
      }

      // Anything else can be handled by SSAUpdater.
      SSA.RewriteUse(U);
    }
  }

  // NewHeader is now the header of the loop.
  L->moveToHeader(NewHeader);

  // Move the original header to the bottom of the loop, where it now more
  // naturally belongs. This isn't necessary for correctness, and CodeGen can
  // usually reorder blocks on its own to fix things like this up, but it's
  // still nice to keep the IR readable.
  //
  // The original header should have only one predecessor at this point, since
  // we checked that the loop had a proper preheader and unique backedge before
  // we started.
  assert(OrigHeader->getSinglePredecessor() &&
         "Original loop header has too many predecessors after loop rotation!");
  OrigHeader->moveAfter(OrigHeader->getSinglePredecessor());

  // Also, since this original header only has one predecessor, zap its
  // PHI nodes, which are now trivial.
  FoldSingleEntryPHINodes(OrigHeader);

  // TODO: We could just go ahead and merge OrigHeader into its predecessor
  // at this point, if we don't mind updating dominator info.

  // Establish a new preheader, update dominators, etc.
  preserveCanonicalLoopForm(L, OrigHeader, OrigPreHeader, OrigLatch,
                            NewHeader, Exit);

  ++NumRotated;
  return true;
}


/// Update LoopInfo, DominatorTree, and DomFrontiers to reflect the CFG change
/// we just made.  Then split edges as necessary to preserve LoopSimplify form.
void LoopRotate::preserveCanonicalLoopForm(Loop *L, BasicBlock *OrigHeader,
                                           BasicBlock *OrigPreHeader,
                                           BasicBlock *OrigLatch,
                                           BasicBlock *NewHeader,
                                           BasicBlock *Exit) {
  assert(L->getHeader() == NewHeader && "Latch block is our new header");

  if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
    // Since OrigPreheader now has the conditional branch to Exit block, it is
    // the dominator of Exit.
    DT->changeImmediateDominator(Exit, OrigPreHeader);
    DT->changeImmediateDominator(NewHeader, OrigPreHeader);
    
    // Update OrigHeader to be dominated by the new header block.
    DT->changeImmediateDominator(OrigHeader, OrigLatch);
  }
  
  // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
  // thus is not a preheader anymore.  Split the edge to form a real preheader.
  BasicBlock *NewPH = SplitCriticalEdge(OrigPreHeader, NewHeader, this);
  NewPH->setName(NewHeader->getName() + ".lr.ph");
  
  // Preserve canonical loop form, which means Exit block should have only one
  // predecessor.
  SplitCriticalEdge(L->getLoopLatch(), Exit, this);

  assert(NewHeader && L->getHeader() == NewHeader &&
         "Invalid loop header after loop rotation");
  assert(L->getLoopPreheader() == NewPH &&
         "Invalid loop preheader after loop rotation");
  assert(L->getLoopLatch() && "Invalid loop latch after loop rotation");
}