text: //===-- Sink.cpp - Code Sinking -------------------------------------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass moves instructions into successor blocks, when possible, so that
  // they aren't executed on paths where their results aren't needed.
  //
  //===----------------------------------------------------------------------===//
  
  #define DEBUG_TYPE "sink"
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/Analysis/AliasAnalysis.h"
  #include "llvm/Analysis/Dominators.h"
  #include "llvm/Analysis/LoopInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/Assembly/Writer.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/Support/CFG.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/raw_ostream.h"
  using namespace llvm;
  
  STATISTIC(NumSunk, "Number of instructions sunk");
  STATISTIC(NumSinkIter, "Number of sinking iterations");
  
  namespace {
    class Sinking : public FunctionPass {
      DominatorTree *DT;
      LoopInfo *LI;
      AliasAnalysis *AA;
  
    public:
      static char ID; // Pass identification
      Sinking() : FunctionPass(ID) {
        initializeSinkingPass(*PassRegistry::getPassRegistry());
      }
  
      virtual bool runOnFunction(Function &F);
  
      virtual void getAnalysisUsage(AnalysisUsage &AU) const {
        AU.setPreservesCFG();
        FunctionPass::getAnalysisUsage(AU);
        AU.addRequired<AliasAnalysis>();
        AU.addRequired<DominatorTree>();
        AU.addRequired<LoopInfo>();
        AU.addPreserved<DominatorTree>();
        AU.addPreserved<LoopInfo>();
      }
    private:
      bool ProcessBlock(BasicBlock &BB);
      bool SinkInstruction(Instruction *I, SmallPtrSet<Instruction *, 8> &Stores);
      bool AllUsesDominatedByBlock(Instruction *Inst, BasicBlock *BB) const;
      bool IsAcceptableTarget(Instruction *Inst, BasicBlock *SuccToSinkTo) const;
    };
  } // end anonymous namespace
  
  char Sinking::ID = 0;
  INITIALIZE_PASS_BEGIN(Sinking, "sink", "Code sinking", false, false)
  INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
  INITIALIZE_PASS_END(Sinking, "sink", "Code sinking", false, false)
  
  FunctionPass *llvm::createSinkingPass() { return new Sinking(); }
  
  /// AllUsesDominatedByBlock - Return true if all uses of the specified value
  /// occur in blocks dominated by the specified block.
  bool Sinking::AllUsesDominatedByBlock(Instruction *Inst,
                                        BasicBlock *BB) const {
    // Ignoring debug uses is necessary so debug info doesn't affect the code.
    // This may leave a referencing dbg_value in the original block, before
    // the definition of the vreg.  Dwarf generator handles this although the
    // user might not get the right info at runtime.
    for (Value::use_iterator I = Inst->use_begin(),
         E = Inst->use_end(); I != E; ++I) {
      // Determine the block of the use.
      Instruction *UseInst = cast<Instruction>(*I);
      BasicBlock *UseBlock = UseInst->getParent();
      if (PHINode *PN = dyn_cast<PHINode>(UseInst)) {
        // PHI nodes use the operand in the predecessor block, not the block with
        // the PHI.
        unsigned Num = PHINode::getIncomingValueNumForOperand(I.getOperandNo());
        UseBlock = PN->getIncomingBlock(Num);
      }
      // Check that it dominates.
      if (!DT->dominates(BB, UseBlock))
        return false;
    }
    return true;
  }
  
  bool Sinking::runOnFunction(Function &F) {
    DT = &getAnalysis<DominatorTree>();
    LI = &getAnalysis<LoopInfo>();
    AA = &getAnalysis<AliasAnalysis>();
  
    bool MadeChange, EverMadeChange = false;
  
    do {
      MadeChange = false;
      DEBUG(dbgs() << "Sinking iteration " << NumSinkIter << "\n");
      // Process all basic blocks.
      for (Function::iterator I = F.begin(), E = F.end();
           I != E; ++I)
        MadeChange |= ProcessBlock(*I);
      EverMadeChange |= MadeChange;
      NumSinkIter++;
    } while (MadeChange);
  
    return EverMadeChange;
  }
  
  bool Sinking::ProcessBlock(BasicBlock &BB) {
    // Can't sink anything out of a block that has less than two successors.
    if (BB.getTerminator()->getNumSuccessors() <= 1 || BB.empty()) return false;
  
    // Don't bother sinking code out of unreachable blocks. In addition to being
    // unprofitable, it can also lead to infinite looping, because in an
    // unreachable loop there may be nowhere to stop.
    if (!DT->isReachableFromEntry(&BB)) return false;
  
    bool MadeChange = false;
  
    // Walk the basic block bottom-up.  Remember if we saw a store.
    BasicBlock::iterator I = BB.end();
    --I;
    bool ProcessedBegin = false;
    SmallPtrSet<Instruction *, 8> Stores;
    do {
      Instruction *Inst = I;  // The instruction to sink.
  
      // Predecrement I (if it's not begin) so that it isn't invalidated by
      // sinking.
      ProcessedBegin = I == BB.begin();
      if (!ProcessedBegin)
        --I;
  
      if (isa<DbgInfoIntrinsic>(Inst))
        continue;
  
      if (SinkInstruction(Inst, Stores))
        ++NumSunk, MadeChange = true;
  
      // If we just processed the first instruction in the block, we're done.
    } while (!ProcessedBegin);
  
    return MadeChange;
  }
  
  static bool isSafeToMove(Instruction *Inst, AliasAnalysis *AA,
                           SmallPtrSet<Instruction *, 8> &Stores) {
  
    if (Inst->mayWriteToMemory()) {
      Stores.insert(Inst);
      return false;
    }
  
    if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
      AliasAnalysis::Location Loc = AA->getLocation(L);
      for (SmallPtrSet<Instruction *, 8>::iterator I = Stores.begin(),
           E = Stores.end(); I != E; ++I)
        if (AA->getModRefInfo(*I, Loc) & AliasAnalysis::Mod)
          return false;
    }
  
    if (isa<TerminatorInst>(Inst) || isa<PHINode>(Inst))
      return false;
  
    return true;
  }
  
  /// IsAcceptableTarget - Return true if it is possible to sink the instruction
  /// in the specified basic block.
  bool Sinking::IsAcceptableTarget(Instruction *Inst,
                                   BasicBlock *SuccToSinkTo) const {
    assert(Inst && "Instruction to be sunk is null");
    assert(SuccToSinkTo && "Candidate sink target is null");
  
    // It is not possible to sink an instruction into its own block.  This can
    // happen with loops.
    if (Inst->getParent() == SuccToSinkTo)
      return false;
  
    // If the block has multiple predecessors, this would introduce computation
    // on different code paths.  We could split the critical edge, but for now we
    // just punt.
    // FIXME: Split critical edges if not backedges.
    if (SuccToSinkTo->getUniquePredecessor() != Inst->getParent()) {
      // We cannot sink a load across a critical edge - there may be stores in
      // other code paths.
      if (!isSafeToSpeculativelyExecute(Inst))
        return false;
  
      // We don't want to sink across a critical edge if we don't dominate the
      // successor. We could be introducing calculations to new code paths.
      if (!DT->dominates(Inst->getParent(), SuccToSinkTo))
        return false;
  
      // Don't sink instructions into a loop.
      Loop *succ = LI->getLoopFor(SuccToSinkTo);
      Loop *cur = LI->getLoopFor(Inst->getParent());
      if (succ != 0 && succ != cur)
        return false;
    }
  
    // Finally, check that all the uses of the instruction are actually
    // dominated by the candidate
    return AllUsesDominatedByBlock(Inst, SuccToSinkTo);
  }
  
  /// SinkInstruction - Determine whether it is safe to sink the specified machine
  /// instruction out of its current block into a successor.
  bool Sinking::SinkInstruction(Instruction *Inst,
                                SmallPtrSet<Instruction *, 8> &Stores) {
    // Check if it's safe to move the instruction.
    if (!isSafeToMove(Inst, AA, Stores))
      return false;
  
    // FIXME: This should include support for sinking instructions within the
    // block they are currently in to shorten the live ranges.  We often get
    // instructions sunk into the top of a large block, but it would be better to
    // also sink them down before their first use in the block.  This xform has to
    // be careful not to *increase* register pressure though, e.g. sinking
    // "x = y + z" down if it kills y and z would increase the live ranges of y
    // and z and only shrink the live range of x.
  
    // SuccToSinkTo - This is the successor to sink this instruction to, once we
    // decide.
    BasicBlock *SuccToSinkTo = 0;
  
    // Instructions can only be sunk if all their uses are in blocks
    // dominated by one of the successors.
    // Look at all the postdominators and see if we can sink it in one.
    DomTreeNode *DTN = DT->getNode(Inst->getParent());
    for (DomTreeNode::iterator I = DTN->begin(), E = DTN->end();
        I != E && SuccToSinkTo == 0; ++I) {
      BasicBlock *Candidate = (*I)->getBlock();
      if ((*I)->getIDom()->getBlock() == Inst->getParent() &&
          IsAcceptableTarget(Inst, Candidate))
        SuccToSinkTo = Candidate;
    }
  
    // If no suitable postdominator was found, look at all the successors and
    // decide which one we should sink to, if any.
    for (succ_iterator I = succ_begin(Inst->getParent()),
        E = succ_end(Inst->getParent()); I != E && SuccToSinkTo == 0; ++I) {
      if (IsAcceptableTarget(Inst, *I))
        SuccToSinkTo = *I;
    }
  
    // If we couldn't find a block to sink to, ignore this instruction.
    if (SuccToSinkTo == 0)
      return false;
  
    DEBUG(dbgs() << "Sink" << *Inst << " (";
          WriteAsOperand(dbgs(), Inst->getParent(), false);
          dbgs() << " -> ";
          WriteAsOperand(dbgs(), SuccToSinkTo, false);
          dbgs() << ")\n");
  
    // Move the instruction.
    Inst->moveBefore(SuccToSinkTo->getFirstInsertionPt());
    return true;
  }
  