LICM.cpp

//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
//
// This pass is a simple loop invariant code motion pass.  An interesting aspect
// of this pass is that it uses alias analysis for two purposes:
//
//  1. Moving loop invariant loads out of loops.  If we can determine that a
//     load inside of a loop never aliases anything stored to, we can hoist it
//     like any other instruction.
//  2. Scalar Promotion of Memory - If there is a store instruction inside of
//     the loop, we try to move the store to happen AFTER the loop instead of
//     inside of the loop.  This can only happen if a few conditions are true:
//       A. The pointer stored through is loop invariant
//       B. There are no stores or loads in the loop which _may_ alias the
//          pointer.  There are no calls in the loop which mod/ref the pointer.
//     If these conditions are true, we can promote the loads and stores in the
//     loop of the pointer to use a temporary alloca'd variable.  We then use
//     the mem2reg functionality to construct the appropriate SSA form for the
//     variable.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Instructions.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Support/CFG.h"
#include "Support/CommandLine.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
#include "llvm/Assembly/Writer.h"
#include <algorithm>

namespace {
  cl::opt<bool>
  DisablePromotion("disable-licm-promotion", cl::Hidden,
                   cl::desc("Disable memory promotion in LICM pass"));

  Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop");
  Statistic<> NumHoistedLoads("licm", "Number of load insts hoisted");
  Statistic<> NumPromoted("licm",
                          "Number of memory locations promoted to registers");

  struct LICM : public FunctionPass, public InstVisitor<LICM> {
    virtual bool runOnFunction(Function &F);

    /// This transformation requires natural loop information & requires that
    /// loop preheaders be inserted into the CFG...
    ///
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequiredID(LoopSimplifyID);
      AU.addRequired<LoopInfo>();
      AU.addRequired<DominatorTree>();
      AU.addRequired<DominanceFrontier>();  // For scalar promotion (mem2reg)
      AU.addRequired<AliasAnalysis>();
    }

  private:
    LoopInfo      *LI;       // Current LoopInfo
    AliasAnalysis *AA;       // Current AliasAnalysis information
    DominanceFrontier *DF;   // Current Dominance Frontier
    bool Changed;            // Set to true when we change anything.
    BasicBlock *Preheader;   // The preheader block of the current loop...
    Loop *CurLoop;           // The current loop we are working on...
    AliasSetTracker *CurAST; // AliasSet information for the current loop...
    DominatorTree *DT;       // Dominator Tree for the current Loop...

    /// visitLoop - Hoist expressions out of the specified loop...    
    ///
    void visitLoop(Loop *L, AliasSetTracker &AST);

    /// HoistRegion - Walk the specified region of the CFG (defined by all
    /// blocks dominated by the specified block, and that are in the current
    /// loop) in depth first order w.r.t the DominatorTree.  This allows us to
    /// visit definitions before uses, allowing us to hoist a loop body in one
    /// pass without iteration.
    ///
    void HoistRegion(DominatorTree::Node *N);

    /// inSubLoop - Little predicate that returns true if the specified basic
    /// block is in a subloop of the current one, not the current one itself.
    ///
    bool inSubLoop(BasicBlock *BB) {
      assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
      for (unsigned i = 0, e = CurLoop->getSubLoops().size(); i != e; ++i)
        if (CurLoop->getSubLoops()[i]->contains(BB))
          return true;  // A subloop actually contains this block!
      return false;
    }

    /// hoist - When an instruction is found to only use loop invariant operands
    /// that is safe to hoist, this instruction is called to do the dirty work.
    ///
    void hoist(Instruction &I);

    /// SafeToHoist - Only hoist an instruction if it is not a trapping
    /// instruction or if it is a trapping instruction and is guaranteed to
    /// execute.
    ///
    bool SafeToHoist(Instruction &I);

    /// pointerInvalidatedByLoop - Return true if the body of this loop may
    /// store into the memory location pointed to by V.
    /// 
    bool pointerInvalidatedByLoop(Value *V) {
      // Check to see if any of the basic blocks in CurLoop invalidate *V.
      return CurAST->getAliasSetForPointer(V, 0).isMod();
    }

    /// isLoopInvariant - Return true if the specified value is loop invariant
    ///
    inline bool isLoopInvariant(Value *V) {
      if (Instruction *I = dyn_cast<Instruction>(V))
        return !CurLoop->contains(I->getParent());
      return true;  // All non-instructions are loop invariant
    }

    /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
    /// to scalars as we can.
    ///
    void PromoteValuesInLoop();

    /// findPromotableValuesInLoop - Check the current loop for stores to
    /// definite pointers, which are not loaded and stored through may aliases.
    /// If these are found, create an alloca for the value, add it to the
    /// PromotedValues list, and keep track of the mapping from value to
    /// alloca...
    ///
    void findPromotableValuesInLoop(
                   std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
                                    std::map<Value*, AllocaInst*> &Val2AlMap);
    

    /// Instruction visitation handlers... these basically control whether or
    /// not the specified instruction types are hoisted.
    ///
    friend class InstVisitor<LICM>;
    void visitBinaryOperator(Instruction &I) {
      if (isLoopInvariant(I.getOperand(0)) &&
          isLoopInvariant(I.getOperand(1)) && SafeToHoist(I))
        hoist(I);
    }
    void visitCastInst(CastInst &CI) {
      Instruction &I = (Instruction&)CI;
      if (isLoopInvariant(I.getOperand(0)) && SafeToHoist(CI)) hoist(I);
    }
    void visitShiftInst(ShiftInst &I) { visitBinaryOperator((Instruction&)I); }

    void visitLoadInst(LoadInst &LI);

    void visitGetElementPtrInst(GetElementPtrInst &GEPI) {
      Instruction &I = (Instruction&)GEPI;
      for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
        if (!isLoopInvariant(I.getOperand(i))) return;
      if(SafeToHoist(GEPI))
        hoist(I);
    }
  };

  RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion");
}

Pass *createLICMPass() { return new LICM(); }

/// runOnFunction - For LICM, this simply traverses the loop structure of the
/// function, hoisting expressions out of loops if possible.
///
bool LICM::runOnFunction(Function &) {
  Changed = false;

  // Get our Loop and Alias Analysis information...
  LI = &getAnalysis<LoopInfo>();
  AA = &getAnalysis<AliasAnalysis>();
  DF = &getAnalysis<DominanceFrontier>();
  DT = &getAnalysis<DominatorTree>();

  // Hoist expressions out of all of the top-level loops.
  const std::vector<Loop*> &TopLevelLoops = LI->getTopLevelLoops();
  for (std::vector<Loop*>::const_iterator I = TopLevelLoops.begin(),
         E = TopLevelLoops.end(); I != E; ++I) {
    AliasSetTracker AST(*AA);
    LICM::visitLoop(*I, AST);
  }
  return Changed;
}


/// visitLoop - Hoist expressions out of the specified loop...    
///
void LICM::visitLoop(Loop *L, AliasSetTracker &AST) {
  // Recurse through all subloops before we process this loop...
  for (std::vector<Loop*>::const_iterator I = L->getSubLoops().begin(),
         E = L->getSubLoops().end(); I != E; ++I) {
    AliasSetTracker SubAST(*AA);
    LICM::visitLoop(*I, SubAST);

    // Incorporate information about the subloops into this loop...
    AST.add(SubAST);
  }
  CurLoop = L;
  CurAST = &AST;

  // Get the preheader block to move instructions into...
  Preheader = L->getLoopPreheader();
  assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");

  // Loop over the body of this loop, looking for calls, invokes, and stores.
  // Because subloops have already been incorporated into AST, we skip blocks in
  // subloops.
  //
  const std::vector<BasicBlock*> &LoopBBs = L->getBlocks();
  for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
         E = LoopBBs.end(); I != E; ++I)
    if (LI->getLoopFor(*I) == L)        // Ignore blocks in subloops...
      AST.add(**I);                     // Incorporate the specified basic block

  // We want to visit all of the instructions in this loop... that are not parts
  // of our subloops (they have already had their invariants hoisted out of
  // their loop, into this loop, so there is no need to process the BODIES of
  // the subloops).
  //
  // Traverse the body of the loop in depth first order on the dominator tree so
  // that we are guaranteed to see definitions before we see uses.  This allows
  // us to perform the LICM transformation in one pass, without iteration.
  //
  HoistRegion(DT->getNode(L->getHeader()));

  // Now that all loop invariants have been removed from the loop, promote any
  // memory references to scalars that we can...
  if (!DisablePromotion)
    PromoteValuesInLoop();

  // Clear out loops state information for the next iteration
  CurLoop = 0;
  Preheader = 0;
}

/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
/// dominated by the specified block, and that are in the current loop) in depth
/// first order w.r.t the DominatorTree.  This allows us to visit definitions
/// before uses, allowing us to hoist a loop body in one pass without iteration.
///
void LICM::HoistRegion(DominatorTree::Node *N) {
  assert(N != 0 && "Null dominator tree node?");

  // If this subregion is not in the top level loop at all, exit.
  if (!CurLoop->contains(N->getBlock())) return;

  // Only need to hoist the contents of this block if it is not part of a
  // subloop (which would already have been hoisted)
  if (!inSubLoop(N->getBlock()))
    visit(*N->getBlock());

  const std::vector<DominatorTree::Node*> &Children = N->getChildren();
  for (unsigned i = 0, e = Children.size(); i != e; ++i)
    HoistRegion(Children[i]);
}


/// hoist - When an instruction is found to only use loop invariant operands
/// that is safe to hoist, this instruction is called to do the dirty work.
///
void LICM::hoist(Instruction &Inst) {
  DEBUG(std::cerr << "LICM hoisting to";
        WriteAsOperand(std::cerr, Preheader, false);
        std::cerr << ": " << Inst);

  // Remove the instruction from its current basic block... but don't delete the
  // instruction.
  Inst.getParent()->getInstList().remove(&Inst);

  // Insert the new node in Preheader, before the terminator.
  Preheader->getInstList().insert(Preheader->getTerminator(), &Inst);
  
  ++NumHoisted;
  Changed = true;
}

/// SafeToHoist - Only hoist an instruction if it is not a trapping instruction
/// or if it is a trapping instruction and is guaranteed to execute
///
bool LICM::SafeToHoist(Instruction &Inst) {

  //If it is a trapping instruction, then check if its guaranteed to execute.
  if(Inst.isTrapping()) {

    //Get the instruction's basic block.
    BasicBlock *InstBB = Inst.getParent();
    
    //Get the Dominator Tree Node for the instruction's basic block/
    DominatorTree::Node *InstDTNode = DT->getNode(InstBB);

    //Get the exit blocks for the current loop.
    const std::vector<BasicBlock* > &ExitBlocks = CurLoop->getExitBlocks();

    //For each exit block, get the DT node and walk up the DT until
    //the instruction's basic block is found or we exit the loop.
    for(unsigned i=0; i < ExitBlocks.size(); ++i) {
      DominatorTree::Node *IDom = DT->getNode(ExitBlocks[i]);
      
      while(IDom != InstDTNode) {
 
        //Get next Immediate Dominator.
        IDom = IDom->getIDom();

        //See if we exited the loop.
        if(!CurLoop->contains(IDom->getBlock()))
          return false;
      }
    }
  }
  
  return true;
}


void LICM::visitLoadInst(LoadInst &LI) {
  if (isLoopInvariant(LI.getOperand(0)) && !LI.isVolatile() &&
      !pointerInvalidatedByLoop(LI.getOperand(0)) && SafeToHoist(LI)) {
    hoist(LI);
    ++NumHoistedLoads;
  }
}

/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
/// stores out of the loop and moving loads to before the loop.  We do this by
/// looping over the stores in the loop, looking for stores to Must pointers
/// which are loop invariant.  We promote these memory locations to use allocas
/// instead.  These allocas can easily be raised to register values by the
/// PromoteMem2Reg functionality.
///
void LICM::PromoteValuesInLoop() {
  // PromotedValues - List of values that are promoted out of the loop.  Each
  // value has an alloca instruction for it, and a canonical version of the
  // pointer.
  std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
  std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca

  findPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
  if (ValueToAllocaMap.empty()) return;   // If there are values to promote...

  Changed = true;
  NumPromoted += PromotedValues.size();

  // Emit a copy from the value into the alloca'd value in the loop preheader
  TerminatorInst *LoopPredInst = Preheader->getTerminator();
  for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
    // Load from the memory we are promoting...
    LoadInst *LI = new LoadInst(PromotedValues[i].second, 
                                PromotedValues[i].second->getName()+".promoted",
                                LoopPredInst);
    // Store into the temporary alloca...
    new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
  }
  
  // Scan the basic blocks in the loop, replacing uses of our pointers with
  // uses of the allocas in question.  If we find a branch that exits the
  // loop, make sure to put reload code into all of the successors of the
  // loop.
  //
  const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
  for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
         E = LoopBBs.end(); I != E; ++I) {
    // Rewrite all loads and stores in the block of the pointer...
    for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
         II != E; ++II) {
      if (LoadInst *L = dyn_cast<LoadInst>(II)) {
        std::map<Value*, AllocaInst*>::iterator
          I = ValueToAllocaMap.find(L->getOperand(0));
        if (I != ValueToAllocaMap.end())
          L->setOperand(0, I->second);    // Rewrite load instruction...
      } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
        std::map<Value*, AllocaInst*>::iterator
          I = ValueToAllocaMap.find(S->getOperand(1));
        if (I != ValueToAllocaMap.end())
          S->setOperand(1, I->second);    // Rewrite store instruction...
      }
    }

    // Check to see if any successors of this block are outside of the loop.
    // If so, we need to copy the value from the alloca back into the memory
    // location...
    //
    for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
      if (!CurLoop->contains(*SI)) {
        // Copy all of the allocas into their memory locations...
        BasicBlock::iterator BI = (*SI)->begin();
        while (isa<PHINode>(*BI))
          ++BI;             // Skip over all of the phi nodes in the block...
        Instruction *InsertPos = BI;
        for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
          // Load from the alloca...
          LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
          // Store into the memory we promoted...
          new StoreInst(LI, PromotedValues[i].second, InsertPos);
        }
      }
  }

  // Now that we have done the deed, use the mem2reg functionality to promote
  // all of the new allocas we just created into real SSA registers...
  //
  std::vector<AllocaInst*> PromotedAllocas;
  PromotedAllocas.reserve(PromotedValues.size());
  for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
    PromotedAllocas.push_back(PromotedValues[i].first);
  PromoteMemToReg(PromotedAllocas, *DT, *DF, AA->getTargetData());
}

/// findPromotableValuesInLoop - Check the current loop for stores to definite
/// pointers, which are not loaded and stored through may aliases.  If these are
/// found, create an alloca for the value, add it to the PromotedValues list,
/// and keep track of the mapping from value to alloca...
///
void LICM::findPromotableValuesInLoop(
                   std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
                             std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
  Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();

  // Loop over all of the alias sets in the tracker object...
  for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
       I != E; ++I) {
    AliasSet &AS = *I;
    // We can promote this alias set if it has a store, if it is a "Must" alias
    // set, and if the pointer is loop invariant.
    if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
        isLoopInvariant(AS.begin()->first)) {
      assert(AS.begin() != AS.end() &&
             "Must alias set should have at least one pointer element in it!");
      Value *V = AS.begin()->first;

      // Check that all of the pointers in the alias set have the same type.  We
      // cannot (yet) promote a memory location that is loaded and stored in
      // different sizes.
      bool PointerOk = true;
      for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
        if (V->getType() != I->first->getType()) {
          PointerOk = false;
          break;
        }

      if (PointerOk) {
        const Type *Ty = cast<PointerType>(V->getType())->getElementType();
        AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
        PromotedValues.push_back(std::make_pair(AI, V));
        
        for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
          ValueToAllocaMap.insert(std::make_pair(I->first, AI));
        
        DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n");
      }
    }
  }
}