lib/Transforms/Scalar/LICM.cpp - llvm - Git at Google

 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file was developed by the LLVM research group and is distributed under
 // the University of Illinois Open Source License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // 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");
       }
     }
   }
 }
	//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file was developed by the LLVM research group and is distributed under
	// the University of Illinois Open Source License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// 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");
	}
	}
	}
	}