//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass performs loop invariant code motion, attempting to remove as much // code from the body of a loop as possible. It does this by either hoisting // code into the preheader block, or by sinking code to the exit blocks if it is // safe. This pass also promotes must-aliased memory locations in the loop to // live in registers, thus hoisting and sinking "invariant" loads and stores. // // This pass uses alias analysis for two purposes: // // 1. Moving loop invariant loads and calls out of loops. If we can determine // that a load or call inside of a loop never aliases anything stored to, // we can hoist it or sink 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. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "licm" #include "llvm/Transforms/Scalar.h" #include "llvm/Constants.h" #include "llvm/DerivedTypes.h" #include "llvm/Instructions.h" #include "llvm/Target/TargetData.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/AliasSetTracker.h" #include "llvm/Analysis/Dominators.h" #include "llvm/Analysis/ScalarEvolution.h" #include "llvm/Transforms/Utils/PromoteMemToReg.h" #include "llvm/Support/CFG.h" #include "llvm/Support/Compiler.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/ADT/Statistic.h" #include <algorithm> using namespace llvm; STATISTIC(NumSunk , "Number of instructions sunk out of loop"); STATISTIC(NumHoisted , "Number of instructions hoisted out of loop"); STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk"); STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk"); STATISTIC(NumPromoted , "Number of memory locations promoted to registers"); static cl::opt<bool> DisablePromotion("disable-licm-promotion", cl::Hidden, cl::desc("Disable memory promotion in LICM pass")); // This feature is currently disabled by default because CodeGen is not yet // capable of rematerializing these constants in PIC mode, so it can lead to // degraded performance. Compile test/CodeGen/X86/remat-constant.ll with // -relocation-model=pic to see an example of this. static cl::opt<bool> EnableLICMConstantMotion("enable-licm-constant-variables", cl::Hidden, cl::desc("Enable hoisting/sinking of constant " "global variables")); namespace { struct VISIBILITY_HIDDEN LICM : public LoopPass { static char ID; // Pass identification, replacement for typeid LICM() : LoopPass(&ID) {} virtual bool runOnLoop(Loop *L, LPPassManager &LPM); /// 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>(); AU.addPreserved<ScalarEvolution>(); AU.addPreserved<DominanceFrontier>(); } bool doFinalization() { // Free the values stored in the map for (std::map<Loop *, AliasSetTracker *>::iterator I = LoopToAliasMap.begin(), E = LoopToAliasMap.end(); I != E; ++I) delete I->second; LoopToAliasMap.clear(); return false; } private: // Various analyses that we use... AliasAnalysis *AA; // Current AliasAnalysis information LoopInfo *LI; // Current LoopInfo DominatorTree *DT; // Dominator Tree for the current Loop... DominanceFrontier *DF; // Current Dominance Frontier // State that is updated as we process loops 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... std::map<Loop *, AliasSetTracker *> LoopToAliasMap; /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L); /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias /// set. void deleteAnalysisValue(Value *V, Loop *L); /// SinkRegion - Walk the specified region of the CFG (defined by all blocks /// dominated by the specified block, and that are in the current loop) in /// reverse depth first order w.r.t the DominatorTree. This allows us to /// visit uses before definitions, allowing us to sink a loop body in one /// pass without iteration. /// void SinkRegion(DomTreeNode *N); /// 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(DomTreeNode *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 (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I) if ((*I)->contains(BB)) return true; // A subloop actually contains this block! return false; } /// isExitBlockDominatedByBlockInLoop - This method checks to see if the /// specified exit block of the loop is dominated by the specified block /// that is in the body of the loop. We use these constraints to /// dramatically limit the amount of the dominator tree that needs to be /// searched. bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock, BasicBlock *BlockInLoop) const { // If the block in the loop is the loop header, it must be dominated! BasicBlock *LoopHeader = CurLoop->getHeader(); if (BlockInLoop == LoopHeader) return true; DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop); DomTreeNode *IDom = DT->getNode(ExitBlock); // Because the exit block is not in the loop, we know we have to get _at // least_ its immediate dominator. do { // Get next Immediate Dominator. IDom = IDom->getIDom(); // If we have got to the header of the loop, then the instructions block // did not dominate the exit node, so we can't hoist it. if (IDom->getBlock() == LoopHeader) return false; } while (IDom != BlockInLoopNode); return true; } /// sink - When an instruction is found to only be used outside of the loop, /// this function moves it to the exit blocks and patches up SSA form as /// needed. /// void sink(Instruction &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 hoist(Instruction &I); /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it /// is not a trapping instruction or if it is a trapping instruction and is /// guaranteed to execute. /// bool isSafeToExecuteUnconditionally(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, unsigned Size) { // Check to see if any of the basic blocks in CurLoop invalidate *V. return CurAST->getAliasSetForPointer(V, Size).isMod(); } bool canSinkOrHoistInst(Instruction &I); bool isLoopInvariantInst(Instruction &I); bool isNotUsedInLoop(Instruction &I); /// 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); }; } char LICM::ID = 0; static RegisterPass<LICM> X("licm", "Loop Invariant Code Motion"); Pass *llvm::createLICMPass() { return new LICM(); } /// Hoist expressions out of the specified loop. Note, alias info for inner /// loop is not preserved so it is not a good idea to run LICM multiple /// times on one loop. /// bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) { Changed = false; // Get our Loop and Alias Analysis information... LI = &getAnalysis<LoopInfo>(); AA = &getAnalysis<AliasAnalysis>(); DF = &getAnalysis<DominanceFrontier>(); DT = &getAnalysis<DominatorTree>(); CurAST = new AliasSetTracker(*AA); // Collect Alias info from subloops for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end(); LoopItr != LoopItrE; ++LoopItr) { Loop *InnerL = *LoopItr; AliasSetTracker *InnerAST = LoopToAliasMap[InnerL]; assert (InnerAST && "Where is my AST?"); // What if InnerLoop was modified by other passes ? CurAST->add(*InnerAST); } CurLoop = L; // 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. // for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E; ++I) { BasicBlock *BB = *I; if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops... CurAST->add(*BB); // 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 sink instructions in one pass, without iteration. After sinking // instructions, we perform another pass to hoist them out of the loop. // SinkRegion(DT->getNode(L->getHeader())); 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; LoopToAliasMap[L] = CurAST; return Changed; } /// SinkRegion - Walk the specified region of the CFG (defined by all blocks /// dominated by the specified block, and that are in the current loop) in /// reverse depth first order w.r.t the DominatorTree. This allows us to visit /// uses before definitions, allowing us to sink a loop body in one pass without /// iteration. /// void LICM::SinkRegion(DomTreeNode *N) { assert(N != 0 && "Null dominator tree node?"); BasicBlock *BB = N->getBlock(); // If this subregion is not in the top level loop at all, exit. if (!CurLoop->contains(BB)) return; // We are processing blocks in reverse dfo, so process children first... const std::vector<DomTreeNode*> &Children = N->getChildren(); for (unsigned i = 0, e = Children.size(); i != e; ++i) SinkRegion(Children[i]); // Only need to process the contents of this block if it is not part of a // subloop (which would already have been processed). if (inSubLoop(BB)) return; for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) { Instruction &I = *--II; // Check to see if we can sink this instruction to the exit blocks // of the loop. We can do this if the all users of the instruction are // outside of the loop. In this case, it doesn't even matter if the // operands of the instruction are loop invariant. // if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) { ++II; sink(I); } } } /// 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(DomTreeNode *N) { assert(N != 0 && "Null dominator tree node?"); BasicBlock *BB = N->getBlock(); // If this subregion is not in the top level loop at all, exit. if (!CurLoop->contains(BB)) return; // Only need to process the contents of this block if it is not part of a // subloop (which would already have been processed). if (!inSubLoop(BB)) for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) { Instruction &I = *II++; // Try hoisting the instruction out to the preheader. We can only do this // if all of the operands of the instruction are loop invariant and if it // is safe to hoist the instruction. // if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) && isSafeToExecuteUnconditionally(I)) hoist(I); } const std::vector<DomTreeNode*> &Children = N->getChildren(); for (unsigned i = 0, e = Children.size(); i != e; ++i) HoistRegion(Children[i]); } /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this /// instruction. /// bool LICM::canSinkOrHoistInst(Instruction &I) { // Loads have extra constraints we have to verify before we can hoist them. if (LoadInst *LI = dyn_cast<LoadInst>(&I)) { if (LI->isVolatile()) return false; // Don't hoist volatile loads! // Loads from constant memory are always safe to move, even if they end up // in the same alias set as something that ends up being modified. if (EnableLICMConstantMotion && AA->pointsToConstantMemory(LI->getOperand(0))) return true; // Don't hoist loads which have may-aliased stores in loop. unsigned Size = 0; if (LI->getType()->isSized()) Size = AA->getTargetData().getTypeStoreSize(LI->getType()); return !pointerInvalidatedByLoop(LI->getOperand(0), Size); } else if (CallInst *CI = dyn_cast<CallInst>(&I)) { // Handle obvious cases efficiently. AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI); if (Behavior == AliasAnalysis::DoesNotAccessMemory) return true; else if (Behavior == AliasAnalysis::OnlyReadsMemory) { // If this call only reads from memory and there are no writes to memory // in the loop, we can hoist or sink the call as appropriate. bool FoundMod = false; for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); I != E; ++I) { AliasSet &AS = *I; if (!AS.isForwardingAliasSet() && AS.isMod()) { FoundMod = true; break; } } if (!FoundMod) return true; } // FIXME: This should use mod/ref information to see if we can hoist or sink // the call. return false; } // Otherwise these instructions are hoistable/sinkable return isa<BinaryOperator>(I) || isa<CastInst>(I) || isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) || isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) || isa<ShuffleVectorInst>(I); } /// isNotUsedInLoop - Return true if the only users of this instruction are /// outside of the loop. If this is true, we can sink the instruction to the /// exit blocks of the loop. /// bool LICM::isNotUsedInLoop(Instruction &I) { for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) { Instruction *User = cast<Instruction>(*UI); if (PHINode *PN = dyn_cast<PHINode>(User)) { // PHI node uses occur in predecessor blocks! for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingValue(i) == &I) if (CurLoop->contains(PN->getIncomingBlock(i))) return false; } else if (CurLoop->contains(User->getParent())) { return false; } } return true; } /// isLoopInvariantInst - Return true if all operands of this instruction are /// loop invariant. We also filter out non-hoistable instructions here just for /// efficiency. /// bool LICM::isLoopInvariantInst(Instruction &I) { // The instruction is loop invariant if all of its operands are loop-invariant for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) if (!CurLoop->isLoopInvariant(I.getOperand(i))) return false; // If we got this far, the instruction is loop invariant! return true; } /// sink - When an instruction is found to only be used outside of the loop, /// this function moves it to the exit blocks and patches up SSA form as needed. /// This method is guaranteed to remove the original instruction from its /// position, and may either delete it or move it to outside of the loop. /// void LICM::sink(Instruction &I) { DOUT << "LICM sinking instruction: " << I; SmallVector<BasicBlock*, 8> ExitBlocks; CurLoop->getExitBlocks(ExitBlocks); if (isa<LoadInst>(I)) ++NumMovedLoads; else if (isa<CallInst>(I)) ++NumMovedCalls; ++NumSunk; Changed = true; // The case where there is only a single exit node of this loop is common // enough that we handle it as a special (more efficient) case. It is more // efficient to handle because there are no PHI nodes that need to be placed. if (ExitBlocks.size() == 1) { if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) { // Instruction is not used, just delete it. CurAST->deleteValue(&I); if (!I.use_empty()) // If I has users in unreachable blocks, eliminate. I.replaceAllUsesWith(UndefValue::get(I.getType())); I.eraseFromParent(); } else { // Move the instruction to the start of the exit block, after any PHI // nodes in it. I.removeFromParent(); BasicBlock::iterator InsertPt = ExitBlocks[0]->getFirstNonPHI(); ExitBlocks[0]->getInstList().insert(InsertPt, &I); } } else if (ExitBlocks.empty()) { // The instruction is actually dead if there ARE NO exit blocks. CurAST->deleteValue(&I); if (!I.use_empty()) // If I has users in unreachable blocks, eliminate. I.replaceAllUsesWith(UndefValue::get(I.getType())); I.eraseFromParent(); } else { // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to // do all of the hard work of inserting PHI nodes as necessary. We convert // the value into a stack object to get it to do this. // Firstly, we create a stack object to hold the value... AllocaInst *AI = 0; if (I.getType() != Type::VoidTy) { AI = new AllocaInst(I.getType(), 0, I.getName(), I.getParent()->getParent()->getEntryBlock().begin()); CurAST->add(AI); } // Secondly, insert load instructions for each use of the instruction // outside of the loop. while (!I.use_empty()) { Instruction *U = cast<Instruction>(I.use_back()); // If the user is a PHI Node, we actually have to insert load instructions // in all predecessor blocks, not in the PHI block itself! if (PHINode *UPN = dyn_cast<PHINode>(U)) { // Only insert into each predecessor once, so that we don't have // different incoming values from the same block! std::map<BasicBlock*, Value*> InsertedBlocks; for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i) if (UPN->getIncomingValue(i) == &I) { BasicBlock *Pred = UPN->getIncomingBlock(i); Value *&PredVal = InsertedBlocks[Pred]; if (!PredVal) { // Insert a new load instruction right before the terminator in // the predecessor block. PredVal = new LoadInst(AI, "", Pred->getTerminator()); CurAST->add(cast<LoadInst>(PredVal)); } UPN->setIncomingValue(i, PredVal); } } else { LoadInst *L = new LoadInst(AI, "", U); U->replaceUsesOfWith(&I, L); CurAST->add(L); } } // Thirdly, insert a copy of the instruction in each exit block of the loop // that is dominated by the instruction, storing the result into the memory // location. Be careful not to insert the instruction into any particular // basic block more than once. std::set<BasicBlock*> InsertedBlocks; BasicBlock *InstOrigBB = I.getParent(); for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { BasicBlock *ExitBlock = ExitBlocks[i]; if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) { // If we haven't already processed this exit block, do so now. if (InsertedBlocks.insert(ExitBlock).second) { // Insert the code after the last PHI node... BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI(); // If this is the first exit block processed, just move the original // instruction, otherwise clone the original instruction and insert // the copy. Instruction *New; if (InsertedBlocks.size() == 1) { I.removeFromParent(); ExitBlock->getInstList().insert(InsertPt, &I); New = &I; } else { New = I.clone(); CurAST->copyValue(&I, New); if (!I.getName().empty()) New->setName(I.getName()+".le"); ExitBlock->getInstList().insert(InsertPt, New); } // Now that we have inserted the instruction, store it into the alloca if (AI) new StoreInst(New, AI, InsertPt); } } } // If the instruction doesn't dominate any exit blocks, it must be dead. if (InsertedBlocks.empty()) { CurAST->deleteValue(&I); I.eraseFromParent(); } // Finally, promote the fine value to SSA form. if (AI) { std::vector<AllocaInst*> Allocas; Allocas.push_back(AI); PromoteMemToReg(Allocas, *DT, *DF, CurAST); } } } /// 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 &I) { DOUT << "LICM hoisting to " << Preheader->getName() << ": " << I; // Remove the instruction from its current basic block... but don't delete the // instruction. I.removeFromParent(); // Insert the new node in Preheader, before the terminator. Preheader->getInstList().insert(Preheader->getTerminator(), &I); if (isa<LoadInst>(I)) ++NumMovedLoads; else if (isa<CallInst>(I)) ++NumMovedCalls; ++NumHoisted; Changed = true; } /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is /// not a trapping instruction or if it is a trapping instruction and is /// guaranteed to execute. /// bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) { // If it is not a trapping instruction, it is always safe to hoist. if (!Inst.isTrapping()) return true; // Otherwise we have to check to make sure that the instruction dominates all // of the exit blocks. If it doesn't, then there is a path out of the loop // which does not execute this instruction, so we can't hoist it. // If the instruction is in the header block for the loop (which is very // common), it is always guaranteed to dominate the exit blocks. Since this // is a common case, and can save some work, check it now. if (Inst.getParent() == CurLoop->getHeader()) return true; // It's always safe to load from a global or alloca. if (isa<LoadInst>(Inst)) if (isa<AllocationInst>(Inst.getOperand(0)) || isa<GlobalVariable>(Inst.getOperand(0))) return true; // Get the exit blocks for the current loop. SmallVector<BasicBlock*, 8> ExitBlocks; CurLoop->getExitBlocks(ExitBlocks); // 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, e = ExitBlocks.size(); i != e; ++i) if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent())) return false; return true; } /// 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(); std::vector<Value*> PointerValueNumbers; // 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) { Value *Ptr = PromotedValues[i].second; // If we are promoting a pointer value, update alias information for the // inserted load. Value *LoadValue = 0; if (isa<PointerType>(cast<PointerType>(Ptr->getType())->getElementType())) { // Locate a load or store through the pointer, and assign the same value // to LI as we are loading or storing. Since we know that the value is // stored in this loop, this will always succeed. for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end(); UI != E; ++UI) if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { LoadValue = LI; break; } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) { if (SI->getOperand(1) == Ptr) { LoadValue = SI->getOperand(0); break; } } assert(LoadValue && "No store through the pointer found!"); PointerValueNumbers.push_back(LoadValue); // Remember this for later. } // Load from the memory we are promoting. LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst); if (LoadValue) CurAST->copyValue(LoadValue, LI); // 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. // for (Loop::block_iterator I = CurLoop->block_begin(), E = CurLoop->block_end(); I != E; ++I) { BasicBlock *BB = *I; // Rewrite all loads and stores in the block of the pointer... for (BasicBlock::iterator II = BB->begin(), E = BB->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... } } } // Now that the body of the loop uses the allocas instead of the original // memory locations, insert code to copy the alloca value back into the // original memory location on all exits from the loop. Note that we only // want to insert one copy of the code in each exit block, though the loop may // exit to the same block more than once. // SmallPtrSet<BasicBlock*, 16> ProcessedBlocks; SmallVector<BasicBlock*, 8> ExitBlocks; CurLoop->getExitBlocks(ExitBlocks); for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { if (!ProcessedBlocks.insert(ExitBlocks[i])) continue; // Copy all of the allocas into their memory locations. BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI(); Instruction *InsertPos = BI; unsigned PVN = 0; for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) { // Load from the alloca. LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos); // If this is a pointer type, update alias info appropriately. if (isa<PointerType>(LI->getType())) CurAST->copyValue(PointerValueNumbers[PVN++], LI); // 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, CurAST); } /// FindPromotableValuesInLoop - Check the current loop for stores to definite /// pointers, which are not loaded and stored through may aliases and are safe /// for promotion. 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, if the pointer is loop invariant, and if we are not eliminating any // volatile loads or stores. if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() || AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue())) continue; assert(!AS.empty() && "Must alias set should have at least one pointer element in it!"); Value *V = AS.begin()->getValue(); // 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->getValue()->getType()) { PointerOk = false; break; } if (!PointerOk) continue; } // It isn't safe to promote a load/store from the loop if the load/store is // conditional. For example, turning: // // for () { if (c) *P += 1; } // // into: // // tmp = *P; for () { if (c) tmp +=1; } *P = tmp; // // is not safe, because *P may only be valid to access if 'c' is true. // // It is safe to promote P if all uses are direct load/stores and if at // least one is guaranteed to be executed. bool GuaranteedToExecute = false; bool InvalidInst = false; for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE; ++UI) { // Ignore instructions not in this loop. Instruction *Use = dyn_cast<Instruction>(*UI); if (!Use || !CurLoop->contains(Use->getParent())) continue; if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) { InvalidInst = true; break; } if (!GuaranteedToExecute) GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use); } // If there is an non-load/store instruction in the loop, we can't promote // it. If there isn't a guaranteed-to-execute instruction, we can't // promote. if (InvalidInst || !GuaranteedToExecute) continue; 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)); // Update the AST and alias analysis. CurAST->copyValue(V, AI); for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I) ValueToAllocaMap.insert(std::make_pair(I->getValue(), AI)); DOUT << "LICM: Promoting value: " << *V << "\n"; } } /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) { AliasSetTracker *AST = LoopToAliasMap[L]; if (!AST) return; AST->copyValue(From, To); } /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias /// set. void LICM::deleteAnalysisValue(Value *V, Loop *L) { AliasSetTracker *AST = LoopToAliasMap[L]; if (!AST) return; AST->deleteValue(V); }