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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/LICM.cpp')
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diff --git a/contrib/llvm/lib/Transforms/Scalar/LICM.cpp b/contrib/llvm/lib/Transforms/Scalar/LICM.cpp new file mode 100644 index 0000000..7347395 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/LICM.cpp @@ -0,0 +1,881 @@ +//===-- 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/IntrinsicInst.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/CommandLine.h" +#include "llvm/Support/raw_ostream.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")); + +namespace { + struct 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>(); + AU.addPreservedID(LoopSimplifyID); + } + + 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. + IDom = IDom->getIDom(); + + while (IDom && IDom != BlockInLoopNode) { + // 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; + + // Get next Immediate Dominator. + IDom = IDom->getIDom(); + }; + + 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(); + + // 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. + // + if (L->hasDedicatedExits()) + SinkRegion(DT->getNode(L->getHeader())); + if (Preheader) + 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 && Preheader && L->hasDedicatedExits()) + 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 (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->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)) { + 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) { + DEBUG(dbgs() << "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 has users in unreachable blocks, eliminate. + // If I is not void type then replaceAllUsesWith undef. + // This allows ValueHandlers and custom metadata to adjust itself. + if (!I.getType()->isVoidTy()) + 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 has users in unreachable blocks, eliminate. + // If I is not void type then replaceAllUsesWith undef. + // This allows ValueHandlers and custom metadata to adjust itself. + if (!I.getType()->isVoidTy()) + 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()->isVoidTy()) { + 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) { + DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " + << I << "\n"); + + // 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.isSafeToSpeculativelyExecute()) + 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; + + // 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 (cast<PointerType>(Ptr->getType())->getElementType()->isPointerTy()) { + // 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) { + User *U = *UI; + if (LoadInst *LI = dyn_cast<LoadInst>(U)) { + LoadValue = LI; + break; + } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) { + 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 (LI->getType()->isPointerTy()) + 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)) + 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)); + + DEBUG(dbgs() << "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); +} |
