//===-- 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 SSAUpdater to construct the appropriate SSA form for the value. // //===----------------------------------------------------------------------===// #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/LLVMContext.h" #include "llvm/Analysis/AliasAnalysis.h" #include "llvm/Analysis/AliasSetTracker.h" #include "llvm/Analysis/ConstantFolding.h" #include "llvm/Analysis/LoopInfo.h" #include "llvm/Analysis/LoopPass.h" #include "llvm/Analysis/Dominators.h" #include "llvm/Analysis/ValueTracking.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/SSAUpdater.h" #include "llvm/Target/TargetData.h" #include "llvm/Target/TargetLibraryInfo.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) { initializeLICMPass(*PassRegistry::getPassRegistry()); } 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.addRequired<DominatorTree>(); AU.addRequired<LoopInfo>(); AU.addRequiredID(LoopSimplifyID); AU.addRequired<AliasAnalysis>(); AU.addPreserved<AliasAnalysis>(); AU.addPreserved("scalar-evolution"); AU.addPreservedID(LoopSimplifyID); AU.addRequired<TargetLibraryInfo>(); } bool doFinalization() { assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets"); return false; } private: AliasAnalysis *AA; // Current AliasAnalysis information LoopInfo *LI; // Current LoopInfo DominatorTree *DT; // Dominator Tree for the current Loop. TargetData *TD; // TargetData for constant folding. TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding. // 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... bool MayThrow; // The current loop contains an instruction which // may throw, thus preventing code motion of // instructions with side effects. DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap; /// 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"); return LI->getLoopFor(BB) != CurLoop; } /// 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); /// isGuaranteedToExecute - Check that the instruction is guaranteed to /// execute. /// bool isGuaranteedToExecute(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, uint64_t Size, const MDNode *TBAAInfo) { // Check to see if any of the basic blocks in CurLoop invalidate *V. return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod(); } bool canSinkOrHoistInst(Instruction &I); bool isNotUsedInLoop(Instruction &I); void PromoteAliasSet(AliasSet &AS, SmallVectorImpl<BasicBlock*> &ExitBlocks, SmallVectorImpl<Instruction*> &InsertPts); }; } char LICM::ID = 0; INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false) INITIALIZE_PASS_DEPENDENCY(DominatorTree) INITIALIZE_PASS_DEPENDENCY(LoopInfo) INITIALIZE_PASS_DEPENDENCY(LoopSimplify) INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo) INITIALIZE_AG_DEPENDENCY(AliasAnalysis) INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false) 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>(); DT = &getAnalysis<DominatorTree>(); TD = getAnalysisIfAvailable<TargetData>(); TLI = &getAnalysis<TargetLibraryInfo>(); 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 = LoopToAliasSetMap[InnerL]; assert(InnerAST && "Where is my AST?"); // What if InnerLoop was modified by other passes ? CurAST->add(*InnerAST); // Once we've incorporated the inner loop's AST into ours, we don't need the // subloop's anymore. delete InnerAST; LoopToAliasSetMap.erase(InnerL); } 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 } MayThrow = false; // TODO: We've already searched for instructions which may throw in subloops. // We may want to reuse this information. for (Loop::block_iterator BB = L->block_begin(), BBE = L->block_end(); (BB != BBE) && !MayThrow ; ++BB) for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); (I != E) && !MayThrow; ++I) MayThrow |= I->mayThrow(); // 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()) { SmallVector<BasicBlock *, 8> ExitBlocks; SmallVector<Instruction *, 8> InsertPts; // Loop over all of the alias sets in the tracker object. for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); I != E; ++I) PromoteAliasSet(*I, ExitBlocks, InsertPts); } // Clear out loops state information for the next iteration CurLoop = 0; Preheader = 0; // If this loop is nested inside of another one, save the alias information // for when we process the outer loop. if (L->getParentLoop()) LoopToAliasSetMap[L] = CurAST; else delete 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; // If the instruction is dead, we would try to sink it because it isn't used // in the loop, instead, just delete it. if (isInstructionTriviallyDead(&I, TLI)) { DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n'); ++II; CurAST->deleteValue(&I); I.eraseFromParent(); Changed = true; continue; } // 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 constant folding this instruction. If all the operands are // constants, it is technically hoistable, but it would be better to just // fold it. if (Constant *C = ConstantFoldInstruction(&I, TD, TLI)) { DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n'); CurAST->copyValue(&I, C); CurAST->deleteValue(&I); I.replaceAllUsesWith(C); I.eraseFromParent(); continue; } // 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 (CurLoop->hasLoopInvariantOperands(&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->isUnordered()) return false; // Don't hoist volatile/atomic 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; if (LI->getMetadata("invariant.load")) return true; // Don't hoist loads which have may-aliased stores in loop. uint64_t Size = 0; if (LI->getType()->isSized()) Size = AA->getTypeStoreSize(LI->getType()); return !pointerInvalidatedByLoop(LI->getOperand(0), Size, LI->getMetadata(LLVMContext::MD_tbaa)); } else if (CallInst *CI = dyn_cast<CallInst>(&I)) { // Don't sink or hoist dbg info; it's legal, but not useful. if (isa<DbgInfoIntrinsic>(I)) return false; // Handle simple cases by querying alias analysis. AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI); if (Behavior == AliasAnalysis::DoesNotAccessMemory) return true; if (AliasAnalysis::onlyReadsMemory(Behavior)) { // 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; } // Only these instructions are hoistable/sinkable. bool HoistableKind = (isa<BinaryOperator>(I) || isa<CastInst>(I) || isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) || isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) || isa<ShuffleVectorInst>(I)); if (!HoistableKind) return false; return isSafeToExecuteUnconditionally(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; } /// 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 << "\n"); SmallVector<BasicBlock*, 8> ExitBlocks; CurLoop->getUniqueExitBlocks(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 (!DT->dominates(I.getParent(), ExitBlocks[0])) { // 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.use_empty()) 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.moveBefore(ExitBlocks[0]->getFirstInsertionPt()); // This instruction is no longer in the AST for the current loop, because // we just sunk it out of the loop. If we just sunk it into an outer // loop, we will rediscover the operation when we process it. CurAST->deleteValue(&I); } return; } 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.use_empty()) I.replaceAllUsesWith(UndefValue::get(I.getType())); I.eraseFromParent(); return; } // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the // hard work of inserting PHI nodes as necessary. SmallVector<PHINode*, 8> NewPHIs; SSAUpdater SSA(&NewPHIs); if (!I.use_empty()) SSA.Initialize(I.getType(), I.getName()); // Insert a copy of the instruction in each exit block of the loop that is // dominated by the instruction. Each exit block is known to only be in the // ExitBlocks list once. BasicBlock *InstOrigBB = I.getParent(); unsigned NumInserted = 0; for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { BasicBlock *ExitBlock = ExitBlocks[i]; if (!DT->dominates(InstOrigBB, ExitBlock)) continue; // Insert the code after the last PHI node. BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt(); // 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 (NumInserted++ == 0) { I.moveBefore(InsertPt); New = &I; } else { New = I.clone(); if (!I.getName().empty()) New->setName(I.getName()+".le"); ExitBlock->getInstList().insert(InsertPt, New); } // Now that we have inserted the instruction, inform SSAUpdater. if (!I.use_empty()) SSA.AddAvailableValue(ExitBlock, New); } // If the instruction doesn't dominate any exit blocks, it must be dead. if (NumInserted == 0) { CurAST->deleteValue(&I); if (!I.use_empty()) I.replaceAllUsesWith(UndefValue::get(I.getType())); I.eraseFromParent(); return; } // Next, rewrite uses of the instruction, inserting PHI nodes as needed. for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) { // Grab the use before incrementing the iterator. Use &U = UI.getUse(); // Increment the iterator before removing the use from the list. ++UI; SSA.RewriteUseAfterInsertions(U); } // Update CurAST for NewPHIs if I had pointer type. if (I.getType()->isPointerTy()) for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) CurAST->copyValue(&I, NewPHIs[i]); // Finally, remove the instruction from CurAST. It is no longer in the loop. CurAST->deleteValue(&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 &I) { DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " << I << "\n"); // Move the new node to the Preheader, before its terminator. I.moveBefore(Preheader->getTerminator()); 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 (isSafeToSpeculativelyExecute(&Inst)) return true; return isGuaranteedToExecute(Inst); } bool LICM::isGuaranteedToExecute(Instruction &Inst) { // Somewhere in this loop there is an instruction which may throw and make us // exit the loop. if (MayThrow) return false; // 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); // Verify that the block dominates each of the exit blocks of the loop. for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) if (!DT->dominates(Inst.getParent(), ExitBlocks[i])) return false; // As a degenerate case, if the loop is statically infinite then we haven't // proven anything since there are no exit blocks. if (ExitBlocks.empty()) return false; return true; } namespace { class LoopPromoter : public LoadAndStorePromoter { Value *SomePtr; // Designated pointer to store to. SmallPtrSet<Value*, 4> &PointerMustAliases; SmallVectorImpl<BasicBlock*> &LoopExitBlocks; SmallVectorImpl<Instruction*> &LoopInsertPts; AliasSetTracker &AST; DebugLoc DL; int Alignment; public: LoopPromoter(Value *SP, const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S, SmallPtrSet<Value*, 4> &PMA, SmallVectorImpl<BasicBlock*> &LEB, SmallVectorImpl<Instruction*> &LIP, AliasSetTracker &ast, DebugLoc dl, int alignment) : LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA), LoopExitBlocks(LEB), LoopInsertPts(LIP), AST(ast), DL(dl), Alignment(alignment) {} virtual bool isInstInList(Instruction *I, const SmallVectorImpl<Instruction*> &) const { Value *Ptr; if (LoadInst *LI = dyn_cast<LoadInst>(I)) Ptr = LI->getOperand(0); else Ptr = cast<StoreInst>(I)->getPointerOperand(); return PointerMustAliases.count(Ptr); } virtual void doExtraRewritesBeforeFinalDeletion() const { // Insert stores after in the loop exit blocks. Each exit block gets a // store of the live-out values that feed them. Since we've already told // the SSA updater about the defs in the loop and the preheader // definition, it is all set and we can start using it. for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) { BasicBlock *ExitBlock = LoopExitBlocks[i]; Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock); Instruction *InsertPos = LoopInsertPts[i]; StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos); NewSI->setAlignment(Alignment); NewSI->setDebugLoc(DL); } } virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const { // Update alias analysis. AST.copyValue(LI, V); } virtual void instructionDeleted(Instruction *I) const { AST.deleteValue(I); } }; } // end anon namespace /// PromoteAliasSet - 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. /// void LICM::PromoteAliasSet(AliasSet &AS, SmallVectorImpl<BasicBlock*> &ExitBlocks, SmallVectorImpl<Instruction*> &InsertPts) { // 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())) return; assert(!AS.empty() && "Must alias set should have at least one pointer element in it!"); Value *SomePtr = AS.begin()->getValue(); // 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; SmallVector<Instruction*, 64> LoopUses; SmallPtrSet<Value*, 4> PointerMustAliases; // We start with an alignment of one and try to find instructions that allow // us to prove better alignment. unsigned Alignment = 1; // 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. for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) { Value *ASIV = ASI->getValue(); PointerMustAliases.insert(ASIV); // 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. if (SomePtr->getType() != ASIV->getType()) return; for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end(); UI != UE; ++UI) { // Ignore instructions that are outside the loop. Instruction *Use = dyn_cast<Instruction>(*UI); if (!Use || !CurLoop->contains(Use)) continue; // If there is an non-load/store instruction in the loop, we can't promote // it. if (LoadInst *load = dyn_cast<LoadInst>(Use)) { assert(!load->isVolatile() && "AST broken"); if (!load->isSimple()) return; } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) { // Stores *of* the pointer are not interesting, only stores *to* the // pointer. if (Use->getOperand(1) != ASIV) continue; assert(!store->isVolatile() && "AST broken"); if (!store->isSimple()) return; // Note that we only check GuaranteedToExecute inside the store case // so that we do not introduce stores where they did not exist before // (which would break the LLVM concurrency model). // If the alignment of this instruction allows us to specify a more // restrictive (and performant) alignment and if we are sure this // instruction will be executed, update the alignment. // Larger is better, with the exception of 0 being the best alignment. unsigned InstAlignment = store->getAlignment(); if ((InstAlignment > Alignment || InstAlignment == 0) && (Alignment != 0)) if (isGuaranteedToExecute(*Use)) { GuaranteedToExecute = true; Alignment = InstAlignment; } if (!GuaranteedToExecute) GuaranteedToExecute = isGuaranteedToExecute(*Use); } else return; // Not a load or store. LoopUses.push_back(Use); } } // If there isn't a guaranteed-to-execute instruction, we can't promote. if (!GuaranteedToExecute) return; // Otherwise, this is safe to promote, lets do it! DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n'); Changed = true; ++NumPromoted; // Grab a debug location for the inserted loads/stores; given that the // inserted loads/stores have little relation to the original loads/stores, // this code just arbitrarily picks a location from one, since any debug // location is better than none. DebugLoc DL = LoopUses[0]->getDebugLoc(); // Figure out the loop exits and their insertion points, if this is the // first promotion. if (ExitBlocks.empty()) { CurLoop->getUniqueExitBlocks(ExitBlocks); InsertPts.resize(ExitBlocks.size()); for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt(); } // We use the SSAUpdater interface to insert phi nodes as required. SmallVector<PHINode*, 16> NewPHIs; SSAUpdater SSA(&NewPHIs); LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks, InsertPts, *CurAST, DL, Alignment); // Set up the preheader to have a definition of the value. It is the live-out // value from the preheader that uses in the loop will use. LoadInst *PreheaderLoad = new LoadInst(SomePtr, SomePtr->getName()+".promoted", Preheader->getTerminator()); PreheaderLoad->setAlignment(Alignment); PreheaderLoad->setDebugLoc(DL); SSA.AddAvailableValue(Preheader, PreheaderLoad); // Rewrite all the loads in the loop and remember all the definitions from // stores in the loop. Promoter.run(LoopUses); // If the SSAUpdater didn't use the load in the preheader, just zap it now. if (PreheaderLoad->use_empty()) PreheaderLoad->eraseFromParent(); } /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) { AliasSetTracker *AST = LoopToAliasSetMap.lookup(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 = LoopToAliasSetMap.lookup(L); if (!AST) return; AST->deleteValue(V); }