//===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and is distributed under // the University of Illinois Open Source License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This file promote memory references to be register references. It promotes // alloca instructions which only have loads and stores as uses (or that have // PHI nodes which are only loaded from). An alloca is transformed by using // dominator frontiers to place PHI nodes, then traversing the function in // depth-first order to rewrite loads and stores as appropriate. This is just // the standard SSA construction algorithm to construct "pruned" SSA form. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Utils/PromoteMemToReg.h" #include "llvm/Analysis/Dominators.h" #include "llvm/iMemory.h" #include "llvm/iPHINode.h" #include "llvm/iOther.h" #include "llvm/Function.h" #include "llvm/Constant.h" #include "llvm/Support/CFG.h" #include "Support/StringExtras.h" using namespace llvm; /// isAllocaPromotable - Return true if this alloca is legal for promotion. /// This is true if there are only loads and stores to the alloca... of if there /// is a PHI node using the address which can be trivially transformed. /// bool llvm::isAllocaPromotable(const AllocaInst *AI, const TargetData &TD) { // FIXME: If the memory unit is of pointer or integer type, we can permit // assignments to subsections of the memory unit. // Only allow direct loads and stores... for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end(); UI != UE; ++UI) // Loop over all of the uses of the alloca if (isa(*UI)) { // noop } else if (const StoreInst *SI = dyn_cast(*UI)) { if (SI->getOperand(0) == AI) return false; // Don't allow a store OF the AI, only INTO the AI. } else if (const PHINode *PN = dyn_cast(*UI)) { // We only support PHI nodes in a few simple cases. The PHI node is only // allowed to have one use, which must be a load instruction, and can only // use alloca instructions (no random pointers). Also, there cannot be // any accesses to AI between the PHI node and the use of the PHI. if (!PN->hasOneUse()) return false; // Our transformation causes the unconditional loading of all pointer // operands to the PHI node. Because this could cause a fault if there is // a critical edge in the CFG and if one of the pointers is illegal, we // refuse to promote PHI nodes unless they are obviously safe. For now, // obviously safe means that all of the operands are allocas. // // If we wanted to extend this code to break critical edges, this // restriction could be relaxed, and we could even handle uses of the PHI // node that are volatile loads or stores. // for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (!isa(PN->getIncomingValue(i))) return false; // Now make sure the one user instruction is in the same basic block as // the PHI, and that there are no loads or stores between the PHI node and // the access. BasicBlock::const_iterator UI = cast(PN->use_back()); if (!isa(UI) || cast(UI)->isVolatile()) return false; // Scan looking for memory accesses. for (--UI; !isa(UI); --UI) if (isa(UI) || isa(UI) || isa(UI)) return false; // If we got this far, we can promote the PHI use. } else { return false; // Not a load, store, or promotable PHI? } return true; } namespace { struct PromoteMem2Reg { // Allocas - The alloca instructions being promoted std::vector Allocas; DominatorTree &DT; DominanceFrontier &DF; const TargetData &TD; // AllocaLookup - Reverse mapping of Allocas std::map AllocaLookup; // NewPhiNodes - The PhiNodes we're adding. std::map > NewPhiNodes; // Visited - The set of basic blocks the renamer has already visited. std::set Visited; public: PromoteMem2Reg(const std::vector &A, DominatorTree &dt, DominanceFrontier &df, const TargetData &td) : Allocas(A), DT(dt), DF(df), TD(td) {} void run(); private: void MarkDominatingPHILive(BasicBlock *BB, unsigned AllocaNum, std::set &DeadPHINodes); void PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI); void PromoteLocallyUsedAllocas(BasicBlock *BB, const std::vector &AIs); void RenamePass(BasicBlock *BB, BasicBlock *Pred, std::vector &IncVals); bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version, std::set &InsertedPHINodes); }; } // end of anonymous namespace void PromoteMem2Reg::run() { Function &F = *DF.getRoot()->getParent(); // LocallyUsedAllocas - Keep track of all of the alloca instructions which are // only used in a single basic block. These instructions can be efficiently // promoted by performing a single linear scan over that one block. Since // individual basic blocks are sometimes large, we group together all allocas // that are live in a single basic block by the basic block they are live in. std::map > LocallyUsedAllocas; for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) { AllocaInst *AI = Allocas[AllocaNum]; assert(isAllocaPromotable(AI, TD) && "Cannot promote non-promotable alloca!"); assert(AI->getParent()->getParent() == &F && "All allocas should be in the same function, which is same as DF!"); if (AI->use_empty()) { // If there are no uses of the alloca, just delete it now. AI->getParent()->getInstList().erase(AI); // Remove the alloca from the Allocas list, since it has been processed Allocas[AllocaNum] = Allocas.back(); Allocas.pop_back(); --AllocaNum; continue; } // Calculate the set of read and write-locations for each alloca. This is // analogous to finding the 'uses' and 'definitions' of each variable. std::vector DefiningBlocks; std::vector UsingBlocks; BasicBlock *OnlyBlock = 0; bool OnlyUsedInOneBlock = true; // As we scan the uses of the alloca instruction, keep track of stores, and // decide whether all of the loads and stores to the alloca are within the // same basic block. RestartUseScan: for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E;++U){ Instruction *User = cast(*U); if (StoreInst *SI = dyn_cast(User)) { // Remember the basic blocks which define new values for the alloca DefiningBlocks.push_back(SI->getParent()); } else if (LoadInst *LI = dyn_cast(User)) { // Otherwise it must be a load instruction, keep track of variable reads UsingBlocks.push_back(LI->getParent()); } else { // Because of the restrictions we placed on PHI node uses above, the PHI // node reads the block in any using predecessors. Transform the PHI of // addresses into a PHI of loaded values. PHINode *PN = cast(User); assert(PN->hasOneUse() && "Cannot handle PHI Node with != 1 use!"); LoadInst *PNUser = cast(PN->use_back()); std::string PNUserName = PNUser->getName(); PNUser->setName(""); // Create the new PHI node and insert load instructions as appropriate. PHINode *NewPN = new PHINode(AI->getAllocatedType(), PNUserName, PN); std::map NewLoads; for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { BasicBlock *Pred = PN->getIncomingBlock(i); LoadInst *&NewLoad = NewLoads[Pred]; if (NewLoad == 0) // Insert the new load in the predecessor NewLoad = new LoadInst(PN->getIncomingValue(i), PN->getIncomingValue(i)->getName()+".val", Pred->getTerminator()); NewPN->addIncoming(NewLoad, Pred); } // Remove the old load. PNUser->replaceAllUsesWith(NewPN); PNUser->getParent()->getInstList().erase(PNUser); // Remove the old PHI node. PN->getParent()->getInstList().erase(PN); // Restart our scan of uses... DefiningBlocks.clear(); UsingBlocks.clear(); goto RestartUseScan; } if (OnlyUsedInOneBlock) { if (OnlyBlock == 0) OnlyBlock = User->getParent(); else if (OnlyBlock != User->getParent()) OnlyUsedInOneBlock = false; } } // If the alloca is only read and written in one basic block, just perform a // linear sweep over the block to eliminate it. if (OnlyUsedInOneBlock) { LocallyUsedAllocas[OnlyBlock].push_back(AI); // Remove the alloca from the Allocas list, since it will be processed. Allocas[AllocaNum] = Allocas.back(); Allocas.pop_back(); --AllocaNum; continue; } // Compute the locations where PhiNodes need to be inserted. Look at the // dominance frontier of EACH basic-block we have a write in. // unsigned CurrentVersion = 0; std::set InsertedPHINodes; while (!DefiningBlocks.empty()) { BasicBlock *BB = DefiningBlocks.back(); DefiningBlocks.pop_back(); // Look up the DF for this write, add it to PhiNodes DominanceFrontier::const_iterator it = DF.find(BB); if (it != DF.end()) { const DominanceFrontier::DomSetType &S = it->second; for (DominanceFrontier::DomSetType::iterator P = S.begin(),PE = S.end(); P != PE; ++P) if (QueuePhiNode(*P, AllocaNum, CurrentVersion, InsertedPHINodes)) DefiningBlocks.push_back(*P); } } // Now that we have inserted PHI nodes along the Iterated Dominance Frontier // of the writes to the variable, scan through the reads of the variable, // marking PHI nodes which are actually necessary as alive (by removing them // from the InsertedPHINodes set). This is not perfect: there may PHI // marked alive because of loads which are dominated by stores, but there // will be no unmarked PHI nodes which are actually used. // for (unsigned i = 0, e = UsingBlocks.size(); i != e; ++i) MarkDominatingPHILive(UsingBlocks[i], AllocaNum, InsertedPHINodes); UsingBlocks.clear(); // If there are any PHI nodes which are now known to be dead, remove them! for (std::set::iterator I = InsertedPHINodes.begin(), E = InsertedPHINodes.end(); I != E; ++I) { PHINode *PN = *I; std::vector &BBPNs = NewPhiNodes[PN->getParent()]; BBPNs[AllocaNum] = 0; // Check to see if we just removed the last inserted PHI node from this // basic block. If so, remove the entry for the basic block. bool HasOtherPHIs = false; for (unsigned i = 0, e = BBPNs.size(); i != e; ++i) if (BBPNs[i]) { HasOtherPHIs = true; break; } if (!HasOtherPHIs) NewPhiNodes.erase(PN->getParent()); PN->getParent()->getInstList().erase(PN); } // Keep the reverse mapping of the 'Allocas' array. AllocaLookup[Allocas[AllocaNum]] = AllocaNum; } // Process all allocas which are only used in a single basic block. for (std::map >::iterator I = LocallyUsedAllocas.begin(), E = LocallyUsedAllocas.end(); I != E; ++I){ const std::vector &Allocas = I->second; assert(!Allocas.empty() && "empty alloca list??"); // It's common for there to only be one alloca in the list. Handle it // efficiently. if (Allocas.size() == 1) PromoteLocallyUsedAlloca(I->first, Allocas[0]); else PromoteLocallyUsedAllocas(I->first, Allocas); } if (Allocas.empty()) return; // All of the allocas must have been trivial! // Set the incoming values for the basic block to be null values for all of // the alloca's. We do this in case there is a load of a value that has not // been stored yet. In this case, it will get this null value. // std::vector Values(Allocas.size()); for (unsigned i = 0, e = Allocas.size(); i != e; ++i) Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType()); // Walks all basic blocks in the function performing the SSA rename algorithm // and inserting the phi nodes we marked as necessary // RenamePass(F.begin(), 0, Values); // The renamer uses the Visited set to avoid infinite loops. Clear it now. Visited.clear(); // Remove the allocas themselves from the function... for (unsigned i = 0, e = Allocas.size(); i != e; ++i) { Instruction *A = Allocas[i]; // If there are any uses of the alloca instructions left, they must be in // sections of dead code that were not processed on the dominance frontier. // Just delete the users now. // if (!A->use_empty()) A->replaceAllUsesWith(Constant::getNullValue(A->getType())); A->getParent()->getInstList().erase(A); } // At this point, the renamer has added entries to PHI nodes for all reachable // code. Unfortunately, there may be blocks which are not reachable, which // the renamer hasn't traversed. If this is the case, the PHI nodes may not // have incoming values for all predecessors. Loop over all PHI nodes we have // created, inserting null constants if they are missing any incoming values. // for (std::map >::iterator I = NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E; ++I) { std::vector Preds(pred_begin(I->first), pred_end(I->first)); std::vector &PNs = I->second; assert(!PNs.empty() && "Empty PHI node list??"); // Only do work here if there the PHI nodes are missing incoming values. We // know that all PHI nodes that were inserted in a block will have the same // number of incoming values, so we can just check any PHI node. PHINode *FirstPHI; for (unsigned i = 0; (FirstPHI = PNs[i]) == 0; ++i) /*empty*/; if (Preds.size() != FirstPHI->getNumIncomingValues()) { // Ok, now we know that all of the PHI nodes are missing entries for some // basic blocks. Start by sorting the incoming predecessors for efficient // access. std::sort(Preds.begin(), Preds.end()); // Now we loop through all BB's which have entries in FirstPHI and remove // them from the Preds list. for (unsigned i = 0, e = FirstPHI->getNumIncomingValues(); i != e; ++i) { // Do a log(n) search of the Preds list for the entry we want. std::vector::iterator EntIt = std::lower_bound(Preds.begin(), Preds.end(), FirstPHI->getIncomingBlock(i)); assert(EntIt != Preds.end() && *EntIt == FirstPHI->getIncomingBlock(i)&& "PHI node has entry for a block which is not a predecessor!"); // Remove the entry Preds.erase(EntIt); } // At this point, the blocks left in the preds list must have dummy // entries inserted into every PHI nodes for the block. for (unsigned i = 0, e = PNs.size(); i != e; ++i) if (PHINode *PN = PNs[i]) { Value *NullVal = Constant::getNullValue(PN->getType()); for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred) PN->addIncoming(NullVal, Preds[pred]); } } } } // MarkDominatingPHILive - Mem2Reg wants to construct "pruned" SSA form, not // "minimal" SSA form. To do this, it inserts all of the PHI nodes on the IDF // as usual (inserting the PHI nodes in the DeadPHINodes set), then processes // each read of the variable. For each block that reads the variable, this // function is called, which removes used PHI nodes from the DeadPHINodes set. // After all of the reads have been processed, any PHI nodes left in the // DeadPHINodes set are removed. // void PromoteMem2Reg::MarkDominatingPHILive(BasicBlock *BB, unsigned AllocaNum, std::set &DeadPHINodes) { // Scan the immediate dominators of this block looking for a block which has a // PHI node for Alloca num. If we find it, mark the PHI node as being alive! for (DominatorTree::Node *N = DT[BB]; N; N = N->getIDom()) { BasicBlock *DomBB = N->getBlock(); std::map >::iterator I = NewPhiNodes.find(DomBB); if (I != NewPhiNodes.end() && I->second[AllocaNum]) { // Ok, we found an inserted PHI node which dominates this value. PHINode *DominatingPHI = I->second[AllocaNum]; // Find out if we previously thought it was dead. std::set::iterator DPNI = DeadPHINodes.find(DominatingPHI); if (DPNI != DeadPHINodes.end()) { // Ok, until now, we thought this PHI node was dead. Mark it as being // alive/needed. DeadPHINodes.erase(DPNI); // Now that we have marked the PHI node alive, also mark any PHI nodes // which it might use as being alive as well. for (pred_iterator PI = pred_begin(DomBB), PE = pred_end(DomBB); PI != PE; ++PI) MarkDominatingPHILive(*PI, AllocaNum, DeadPHINodes); } } } } /// PromoteLocallyUsedAlloca - Many allocas are only used within a single basic /// block. If this is the case, avoid traversing the CFG and inserting a lot of /// potentially useless PHI nodes by just performing a single linear pass over /// the basic block using the Alloca. /// void PromoteMem2Reg::PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI) { assert(!AI->use_empty() && "There are no uses of the alloca!"); // Handle degenerate cases quickly. if (AI->hasOneUse()) { Instruction *U = cast(AI->use_back()); if (LoadInst *LI = dyn_cast(U)) { // Must be a load of uninitialized value. LI->replaceAllUsesWith(Constant::getNullValue(AI->getAllocatedType())); } else { // Otherwise it must be a store which is never read. assert(isa(U)); } BB->getInstList().erase(U); } else { // Uses of the uninitialized memory location shall get zero... Value *CurVal = Constant::getNullValue(AI->getAllocatedType()); for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { Instruction *Inst = I++; if (LoadInst *LI = dyn_cast(Inst)) { if (LI->getOperand(0) == AI) { // Loads just returns the "current value"... LI->replaceAllUsesWith(CurVal); BB->getInstList().erase(LI); } } else if (StoreInst *SI = dyn_cast(Inst)) { if (SI->getOperand(1) == AI) { // Store updates the "current value"... CurVal = SI->getOperand(0); BB->getInstList().erase(SI); } } } } // After traversing the basic block, there should be no more uses of the // alloca, remove it now. assert(AI->use_empty() && "Uses of alloca from more than one BB??"); AI->getParent()->getInstList().erase(AI); } /// PromoteLocallyUsedAllocas - This method is just like /// PromoteLocallyUsedAlloca, except that it processes multiple alloca /// instructions in parallel. This is important in cases where we have large /// basic blocks, as we don't want to rescan the entire basic block for each /// alloca which is locally used in it (which might be a lot). void PromoteMem2Reg:: PromoteLocallyUsedAllocas(BasicBlock *BB, const std::vector &AIs) { std::map CurValues; for (unsigned i = 0, e = AIs.size(); i != e; ++i) CurValues[AIs[i]] = 0; // Insert with null value for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) { Instruction *Inst = I++; if (LoadInst *LI = dyn_cast(Inst)) { // Is this a load of an alloca we are tracking? if (AllocaInst *AI = dyn_cast(LI->getOperand(0))) { std::map::iterator AIt = CurValues.find(AI); if (AIt != CurValues.end()) { // Loads just returns the "current value"... if (AIt->second == 0) // Uninitialized value?? AIt->second =Constant::getNullValue(AIt->first->getAllocatedType()); LI->replaceAllUsesWith(AIt->second); BB->getInstList().erase(LI); } } } else if (StoreInst *SI = dyn_cast(Inst)) { if (AllocaInst *AI = dyn_cast(SI->getOperand(1))) { std::map::iterator AIt = CurValues.find(AI); if (AIt != CurValues.end()) { // Store updates the "current value"... AIt->second = SI->getOperand(0); BB->getInstList().erase(SI); } } } } } // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific // Alloca returns true if there wasn't already a phi-node for that variable // bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo, unsigned &Version, std::set &InsertedPHINodes) { // Look up the basic-block in question std::vector &BBPNs = NewPhiNodes[BB]; if (BBPNs.empty()) BBPNs.resize(Allocas.size()); // If the BB already has a phi node added for the i'th alloca then we're done! if (BBPNs[AllocaNo]) return false; // Create a PhiNode using the dereferenced type... and add the phi-node to the // BasicBlock. BBPNs[AllocaNo] = new PHINode(Allocas[AllocaNo]->getAllocatedType(), Allocas[AllocaNo]->getName() + "." + utostr(Version++), BB->begin()); InsertedPHINodes.insert(BBPNs[AllocaNo]); return true; } // RenamePass - Recursively traverse the CFG of the function, renaming loads and // stores to the allocas which we are promoting. IncomingVals indicates what // value each Alloca contains on exit from the predecessor block Pred. // void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred, std::vector &IncomingVals) { // If this BB needs a PHI node, update the PHI node for each variable we need // PHI nodes for. std::map >::iterator BBPNI = NewPhiNodes.find(BB); if (BBPNI != NewPhiNodes.end()) { std::vector &BBPNs = BBPNI->second; for (unsigned k = 0; k != BBPNs.size(); ++k) if (PHINode *PN = BBPNs[k]) { // Add this incoming value to the PHI node. PN->addIncoming(IncomingVals[k], Pred); // The currently active variable for this block is now the PHI. IncomingVals[k] = PN; } } // don't revisit nodes if (Visited.count(BB)) return; // mark as visited Visited.insert(BB); for (BasicBlock::iterator II = BB->begin(); !isa(II); ) { Instruction *I = II++; // get the instruction, increment iterator if (LoadInst *LI = dyn_cast(I)) { if (AllocaInst *Src = dyn_cast(LI->getPointerOperand())) { std::map::iterator AI = AllocaLookup.find(Src); if (AI != AllocaLookup.end()) { Value *V = IncomingVals[AI->second]; // walk the use list of this load and replace all uses with r LI->replaceAllUsesWith(V); BB->getInstList().erase(LI); } } } else if (StoreInst *SI = dyn_cast(I)) { // Delete this instruction and mark the name as the current holder of the // value if (AllocaInst *Dest = dyn_cast(SI->getPointerOperand())) { std::map::iterator ai = AllocaLookup.find(Dest); if (ai != AllocaLookup.end()) { // what value were we writing? IncomingVals[ai->second] = SI->getOperand(0); BB->getInstList().erase(SI); } } } } // Recurse to our successors. TerminatorInst *TI = BB->getTerminator(); for (unsigned i = 0; i != TI->getNumSuccessors(); i++) { std::vector OutgoingVals(IncomingVals); RenamePass(TI->getSuccessor(i), BB, OutgoingVals); } } /// PromoteMemToReg - Promote the specified list of alloca instructions into /// scalar registers, inserting PHI nodes as appropriate. This function makes /// use of DominanceFrontier information. This function does not modify the CFG /// of the function at all. All allocas must be from the same function. /// void llvm::PromoteMemToReg(const std::vector &Allocas, DominatorTree &DT, DominanceFrontier &DF, const TargetData &TD) { // If there is nothing to do, bail out... if (Allocas.empty()) return; PromoteMem2Reg(Allocas, DT, DF, TD).run(); }