//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===// // // 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 implements "aggressive" dead code elimination. ADCE is DCe where // values are assumed to be dead until proven otherwise. This is similar to // SCCP, except applied to the liveness of values. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Type.h" #include "llvm/Analysis/PostDominators.h" #include "llvm/iTerminators.h" #include "llvm/iPHINode.h" #include "llvm/Constant.h" #include "llvm/Support/CFG.h" #include "Support/Debug.h" #include "Support/DepthFirstIterator.h" #include "Support/Statistic.h" #include "Support/STLExtras.h" #include namespace { Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed"); Statistic<> NumInstRemoved ("adce", "Number of instructions removed"); //===----------------------------------------------------------------------===// // ADCE Class // // This class does all of the work of Aggressive Dead Code Elimination. // It's public interface consists of a constructor and a doADCE() method. // class ADCE : public FunctionPass { Function *Func; // The function that we are working on std::vector WorkList; // Instructions that just became live std::set LiveSet; // The set of live instructions //===--------------------------------------------------------------------===// // The public interface for this class // public: // Execute the Aggressive Dead Code Elimination Algorithm // virtual bool runOnFunction(Function &F) { Func = &F; bool Changed = doADCE(); assert(WorkList.empty()); LiveSet.clear(); return Changed; } // getAnalysisUsage - We require post dominance frontiers (aka Control // Dependence Graph) virtual void getAnalysisUsage(AnalysisUsage &AU) const { AU.addRequired(); AU.addRequired(); } //===--------------------------------------------------------------------===// // The implementation of this class // private: // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning // true if the function was modified. // bool doADCE(); void markBlockAlive(BasicBlock *BB); // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the // instructions in the specified basic block, dropping references on // instructions that are dead according to LiveSet. bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB); TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI); inline void markInstructionLive(Instruction *I) { if (LiveSet.count(I)) return; DEBUG(std::cerr << "Insn Live: " << I); LiveSet.insert(I); WorkList.push_back(I); } inline void markTerminatorLive(const BasicBlock *BB) { DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator()); markInstructionLive(const_cast(BB->getTerminator())); } }; RegisterOpt X("adce", "Aggressive Dead Code Elimination"); } // End of anonymous namespace Pass *createAggressiveDCEPass() { return new ADCE(); } void ADCE::markBlockAlive(BasicBlock *BB) { // Mark the basic block as being newly ALIVE... and mark all branches that // this block is control dependent on as being alive also... // PostDominanceFrontier &CDG = getAnalysis(); PostDominanceFrontier::const_iterator It = CDG.find(BB); if (It != CDG.end()) { // Get the blocks that this node is control dependent on... const PostDominanceFrontier::DomSetType &CDB = It->second; for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live bind_obj(this, &ADCE::markTerminatorLive)); } // If this basic block is live, and it ends in an unconditional branch, then // the branch is alive as well... if (BranchInst *BI = dyn_cast(BB->getTerminator())) if (BI->isUnconditional()) markTerminatorLive(BB); } // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the // instructions in the specified basic block, dropping references on // instructions that are dead according to LiveSet. bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) { bool Changed = false; for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ) if (!LiveSet.count(I)) { // Is this instruction alive? I->dropAllReferences(); // Nope, drop references... if (PHINode *PN = dyn_cast(I)) { // We don't want to leave PHI nodes in the program that have // #arguments != #predecessors, so we remove them now. // PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); // Delete the instruction... I = BB->getInstList().erase(I); Changed = true; } else { ++I; } } else { ++I; } return Changed; } /// convertToUnconditionalBranch - Transform this conditional terminator /// instruction into an unconditional branch because we don't care which of the /// successors it goes to. This eliminate a use of the condition as well. /// TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) { BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI); BasicBlock *BB = TI->getParent(); // Remove entries from PHI nodes to avoid confusing ourself later... for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i) TI->getSuccessor(i)->removePredecessor(BB); // Delete the old branch itself... BB->getInstList().erase(TI); return NB; } // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning // true if the function was modified. // bool ADCE::doADCE() { bool MadeChanges = false; // Iterate over all of the instructions in the function, eliminating trivially // dead instructions, and marking instructions live that are known to be // needed. Perform the walk in depth first order so that we avoid marking any // instructions live in basic blocks that are unreachable. These blocks will // be eliminated later, along with the instructions inside. // for (df_iterator BBI = df_begin(Func), BBE = df_end(Func); BBI != BBE; ++BBI) { BasicBlock *BB = *BBI; for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) { if (II->mayWriteToMemory() || isa(II) || isa(II)){ markInstructionLive(II); ++II; // Increment the inst iterator if the inst wasn't deleted } else if (isInstructionTriviallyDead(II)) { // Remove the instruction from it's basic block... II = BB->getInstList().erase(II); ++NumInstRemoved; MadeChanges = true; } else { ++II; // Increment the inst iterator if the inst wasn't deleted } } } // Check to ensure we have an exit node for this CFG. If we don't, we won't // have any post-dominance information, thus we cannot perform our // transformations safely. // PostDominatorTree &DT = getAnalysis(); if (DT[&Func->getEntryBlock()] == 0) { WorkList.clear(); return MadeChanges; } DEBUG(std::cerr << "Processing work list\n"); // AliveBlocks - Set of basic blocks that we know have instructions that are // alive in them... // std::set AliveBlocks; // Process the work list of instructions that just became live... if they // became live, then that means that all of their operands are necessary as // well... make them live as well. // while (!WorkList.empty()) { Instruction *I = WorkList.back(); // Get an instruction that became live... WorkList.pop_back(); BasicBlock *BB = I->getParent(); if (!AliveBlocks.count(BB)) { // Basic block not alive yet... AliveBlocks.insert(BB); // Block is now ALIVE! markBlockAlive(BB); // Make it so now! } // PHI nodes are a special case, because the incoming values are actually // defined in the predecessor nodes of this block, meaning that the PHI // makes the predecessors alive. // if (PHINode *PN = dyn_cast(I)) for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI) if (!AliveBlocks.count(*PI)) { AliveBlocks.insert(BB); // Block is now ALIVE! markBlockAlive(*PI); } // Loop over all of the operands of the live instruction, making sure that // they are known to be alive as well... // for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) if (Instruction *Operand = dyn_cast(I->getOperand(op))) markInstructionLive(Operand); } DEBUG( std::cerr << "Current Function: X = Live\n"; for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){ std::cerr << I->getName() << ":\t" << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n"); for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){ if (LiveSet.count(BI)) std::cerr << "X "; std::cerr << *BI; } }); // Find the first postdominator of the entry node that is alive. Make it the // new entry node... // if (AliveBlocks.size() == Func->size()) { // No dead blocks? for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) { // Loop over all of the instructions in the function, telling dead // instructions to drop their references. This is so that the next sweep // over the program can safely delete dead instructions without other dead // instructions still referring to them. // dropReferencesOfDeadInstructionsInLiveBlock(I); // Check to make sure the terminator instruction is live. If it isn't, // this means that the condition that it branches on (we know it is not an // unconditional branch), is not needed to make the decision of where to // go to, because all outgoing edges go to the same place. We must remove // the use of the condition (because it's probably dead), so we convert // the terminator to a conditional branch. // TerminatorInst *TI = I->getTerminator(); if (!LiveSet.count(TI)) convertToUnconditionalBranch(TI); } } else { // If there are some blocks dead... // If the entry node is dead, insert a new entry node to eliminate the entry // node as a special case. // if (!AliveBlocks.count(&Func->front())) { BasicBlock *NewEntry = new BasicBlock(); NewEntry->getInstList().push_back(new BranchInst(&Func->front())); Func->getBasicBlockList().push_front(NewEntry); AliveBlocks.insert(NewEntry); // This block is always alive! LiveSet.insert(NewEntry->getTerminator()); // The branch is live } // Loop over all of the alive blocks in the function. If any successor // blocks are not alive, we adjust the outgoing branches to branch to the // first live postdominator of the live block, adjusting any PHI nodes in // the block to reflect this. // for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) if (AliveBlocks.count(I)) { BasicBlock *BB = I; TerminatorInst *TI = BB->getTerminator(); // If the terminator instruction is alive, but the block it is contained // in IS alive, this means that this terminator is a conditional branch // on a condition that doesn't matter. Make it an unconditional branch // to ONE of the successors. This has the side effect of dropping a use // of the conditional value, which may also be dead. if (!LiveSet.count(TI)) TI = convertToUnconditionalBranch(TI); // Loop over all of the successors, looking for ones that are not alive. // We cannot save the number of successors in the terminator instruction // here because we may remove them if we don't have a postdominator... // for (unsigned i = 0; i != TI->getNumSuccessors(); ++i) if (!AliveBlocks.count(TI->getSuccessor(i))) { // Scan up the postdominator tree, looking for the first // postdominator that is alive, and the last postdominator that is // dead... // PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)]; // There is a special case here... if there IS no post-dominator for // the block we have no owhere to point our branch to. Instead, // convert it to a return. This can only happen if the code // branched into an infinite loop. Note that this may not be // desirable, because we _are_ altering the behavior of the code. // This is a well known drawback of ADCE, so in the future if we // choose to revisit the decision, this is where it should be. // if (LastNode == 0) { // No postdominator! // Call RemoveSuccessor to transmogrify the terminator instruction // to not contain the outgoing branch, or to create a new // terminator if the form fundamentally changes (i.e., // unconditional branch to return). Note that this will change a // branch into an infinite loop into a return instruction! // RemoveSuccessor(TI, i); // RemoveSuccessor may replace TI... make sure we have a fresh // pointer... and e variable. // TI = BB->getTerminator(); // Rescan this successor... --i; } else { PostDominatorTree::Node *NextNode = LastNode->getIDom(); while (!AliveBlocks.count(NextNode->getBlock())) { LastNode = NextNode; NextNode = NextNode->getIDom(); } // Get the basic blocks that we need... BasicBlock *LastDead = LastNode->getBlock(); BasicBlock *NextAlive = NextNode->getBlock(); // Make the conditional branch now go to the next alive block... TI->getSuccessor(i)->removePredecessor(BB); TI->setSuccessor(i, NextAlive); // If there are PHI nodes in NextAlive, we need to add entries to // the PHI nodes for the new incoming edge. The incoming values // should be identical to the incoming values for LastDead. // for (BasicBlock::iterator II = NextAlive->begin(); PHINode *PN = dyn_cast(II); ++II) if (LiveSet.count(PN)) { // Only modify live phi nodes // Get the incoming value for LastDead... int OldIdx = PN->getBasicBlockIndex(LastDead); assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!"); Value *InVal = PN->getIncomingValue(OldIdx); // Add an incoming value for BB now... PN->addIncoming(InVal, BB); } } } // Now loop over all of the instructions in the basic block, telling // dead instructions to drop their references. This is so that the next // sweep over the program can safely delete dead instructions without // other dead instructions still referring to them. // dropReferencesOfDeadInstructionsInLiveBlock(BB); } } // We make changes if there are any dead blocks in the function... if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) { MadeChanges = true; NumBlockRemoved += NumDeadBlocks; } // Loop over all of the basic blocks in the function, removing control flow // edges to live blocks (also eliminating any entries in PHI functions in // referenced blocks). // for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) if (!AliveBlocks.count(BB)) { // Remove all outgoing edges from this basic block and convert the // terminator into a return instruction. std::vector Succs(succ_begin(BB), succ_end(BB)); if (!Succs.empty()) { // Loop over all of the successors, removing this block from PHI node // entries that might be in the block... while (!Succs.empty()) { Succs.back()->removePredecessor(BB); Succs.pop_back(); } // Delete the old terminator instruction... BB->getInstList().pop_back(); const Type *RetTy = Func->getReturnType(); BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ? Constant::getNullValue(RetTy) : 0)); } } // Loop over all of the basic blocks in the function, dropping references of // the dead basic blocks. We must do this after the previous step to avoid // dropping references to PHIs which still have entries... // for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) if (!AliveBlocks.count(BB)) BB->dropAllReferences(); // Now loop through all of the blocks and delete the dead ones. We can safely // do this now because we know that there are no references to dead blocks // (because they have dropped all of their references... we also remove dead // instructions from alive blocks. // for (Function::iterator BI = Func->begin(); BI != Func->end(); ) if (!AliveBlocks.count(BI)) { // Delete dead blocks... BI = Func->getBasicBlockList().erase(BI); } else { // Scan alive blocks... for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); ) if (!LiveSet.count(II)) { // Is this instruction alive? // Nope... remove the instruction from it's basic block... II = BI->getInstList().erase(II); ++NumInstRemoved; MadeChanges = true; } else { ++II; } ++BI; // Increment iterator... } return MadeChanges; }