| //===- 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 <algorithm> |
| |
| 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<Instruction*> WorkList; // Instructions that just became live |
| std::set<Instruction*> 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<PostDominatorTree>(); |
| AU.addRequired<PostDominanceFrontier>(); |
| } |
| |
| |
| //===--------------------------------------------------------------------===// |
| // 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<TerminatorInst*>(BB->getTerminator())); |
| } |
| }; |
| |
| RegisterOpt<ADCE> 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>(); |
| |
| 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<BranchInst>(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<PHINode>(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<Function*> 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<ReturnInst>(II) || isa<UnwindInst>(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<PostDominatorTree>(); |
| 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<BasicBlock*> 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<PHINode>(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<Instruction>(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<PHINode>(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<BasicBlock*> 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; |
| } |