| //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file 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. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "adce" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/Constants.h" |
| #include "llvm/Instructions.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/PostDominators.h" |
| #include "llvm/Support/CFG.h" |
| #include "llvm/Transforms/Utils/BasicBlockUtils.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/ADT/DepthFirstIterator.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/Support/Compiler.h" |
| #include <algorithm> |
| using namespace llvm; |
| |
| STATISTIC(NumBlockRemoved, "Number of basic blocks removed"); |
| STATISTIC(NumInstRemoved , "Number of instructions removed"); |
| STATISTIC(NumCallRemoved , "Number of calls removed"); |
| |
| namespace { |
| //===----------------------------------------------------------------------===// |
| // 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 VISIBILITY_HIDDEN 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: |
| static char ID; // Pass identification, replacement for typeid |
| ADCE() : FunctionPass((intptr_t)&ID) {} |
| |
| // 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 { |
| // We require that all function nodes are unified, because otherwise code |
| // can be marked live that wouldn't necessarily be otherwise. |
| AU.addRequired<UnifyFunctionExitNodes>(); |
| AU.addRequired<AliasAnalysis>(); |
| 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); |
| |
| |
| // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in |
| // the specified basic block, deleting ones that are dead according to |
| // LiveSet. |
| bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB); |
| |
| TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI); |
| |
| inline void markInstructionLive(Instruction *I) { |
| if (!LiveSet.insert(I).second) return; |
| DOUT << "Insn Live: " << *I; |
| WorkList.push_back(I); |
| } |
| |
| inline void markTerminatorLive(const BasicBlock *BB) { |
| DOUT << "Terminator Live: " << *BB->getTerminator(); |
| markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator())); |
| } |
| }; |
| |
| char ADCE::ID = 0; |
| RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination"); |
| } // End of anonymous namespace |
| |
| FunctionPass *llvm::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 (PostDominanceFrontier::DomSetType::const_iterator I = |
| CDB.begin(), E = CDB.end(); I != E; ++I) |
| markTerminatorLive(*I); // Mark all their terminators as live |
| } |
| |
| // 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); |
| } |
| |
| // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the |
| // specified basic block, deleting ones that are dead according to LiveSet. |
| bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) { |
| bool Changed = false; |
| for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) { |
| Instruction *I = II++; |
| if (!LiveSet.count(I)) { // Is this instruction alive? |
| if (!I->use_empty()) |
| I->replaceAllUsesWith(UndefValue::get(I->getType())); |
| |
| // Nope... remove the instruction from it's basic block... |
| if (isa<CallInst>(I)) |
| ++NumCallRemoved; |
| else |
| ++NumInstRemoved; |
| BB->getInstList().erase(I); |
| Changed = true; |
| } |
| } |
| 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; |
| |
| AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); |
| |
| // 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. |
| // |
| std::set<BasicBlock*> ReachableBBs; |
| for (df_ext_iterator<BasicBlock*> |
| BBI = df_ext_begin(&Func->front(), ReachableBBs), |
| BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) { |
| BasicBlock *BB = *BBI; |
| for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) { |
| Instruction *I = II++; |
| if (CallInst *CI = dyn_cast<CallInst>(I)) { |
| if (AA.onlyReadsMemory(CI)) { |
| if (CI->use_empty()) { |
| BB->getInstList().erase(CI); |
| ++NumCallRemoved; |
| } |
| } else { |
| markInstructionLive(I); |
| } |
| } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) || |
| isa<UnwindInst>(I) || isa<UnreachableInst>(I)) { |
| // FIXME: Unreachable instructions should not be marked intrinsically |
| // live here. |
| markInstructionLive(I); |
| } else if (isInstructionTriviallyDead(I)) { |
| // Remove the instruction from it's basic block... |
| BB->getInstList().erase(I); |
| ++NumInstRemoved; |
| } |
| } |
| } |
| |
| // 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; |
| } |
| |
| // Scan the function marking blocks without post-dominance information as |
| // live. Blocks without post-dominance information occur when there is an |
| // infinite loop in the program. Because the infinite loop could contain a |
| // function which unwinds, exits or has side-effects, we don't want to delete |
| // the infinite loop or those blocks leading up to it. |
| for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) |
| if (DT[I] == 0 && ReachableBBs.count(I)) |
| for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI) |
| markInstructionLive((*PI)->getTerminator()); |
| |
| DOUT << "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 (!ReachableBBs.count(BB)) continue; |
| if (AliveBlocks.insert(BB).second) // Basic block not alive yet. |
| 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 (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { |
| // If the incoming edge is clearly dead, it won't have control |
| // dependence information. Do not mark it live. |
| BasicBlock *PredBB = PN->getIncomingBlock(i); |
| if (ReachableBBs.count(PredBB)) { |
| // FIXME: This should mark the control dependent edge as live, not |
| // necessarily the predecessor itself! |
| if (AliveBlocks.insert(PredBB).second) |
| markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE! |
| if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i))) |
| markInstructionLive(Op); |
| } |
| } |
| } else { |
| // 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( |
| DOUT << "Current Function: X = Live\n"; |
| for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){ |
| DOUT << 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)) DOUT << "X "; |
| DOUT << *BI; |
| } |
| }); |
| |
| // All blocks being live is a common case, handle it specially. |
| 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 deleting instructions |
| // to drop their references. |
| deleteDeadInstructionsInLiveBlock(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 an unconditional branch. |
| // |
| TerminatorInst *TI = I->getTerminator(); |
| if (!LiveSet.count(TI)) |
| convertToUnconditionalBranch(TI); |
| } |
| |
| return MadeChanges; |
| } |
| |
| |
| // 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(); |
| new BranchInst(&Func->front(), NewEntry); |
| 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... |
| // |
| DomTreeNode *LastNode = DT[TI->getSuccessor(i)]; |
| DomTreeNode *NextNode = 0; |
| |
| if (LastNode) { |
| NextNode = LastNode->getIDom(); |
| while (!AliveBlocks.count(NextNode->getBlock())) { |
| LastNode = NextNode; |
| NextNode = NextNode->getIDom(); |
| if (NextNode == 0) { |
| LastNode = 0; |
| break; |
| } |
| } |
| } |
| |
| // There is a special case here... if there IS no post-dominator for |
| // the block we have nowhere 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! |
| if (!isa<InvokeInst>(TI)) { |
| // 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. |
| // |
| TI = BB->getTerminator(); |
| |
| // Rescan this successor... |
| --i; |
| } else { |
| |
| } |
| } else { |
| // 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(); |
| isa<PHINode>(II); ++II) { |
| PHINode *PN = cast<PHINode>(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, deleting |
| // dead instructions. This is so that the next sweep over the program |
| // can safely delete dead instructions without other dead instructions |
| // still referring to them. |
| // |
| deleteDeadInstructionsInLiveBlock(BB); |
| } |
| |
| // 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... |
| // |
| std::vector<BasicBlock*> DeadBlocks; |
| for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) |
| if (!AliveBlocks.count(BB)) { |
| // Remove PHI node entries for this block in live successor blocks. |
| for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) |
| if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI)) |
| (*SI)->removePredecessor(BB); |
| |
| BB->dropAllReferences(); |
| MadeChanges = true; |
| DeadBlocks.push_back(BB); |
| } |
| |
| NumBlockRemoved += DeadBlocks.size(); |
| |
| // 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). |
| for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(), |
| E = DeadBlocks.end(); I != E; ++I) |
| Func->getBasicBlockList().erase(*I); |
| |
| return MadeChanges; |
| } |
