//===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This pass is an extremely simple version of the SimplifyCFG pass. Its sole // job is to delete LLVM basic blocks that are not reachable from the entry // node. To do this, it performs a simple depth first traversal of the CFG, // then deletes any unvisited nodes. // // Note that this pass is really a hack. In particular, the instruction // selectors for various targets should just not generate code for unreachable // blocks. Until LLVM has a more systematic way of defining instruction // selectors, however, we cannot really expect them to handle additional // complexity. // //===----------------------------------------------------------------------===// #include "llvm/CodeGen/Passes.h" #include "llvm/ADT/DepthFirstIterator.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/Analysis/Dominators.h" #include "llvm/CodeGen/MachineDominators.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/CodeGen/MachineLoopInfo.h" #include "llvm/CodeGen/MachineModuleInfo.h" #include "llvm/CodeGen/MachineRegisterInfo.h" #include "llvm/IR/Constant.h" #include "llvm/IR/Function.h" #include "llvm/IR/Instructions.h" #include "llvm/IR/Type.h" #include "llvm/Pass.h" #include "llvm/Support/CFG.h" #include "llvm/Target/TargetInstrInfo.h" using namespace llvm; namespace { class UnreachableBlockElim : public FunctionPass { virtual bool runOnFunction(Function &F); public: static char ID; // Pass identification, replacement for typeid UnreachableBlockElim() : FunctionPass(ID) { initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry()); } virtual void getAnalysisUsage(AnalysisUsage &AU) const { AU.addPreserved<DominatorTree>(); } }; } char UnreachableBlockElim::ID = 0; INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim", "Remove unreachable blocks from the CFG", false, false) FunctionPass *llvm::createUnreachableBlockEliminationPass() { return new UnreachableBlockElim(); } bool UnreachableBlockElim::runOnFunction(Function &F) { SmallPtrSet<BasicBlock*, 8> Reachable; // Mark all reachable blocks. for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I) /* Mark all reachable blocks */; // Loop over all dead blocks, remembering them and deleting all instructions // in them. std::vector<BasicBlock*> DeadBlocks; for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) if (!Reachable.count(I)) { BasicBlock *BB = I; DeadBlocks.push_back(BB); while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); BB->getInstList().pop_front(); } for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) (*SI)->removePredecessor(BB); BB->dropAllReferences(); } // Actually remove the blocks now. for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) { DeadBlocks[i]->eraseFromParent(); } return DeadBlocks.size(); } namespace { class UnreachableMachineBlockElim : public MachineFunctionPass { virtual bool runOnMachineFunction(MachineFunction &F); virtual void getAnalysisUsage(AnalysisUsage &AU) const; MachineModuleInfo *MMI; public: static char ID; // Pass identification, replacement for typeid UnreachableMachineBlockElim() : MachineFunctionPass(ID) {} }; } char UnreachableMachineBlockElim::ID = 0; INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination", "Remove unreachable machine basic blocks", false, false) char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID; void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const { AU.addPreserved<MachineLoopInfo>(); AU.addPreserved<MachineDominatorTree>(); MachineFunctionPass::getAnalysisUsage(AU); } bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) { SmallPtrSet<MachineBasicBlock*, 8> Reachable; bool ModifiedPHI = false; MMI = getAnalysisIfAvailable<MachineModuleInfo>(); MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>(); MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>(); // Mark all reachable blocks. for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> > I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I) /* Mark all reachable blocks */; // Loop over all dead blocks, remembering them and deleting all instructions // in them. std::vector<MachineBasicBlock*> DeadBlocks; for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { MachineBasicBlock *BB = I; // Test for deadness. if (!Reachable.count(BB)) { DeadBlocks.push_back(BB); // Update dominator and loop info. if (MLI) MLI->removeBlock(BB); if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB); while (BB->succ_begin() != BB->succ_end()) { MachineBasicBlock* succ = *BB->succ_begin(); MachineBasicBlock::iterator start = succ->begin(); while (start != succ->end() && start->isPHI()) { for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2) if (start->getOperand(i).isMBB() && start->getOperand(i).getMBB() == BB) { start->RemoveOperand(i); start->RemoveOperand(i-1); } start++; } BB->removeSuccessor(BB->succ_begin()); } } } // Actually remove the blocks now. for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) DeadBlocks[i]->eraseFromParent(); // Cleanup PHI nodes. for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { MachineBasicBlock *BB = I; // Prune unneeded PHI entries. SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(), BB->pred_end()); MachineBasicBlock::iterator phi = BB->begin(); while (phi != BB->end() && phi->isPHI()) { for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2) if (!preds.count(phi->getOperand(i).getMBB())) { phi->RemoveOperand(i); phi->RemoveOperand(i-1); ModifiedPHI = true; } if (phi->getNumOperands() == 3) { unsigned Input = phi->getOperand(1).getReg(); unsigned Output = phi->getOperand(0).getReg(); MachineInstr* temp = phi; ++phi; temp->eraseFromParent(); ModifiedPHI = true; if (Input != Output) { MachineRegisterInfo &MRI = F.getRegInfo(); MRI.constrainRegClass(Input, MRI.getRegClass(Output)); MRI.replaceRegWith(Output, Input); } continue; } ++phi; } } F.RenumberBlocks(); return (DeadBlocks.size() || ModifiedPHI); }