1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This pass is an extremely simple version of the SimplifyCFG pass. Its sole 11 // job is to delete LLVM basic blocks that are not reachable from the entry 12 // node. To do this, it performs a simple depth first traversal of the CFG, 13 // then deletes any unvisited nodes. 14 // 15 // Note that this pass is really a hack. In particular, the instruction 16 // selectors for various targets should just not generate code for unreachable 17 // blocks. Until LLVM has a more systematic way of defining instruction 18 // selectors, however, we cannot really expect them to handle additional 19 // complexity. 20 // 21 //===----------------------------------------------------------------------===// 22 23 #include "llvm/CodeGen/Passes.h" 24 #include "llvm/ADT/DepthFirstIterator.h" 25 #include "llvm/ADT/SmallPtrSet.h" 26 #include "llvm/Analysis/Dominators.h" 27 #include "llvm/CodeGen/MachineDominators.h" 28 #include "llvm/CodeGen/MachineFunctionPass.h" 29 #include "llvm/CodeGen/MachineLoopInfo.h" 30 #include "llvm/CodeGen/MachineModuleInfo.h" 31 #include "llvm/CodeGen/MachineRegisterInfo.h" 32 #include "llvm/IR/Constant.h" 33 #include "llvm/IR/Function.h" 34 #include "llvm/IR/Instructions.h" 35 #include "llvm/IR/Type.h" 36 #include "llvm/Pass.h" 37 #include "llvm/Support/CFG.h" 38 #include "llvm/Target/TargetInstrInfo.h" 39 using namespace llvm; 40 41 namespace { 42 class UnreachableBlockElim : public FunctionPass { 43 virtual bool runOnFunction(Function &F); 44 public: 45 static char ID; // Pass identification, replacement for typeid 46 UnreachableBlockElim() : FunctionPass(ID) { 47 initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry()); 48 } 49 50 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 51 AU.addPreserved<DominatorTree>(); 52 } 53 }; 54 } 55 char UnreachableBlockElim::ID = 0; 56 INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim", 57 "Remove unreachable blocks from the CFG", false, false) 58 59 FunctionPass *llvm::createUnreachableBlockEliminationPass() { 60 return new UnreachableBlockElim(); 61 } 62 63 bool UnreachableBlockElim::runOnFunction(Function &F) { 64 SmallPtrSet<BasicBlock*, 8> Reachable; 65 66 // Mark all reachable blocks. 67 for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I = 68 df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I) 69 /* Mark all reachable blocks */; 70 71 // Loop over all dead blocks, remembering them and deleting all instructions 72 // in them. 73 std::vector<BasicBlock*> DeadBlocks; 74 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 75 if (!Reachable.count(I)) { 76 BasicBlock *BB = I; 77 DeadBlocks.push_back(BB); 78 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 79 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); 80 BB->getInstList().pop_front(); 81 } 82 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) 83 (*SI)->removePredecessor(BB); 84 BB->dropAllReferences(); 85 } 86 87 // Actually remove the blocks now. 88 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) { 89 DeadBlocks[i]->eraseFromParent(); 90 } 91 92 return DeadBlocks.size(); 93 } 94 95 96 namespace { 97 class UnreachableMachineBlockElim : public MachineFunctionPass { 98 virtual bool runOnMachineFunction(MachineFunction &F); 99 virtual void getAnalysisUsage(AnalysisUsage &AU) const; 100 MachineModuleInfo *MMI; 101 public: 102 static char ID; // Pass identification, replacement for typeid 103 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {} 104 }; 105 } 106 char UnreachableMachineBlockElim::ID = 0; 107 108 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination", 109 "Remove unreachable machine basic blocks", false, false) 110 111 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID; 112 113 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const { 114 AU.addPreserved<MachineLoopInfo>(); 115 AU.addPreserved<MachineDominatorTree>(); 116 MachineFunctionPass::getAnalysisUsage(AU); 117 } 118 119 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) { 120 SmallPtrSet<MachineBasicBlock*, 8> Reachable; 121 bool ModifiedPHI = false; 122 123 MMI = getAnalysisIfAvailable<MachineModuleInfo>(); 124 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>(); 125 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>(); 126 127 // Mark all reachable blocks. 128 for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> > 129 I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); 130 I != E; ++I) 131 /* Mark all reachable blocks */; 132 133 // Loop over all dead blocks, remembering them and deleting all instructions 134 // in them. 135 std::vector<MachineBasicBlock*> DeadBlocks; 136 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { 137 MachineBasicBlock *BB = I; 138 139 // Test for deadness. 140 if (!Reachable.count(BB)) { 141 DeadBlocks.push_back(BB); 142 143 // Update dominator and loop info. 144 if (MLI) MLI->removeBlock(BB); 145 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB); 146 147 while (BB->succ_begin() != BB->succ_end()) { 148 MachineBasicBlock* succ = *BB->succ_begin(); 149 150 MachineBasicBlock::iterator start = succ->begin(); 151 while (start != succ->end() && start->isPHI()) { 152 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2) 153 if (start->getOperand(i).isMBB() && 154 start->getOperand(i).getMBB() == BB) { 155 start->RemoveOperand(i); 156 start->RemoveOperand(i-1); 157 } 158 159 start++; 160 } 161 162 BB->removeSuccessor(BB->succ_begin()); 163 } 164 } 165 } 166 167 // Actually remove the blocks now. 168 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) 169 DeadBlocks[i]->eraseFromParent(); 170 171 // Cleanup PHI nodes. 172 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { 173 MachineBasicBlock *BB = I; 174 // Prune unneeded PHI entries. 175 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(), 176 BB->pred_end()); 177 MachineBasicBlock::iterator phi = BB->begin(); 178 while (phi != BB->end() && phi->isPHI()) { 179 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2) 180 if (!preds.count(phi->getOperand(i).getMBB())) { 181 phi->RemoveOperand(i); 182 phi->RemoveOperand(i-1); 183 ModifiedPHI = true; 184 } 185 186 if (phi->getNumOperands() == 3) { 187 unsigned Input = phi->getOperand(1).getReg(); 188 unsigned Output = phi->getOperand(0).getReg(); 189 190 MachineInstr* temp = phi; 191 ++phi; 192 temp->eraseFromParent(); 193 ModifiedPHI = true; 194 195 if (Input != Output) { 196 MachineRegisterInfo &MRI = F.getRegInfo(); 197 MRI.constrainRegClass(Input, MRI.getRegClass(Output)); 198 MRI.replaceRegWith(Output, Input); 199 } 200 201 continue; 202 } 203 204 ++phi; 205 } 206 } 207 208 F.RenumberBlocks(); 209 210 return (DeadBlocks.size() || ModifiedPHI); 211 } 212