UnreachableBlockElim.cpp revision 198892
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/Constant.h" 25#include "llvm/Instructions.h" 26#include "llvm/Function.h" 27#include "llvm/Pass.h" 28#include "llvm/Type.h" 29#include "llvm/Analysis/ProfileInfo.h" 30#include "llvm/CodeGen/MachineDominators.h" 31#include "llvm/CodeGen/MachineFunctionPass.h" 32#include "llvm/CodeGen/MachineModuleInfo.h" 33#include "llvm/CodeGen/MachineLoopInfo.h" 34#include "llvm/CodeGen/MachineRegisterInfo.h" 35#include "llvm/Support/CFG.h" 36#include "llvm/Target/TargetInstrInfo.h" 37#include "llvm/ADT/DepthFirstIterator.h" 38#include "llvm/ADT/SmallPtrSet.h" 39using namespace llvm; 40 41namespace { 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 48 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 49 AU.addPreserved<ProfileInfo>(); 50 } 51 }; 52} 53char UnreachableBlockElim::ID = 0; 54static RegisterPass<UnreachableBlockElim> 55X("unreachableblockelim", "Remove unreachable blocks from the CFG"); 56 57FunctionPass *llvm::createUnreachableBlockEliminationPass() { 58 return new UnreachableBlockElim(); 59} 60 61bool UnreachableBlockElim::runOnFunction(Function &F) { 62 SmallPtrSet<BasicBlock*, 8> Reachable; 63 64 // Mark all reachable blocks. 65 for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I = 66 df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I) 67 /* Mark all reachable blocks */; 68 69 // Loop over all dead blocks, remembering them and deleting all instructions 70 // in them. 71 std::vector<BasicBlock*> DeadBlocks; 72 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 73 if (!Reachable.count(I)) { 74 BasicBlock *BB = I; 75 DeadBlocks.push_back(BB); 76 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 77 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); 78 BB->getInstList().pop_front(); 79 } 80 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) 81 (*SI)->removePredecessor(BB); 82 BB->dropAllReferences(); 83 } 84 85 // Actually remove the blocks now. 86 ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>(); 87 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) { 88 if (PI) PI->removeBlock(DeadBlocks[i]); 89 DeadBlocks[i]->eraseFromParent(); 90 } 91 92 return DeadBlocks.size(); 93} 94 95 96namespace { 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} 106char UnreachableMachineBlockElim::ID = 0; 107 108static RegisterPass<UnreachableMachineBlockElim> 109Y("unreachable-mbb-elimination", 110 "Remove unreachable machine basic blocks"); 111 112const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y; 113 114void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const { 115 AU.addPreserved<MachineLoopInfo>(); 116 AU.addPreserved<MachineDominatorTree>(); 117 MachineFunctionPass::getAnalysisUsage(AU); 118} 119 120bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) { 121 SmallPtrSet<MachineBasicBlock*, 8> Reachable; 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() && 152 start->getOpcode() == TargetInstrInfo::PHI) { 153 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2) 154 if (start->getOperand(i).isMBB() && 155 start->getOperand(i).getMBB() == BB) { 156 start->RemoveOperand(i); 157 start->RemoveOperand(i-1); 158 } 159 160 start++; 161 } 162 163 BB->removeSuccessor(BB->succ_begin()); 164 } 165 } 166 } 167 168 // Actually remove the blocks now. 169 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) { 170 MachineBasicBlock *MBB = DeadBlocks[i]; 171 // If there are any labels in the basic block, unregister them from 172 // MachineModuleInfo. 173 if (MMI && !MBB->empty()) { 174 for (MachineBasicBlock::iterator I = MBB->begin(), 175 E = MBB->end(); I != E; ++I) { 176 if (I->isLabel()) 177 // The label ID # is always operand #0, an immediate. 178 MMI->InvalidateLabel(I->getOperand(0).getImm()); 179 } 180 } 181 MBB->eraseFromParent(); 182 } 183 184 // Cleanup PHI nodes. 185 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { 186 MachineBasicBlock *BB = I; 187 // Prune unneeded PHI entries. 188 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(), 189 BB->pred_end()); 190 MachineBasicBlock::iterator phi = BB->begin(); 191 while (phi != BB->end() && 192 phi->getOpcode() == TargetInstrInfo::PHI) { 193 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2) 194 if (!preds.count(phi->getOperand(i).getMBB())) { 195 phi->RemoveOperand(i); 196 phi->RemoveOperand(i-1); 197 } 198 199 if (phi->getNumOperands() == 3) { 200 unsigned Input = phi->getOperand(1).getReg(); 201 unsigned Output = phi->getOperand(0).getReg(); 202 203 MachineInstr* temp = phi; 204 ++phi; 205 temp->eraseFromParent(); 206 207 if (Input != Output) 208 F.getRegInfo().replaceRegWith(Output, Input); 209 210 continue; 211 } 212 213 ++phi; 214 } 215 } 216 217 F.RenumberBlocks(); 218 219 return DeadBlocks.size(); 220} 221